NONDESTRUCTIVE 14 MEV NEUTRON ACTIVATION
ANALYSIS OF JADES
( 玉 之 非 破 壞 性 中 不 活 化 分 析 )
by
Chan Tak Shing
( 陳 德 誠 )
A Thesis submittteCd in Partial Fulfillment
of the Requirements for the Degree of
Master of Philosophy in Physics
The Chinese University of Hong Kong
June 1982
Table of Content
Acknowledgements i
Abstract ii
I. Introduction 1
II. Activation Techinique
2.1 Jade Composition Determinations
2.1.1. Activity Determinations 4
2.1.2. Interference Reactions 6
2.1.3. Optimization of Conditions 11
2.1.4. Compositions 13
2.2 Coincidetuce Counting Applied to the Decay
Scheme of 27Mg
2.2.1. Basic Theory 14
2.2.2. Resolving-Time Determination 17
2.2.3. Accidental Coincidences 18
2.2.4. Coincidence Measurements 24
2.3 Internal Conversion Coefficients 28
III. Experimental
3.1 14 MeV Neutron Generator 31
3.2 Neutron Flux Measurements 32
3.3 The Pneumatic Transfer System 35
3.4 Sample Container 39
3.5 Gamma-Ray Spectrometry
3.5.1. Detectors 42
3.5.2. Electronic Relay Unit 46
36 Standard Samples 47
Page
IV. Results and Discussion
V. Conclusions
Appendixes
A . The Characteristics of Gems
B. Occurrence. of Jades
C. Plates of Jades
References
Page:
52
70
71
75
76
81
1
Acknowledements
I wish to thank my supervl sor, Dr. L. S. Chuang, for
his guidance and encouragement. I also wish to thank
K. S. Sin and S.W. Wong for their technical assistance. The
jades in this project were on loan -through the kindness of
0' Tama Jewellers and the Art Gallery of The Chinese
University of Hong Kong. It is a pleasure to thank
S.Y. Hung for her assistance in taking colour plates of
the jades. Thanks are due to L. Y. Kwok for typing this
thesis. I am also indebted to Dr. T. C. Wong for his
helpful discussions. The financial support of the Physics
Lepartment of the University is grate-ful.ly acknowledged.
Finally, I wish to express my gratitude to my family for
their continued encouragement.
T.S. Chan
June, 1982
11
AbstI',--Lct
Neutrons eneratt.ed from a 14 MeV neutron generator
were used to bombard the various elements of jades, and the
resulting gamma ac. L i vit ices, measured by. a 2-inch x 2-inch
NaI(Tl) scintillation detector coupled to a multichannel
pulse-height analyser, revealed the chemical composition of
the jades. The usefulness and advantages of thi..s analytical
method in the analysis of jades are described.
A simple relay unit which simultaneously initiates
the single-channel analysers, scalers, and multichannel
analyser in coincidence counting was constructed. The
accuracy of this unit is ±0.03 sec.
For comparative measurements of gamma activities,
standards made from chemical mixtures were used. The
accuracy and u.use[ulnc-.ss of these standards are discussed.
A. coincidence technique was introduced to study the
decay scheme of 2 'PvMg, which is a radionuclide resulting from
the nuclear reaction 27Al(n,p)?1M,1g. By measuring the
coincidence between the 0.844 MeV and 1.014 MeV gammas of
27Mg, the time correlation between these two gammas was
investigated.
By an extrapolation method, the internal conversion
coefficients for the gammas of` 7MMMg were found to be
3.63 x 10-' and 4.67 x 10-5 for the 0.84 MeV and the 1.01 MeV
gammas, respectively.
The interference problems encountered in multi-element
determinations, and the solutions to them, are discussed.
1
Chapter 1
Introduction
In recent years, neutron activation analysis has
developed into a powerful analytical method of elemental
analysis, and is widely used in many areas and in various
applications. This technique can frequently offer the
advantages of good sensitivity and selectivity, high
precision and accuracy, and non-destruction of samples.
Coupled with modern techniques such as charged-particle
activation analysis, X-ray fluorescence, and atomic
absorption spectrometry, neutron activation analysis
serves as -one of a series of complementary analytical
methods.
Neutron activation analysis is widely applied in
industrial, environmental, archaeological, and medical-
studies. Perhaps the results of this research will help
to bring the technique to the,commercial field. Strictly,
or mineralogically speaking, jade is divided into two
classes, nephrite and jadeite (Webster, 1975). However,
many greenish tough minerals, which have an appearance
similar to jadeite or nephrite are often, though wrongly,
called jade. From the points of view of both sellers
and customers, it is important and necessary to distinguish
clearly between the green stones, as the price for a
genuine jadeite and an ordinary green stone differs
tremendously. Without going through certain tests, but
instead merely depending upon the external appearance
2
of a given greenish stone, it is di.I:.-i'icult to tell whether
it is really a jade. The tests general l.y employed by
jewellers are the specific gravity test, the hardness test,
the refractive index test, and the absorption spectrum,
test (Liddicoat, 1969). All these tests are nondestructive,
except the hardness test, which may leave a scratch on the
stone. Some minerals, for example smaragdite, chloromelanite,
and the so-called "synthetic jade", may not be distinguished
from jade by the above tests. Nevertheless, if the chemical
compositions of these stones are examined, one may be able
to establish the identities of the stones.
The jades analyzed in this study were each put
near the target of a 14 MeV neutron generator and bombarded
by the fast neutrons. By measuring the gamma rays emitted
from the induced activities, and using the comparator
method, the concentrations of some the elements in the
jades were measured. The gamma-ray spectrum of jadeite
can be easily identified by its two intense 21Mg photopeaks
(formed from aluminum), which 'have energies of 0.844 MeV
and 1.014 MeV, respectively. Besides aluminum, other
elements after being activated can also emit gamma radiation
of about 0.844 MeV (for example, iron) and of about 1.014
MeV (for example, chromium). I f there were a time
coincidence between the gamma rays emitted by 27Mg, the
product nucleus of the 27A1(n, p) 27 Mg reaction, it would
be possible to distinguish the gamma activity of 27Mg
3
from that of other radionuclides that emit gammas of similar
energies. Some workers (Ciuffolotti and Demicheli s, 1 962)
reported that the two 27Mg radiations are in coincidence,
but the present decay scheme (Lederer and Shirley, 1978)
does not show this. With this in mind, we conducted a
reinvestigation of the decay scheme of 27 Mg by studying
the coincidences of the 0.844 MeV and 1.014 MeV radiations,
and their internal conversion coefficients.
In the analysis of the pulse-height spectra,
photopeak areas were obtained by the total peak area
method (Kokta, 1973), which employs a simple baseline
subtraction technique. The counting statistics (Quittner,
1972) obtained for sit. icon, which constitutes about 27%
by weight of jade, were ±1.05% (one standard. deviation),
measuring the 1.779 MeV gamma-ray peak of 2.246-minute
2 6A1.
Chapter II
Activat ion Technique
2.1 J a de Compo sition Peterminations
2.1.1. Activity Peterminat ions
14 MeV neutrons are produced at the target of the
neutron generator (Kaman type 711A) (Karri an, 1973) via the
react ion :
The high-energy neutrons so produced were used to
bombard samples, to produce various radionuclides. The
measured activity of each radionuclide can be related
to the amount of target element in two ways, namely, via
the absolute method, and via the comparator method.
In the absolute method (Garrec, 1969) (Girardi,%
1964), the activity, A, at the end of time tp after an
irradiation for a time duration tj for each radionuclide
is given by:
where N is the total number of atoms of target nuclei (of
a given Z and A) in the sample, a is the cross section of
the target nuclide for a given reaction with neutrons of
5
a given energy, cb is the neutron f lux to which the target
atoms are exposed, f is the total attenuation correction
factor of neutrons and gamma rays, E is the detection
efficiency, and A i s the decay constant of the radionuclide.
The total attenuation correction factor, which is
the product of the neutron attenuation factor and the garnma-
ray attenuation factor, can be calculated by using the
technique developed by Nargolwal l a (1968).
In actual analysis, an accurate absolute neutron
flux value and the attenuation correction factor cannot
be determined easily (Meinke, 1957). By the use of the
comparator method, these difficulties can be overcome.
With it, the mass of the element is determined by comparing
the intensity of the gamma radiation of the sample, as
measured by the area of the appropriate photopeak, with
that of a standard which was irradiated and counted under
the same conditions. Iddings (1964) and Volborth (1963)
have recommended this method as being the more accurate one.
If Au and Ag are, respectively, the activities of
the unknown and of the standard, and if Mu and Ms are the
corresponding masses of the element, then the following
equation holds:
with neutron flux the same for both samples, as well as
all other irradiation and counting conditions. However)
the neutron flux usually varies during the analyses, and so
a neutron m o n i t o r i s u s e d . T h e a b o v e e cj u a t i o n i s t b e n
modified to:
where Rs and R are the neutron monitor counts for the
standard and unknown samp 1 e , respeclively, dur ing 1:he
neutron-irradiation period.
2.1.2 Interference React ions
M a n y a u t h o r s h a v e u s e d t h e n e u t r o n a. c t i v a t i o n
technique to determine the silicon and oxygen contents of
meteorites (Wing, 1964), and, the aluminium and silicon
contents of rocks (Turner, 1956). In advance of any
activation measurements, it is useful to consider what
chemical elements exist in jades. Since jade comes from
minerals of the Earth, we would expect the elements present
in jades to have some correlation with the abundances of
the elements which constitute the Earth's crust (Weast,
1970). From Table 2.1, it can be seen that some eight
elements have abundances greater than 2% by weight of the
Earth's crust. They are oxygen, silicon, aluminium, iron,
calcium, sodium, potassium and magnesium. All of these
are known to be present in some jades (plus Mn and perhaps
Cr), and can be readily activated, using the technique
of 14 MeV neutron activation analysis (Nargolwalla, 1973).
That isj too closely similar in energy to be resolved from
one another with the type of detector used.iJ L
H owev e r, fr om t h e nu c1e ar d at a for the r e ac tion s of
these elements with 14 MeV neutrons (Nargolwalla, 1973),
(Table 2.2), it can be seen that two types of interference
reactions exist. The first type is that in which two or
more activation products emit gamma rays of the same energy
or nearly the same energy, e.g., the activation of 2Al
and 56Fe with 14 MeV neutrons produces different radionuclides,
but both decay with the emission of gamma rays of about
0.844- MeV energy. The type 2 interference is one in which
two different elements, when activated, yield the same
Oli O V r 11radionuclide, e.g., Na produced from both ;A1 and z Mg.
There are two principal ways to solve such
interference problems, namely, by means of half-life
differences of the radionuclides, or by means of the fact
that one of the elements may give rise to a secondv '
photopeak which is free from interferences. Belonging to
the type 1 interference, 2 7Mg has a half-life of 9.4-5
-minutes, whereas 56Mn has a half-life of 154.9 minutes.
After a waiting time of 45 minutes, for example, the
activity of 27Mg will be reduced to only 0.037 of its
E1 emeut Abundances, in %
0
Si
A1
Fe
Ca
Na
K
Mg
Ti
H
P
M n
S
C
CI
Rb
F
Sr
Ba
Zr
Cr
V
Zn
Ot
46. 60
27 . 72
8 . 13
5.00
3 . 63
2 . 83
2 . 59
2 . 09
0.440
0.140
0. 118
0. 100
0. 052
0.0320
0.0314
0.0310
0.0300
0.0300
0.0250
0.0220
0.0200
0.0150
0.0132
0 . 31
Table 2.1
Chemical Composition of the Earth's Crust;
(Elements present 0.01%, from Mason, 1952
9
initial value, wllcrcas that of 5 6Mn will only be reduced
to 0.818 of its initial value. In the case of the type
2 interference cited as an example, the 1.014 MeV ph.otopeak
of 2 7Mg can be used to ascertain the 27 Al contribution
to the 2 Na activity from that due to 2 1+Mg.
TARGET
NUCLIDI, REACTION
PRODUCT
RADIONUCLIDE
and half-lift
TARGET
% ISOTOPIC
ABUNDANCE
14 MeV
O (nib)
MeV OF MAIN y'S (AND
THEIR % EMISSIONS)
160
2 3Na
2 3Na
2 '4Mg
2 5Mg
2 6Mg
27A1
27a:
2 8 Si
29Si
30Si
39r
UI(
41K
4 2Ca
4 4 Ca
4 4 Ca
5 0 Cr
5 2Cr
5 3 Cr
5 5Mn
5 4Fe
5 ,F e
5 7F e
(n,p)
(n,p)
(n,a)
(n,p)
(n,p
(n, a]
(n,p;
(n,a)
(n,p;
(ri,p)
(n,a)
(n, 2n)
(n,p)
(n,a)
(n,p)
(n,p)
(n,a)
(n, 2n)
(n,p)
(n,p)
(n,a)
(n,2n)
(n,p)
(n,p)
16N5 7.13s
23Ne,37.6s
2 °F, 11.0s
24Na,15.02b
2 5 N a, 6 0 s
2 3 N e, 3 7 . 6 s
27Mg, 9.45 m.
24Na,15.021
2 8A1, 2.246m,
! 29A1, 6.52m
2'Mg, 9.4 5n
38K, 7.6 3n
41Ar, 1.83h
3 8 C1,3 7.2m
42K,12.36b
44K,22m
41Ar, 1.83b
4 9Cr,42.Om
52V, 3.755m
53V, 1.55m
52V, 3.755m
5 3Fe . 8.53m
5 6Mn, 2.5821
:) 1 Mn, 1. 59m
99.76
100.00
100.00
78.99
10.00
11.01
100.00
100.00
92.20
4.70
3.10
93.30
6.70
6.70
0.65
2.08
2.08
4.35
83.79
9.50
100.00
5. 80
91.7
2. 19
39
43
150
190
44
7 7
75
116
230
120
70
3.5
49
39
182
36
35
19
94
40
32
15.5
103
75
6.128 (69%)
0.439 (33%)
1.633 (100%)
.1.369 (100%)
2.754 (100%)
.0.391(12.8%),0.586(13%),
V975(12.8%),1.612(8.8%)
0.439 (33%)
0.844 (72%),
1.014 (28%)
1.369 (100%),
2.754 (100%)
1.779 (100%)
0.511 (200%),
1.273 (91%)
0.844 (72%),
U.014 (28%)
0.511 (200%),
2.167 (100%)
1.294 (99.2%)
.1.642 (32.8%),
2.168 (55%)
1.525 (17.9%)
.1.024(10%),1.127(13.5%).
(1.157(85%),2.150(29%),
2.519 (10%)
1.294 (99.2%)
,0.511 (192%),
1.152 (29.5)
1.434 (100%)
.1.006 (88.7%),
(1.287 (11.3%)
1.434 (100%)
.0.377 (43%),
0.511 (196%)
,0.847(99%),1.811(30%),
2. 113 (15.5%)
.0.122 (9.5%) ,
1.692 (2.7%)
Table 2.2
Values taken from G. Crdtmann, 1976
DATA FOR 14 MeV NAA OF JAPE S
2.1.3 0 p t i m i x a t i o n o f Con d i t i on s
NAA = neutron activation analysis
From the above, it can be seen that proper selection
of the irradiation time is important in multi-elemental
analysis by NAA. The half-lives of the significant
activated nuclides in jades differ greatly, ranging from
7.13 seconds to as long as 901 minutes. Due to economic
considerations, and the need to minimize interference
reactions, the irradiation time for each analysis was
chosen to be 600 seconds. In this time, the fraction of
saturation for 27Mg is 0.520; 28A1, 0.954; for 56Mn, 0.0438;
for 24Na, 0.00766. Shortening the irradiation time to, say,
300 seconds, the fractions of saturation of the short-lived
nuclides would not change much: 27Mg to 0.307 and 28A1 to
0.786. However, for the long-lived nuclides 5 6Mn and 2iiNa,
the fractions of saturation would only reach 0.0221 and
0.00384, respectively, and these activities would then be
too low to be detectable in a long-lived background counting
If the irradiation time were lengthened to 1000 seconds, th.it
would essentially double the activities of 56Mn and 24Na,,
but would correspondingly increase the MCA deadtime loss
and cost of the experiment.
Of equal importance, the decay time for the counting
of the activated samples needs careful selection. Owing
to the great abundance of silicon in jade, the Compton
edge and Compton continuum of the 28A1 (ti = 2.246 minutes)
activity introduce a large background for the detection of
photopeaks of other elements. The only way to remove this
interference is to take advantage of the short half-life ol
28A1, by increasing the decay time. However; there is one
element present in jade, chromium.which is of high interest
5 ?
Too long a decay time will decrease the activity of V,
the product of the 52Cr(n,p)52V reaction, whose half-life
is 3.755 minutes.
In order to measure or attempt to measure the
elements silicon, aluminium, iron, magnesium and chromium,
we divided the counting process into two stages. The first
stage was to measure the short-lived radionuclides, while
the second stage was for the measurement of the longer-
lived radionuclides. For stage 1, a decay time of 20 seconds
was chosen, which still allowed sufficient time for the
1 6sample to be put at the counting position and for N
(ti = 7.13 seconds), the product of the 160(n,p)16N
reaction, to decay out sufficiently. This short decay
time was chosen in order that the gamma activity of 5 2V
might be measured efficiently. ' For stage 2, a decay time
of 45 minutes was chosen, by which time the activities of
28A1 and 27Mg were reduced by factors of 9.30 x 107
and 0 .0369, respectively, and their contributions to the
spectrum were thus made negligible.
The selected counting times for both stages were
2000 seconds, in order to obtain better counting statistics
for the activities of 7Mg and the coincidence countings
for its 0.844 McV ancl 1.014 MeV gammas, and for the
activities of the Iong-1ife nuc 1 ide 2 'fNa. Garrec (1969 )
suggested an irradiation time of 10 minutes, a 130-minute
decay time, followed by a 10-minute counting time. However
this lengthy analysing time was not felt to be appropriate
in this work.
Two runs were made for each sample, the time
interval between the analyses being 1 day for jadeite samples,
and 3 days for other jades, since jadeite produces mainly
short-lived radionuclides, 28A1 and 7Mg, whereas other
jades produce significant amounts of the long-lived
radionuclide, 2t+Na.
2.1.4. Compositions
Due to the rather poor energy resolution of Nal(Tl)
scintillation detectors, the 1.811 MeV photopeak of 36Mn,
f~ [T gproduced by the nuclear reaction 1 Fe(n,p)3 5Mn, overlaps
with the 1., 779 MeV photopeak of 28A1, produced by the
i
reaction 28 Si(n,p)28A1. The contribution of 2dAl at
1.779 MeV can be found by a method similar to that suggested
by Grieken (1968), that is:
where k„ is the ratio of the activity of 56Mn at 1.779 MeVFe
to that at 0.847 MeV , ancl f rom act ivat ion of pure iron , the
value of kr, was found to be 0.235 ± 0.008.f i n
3 1 2 8The nuclear reaction of phosphorus, P(n,a)~ Al,
also produces the same 1.779 MeV gamma radiation. However
the amount of phosphorus present in meteorites and rocks
is less than 0.3% in all the known cases (Wing, 1964),
and thus phosphorus should not interfere significantly in
the analysis of jades (see Table 2.2 ).
The compositions of the various jade samples were
determined as follows. In stage 1 counting, both aluminium
(27Mg) and iron (56Mn) contribute to the 0.844 MeV photopeak,
r- r- ry Qand both iron (J Mn) and silicon ( Al) contribute to the
1.779 MeV photopeak. In stage 2 counting, only iron (J 6Mn)
plays a role in the 0.844 MeV photopeak, and only magnesium
(2 ''Na) in the 1.369 MeV photopeak. By subtracting the
c o n t rib u tio n o f J GMn (amo un t determine d f rom s t age 2) t o
the 1.779 MeV peak in stage 1, the amount of silicon can
be obtained. The amount of aluminium may be found in%
a similar manner. Chromium, whose 1.434 MeV photopeak
lies on the Compton edge of the 1.779 MeV gamma of 28A1,
by a spectrum-stripping method, using both graphical and
computer techniques (Kowalski and Isenhour, 1968).
2.2 Coincidence Counting Applied to the Decay Scheme of 27Mg 1
2.2.1 Basic Theor'
Besides 27Mg, a number of other fast-neutron
product radionuclides of common elements also emit gamma
rays of about 0.84 MeV andor 1.01 MeV, and hence can
interfere with the determination of the aluminium content
of jade samp 1 es. I f , however , tlie two gamma rays of 27Mg
are emitted in coincidence, it might be possible to
distinguish the ZMg activity from that of, say, 5b.Mn
and 52V - by use of a coincidence technique. Some workers
(Ciuffolotti and Demiche1is, 1962) c1aimed that the two
gammas of 27Mg are emitted in coincidence, but the recent
(Fig. 2.1) decay scheme of 27Mg (Lederer and Shirley, 1978)
(Ophel and Lawergren, 1963), shows that the two are not in
coincidence. Therefore, in the process of conducting these
jade analyses, the decay scheme of 27Mg was also studied.
Suppose a radionuclide source emits two gamma rays
in cascade , and i. t:s d isintegrat ion rate is no. Dectector
1 will have a countine: rate ni . that is:
n i = nob (2 ~ I T)
where £i is the overall efficiency of detector 1,
Similarly for detector 2,
n 2. = no£; (2 - 2)
If these two detectors are connected in coincidence, the
coincidence count rate measured fSieabahn and Bell. 1965)
will be:
ni? — n o £i£ o (2 - 3)
27Mq12 9-45 mi n
6 7 %s
.33 %
1014
J9-844
2713
28AI
13 2-24 6 mi n
100%
1-779
28Si
14
Fiy. 2.1
Decay Schemes for 2 7Mg and 28A1
The accidental coincidence rate ip is given by:ct
(2 - 4)
where t is the resolving time, which is the time period in
which two pulses can be recorded as a coincidence event.
It can be seen from equation (2 - 4) that even a
small increase in no will result in a relatively large
2increase in n , since n increases with no , while n]2
a a
increases with no- The ratio of the true coincidence
counting rate to accidental coincidence counting rate is
thus :
In order that the accidental coincidence rate he
reduced to a minimum with respect to the true coincidence
rate, the resolving time should be kept as short as possible.
2.2.2 Resolving-Time Determination
There are two methods (Bleuler and Goldsmith, 1960)
for the determination of the resolving time of a counting
system. The first method is to plot the number of
coincidences as a function of delay inserted in one of the
channels, as shown in.Fig. 2.2. Since the coincidence
analyser detects only the leading edge of the input signals,
the width of these signals does not affect the resolving
times (Canberra, 1979). Another method is to plot the
rate of coincidences as a function of the product of the
disintegration rates of two unrelated sources (Fig. 2.3).
In this case, two independent sources are counted with the
two detectors, which are well shielded from each other so
that no coincident radiation can reach both detectors.
The coincidences observed, then, are only due to the finite
resolving time and to a background produced by cosmic rays
and natural radiations.
By means of these two methods, the resolving time
of the coincidence circuit was found to be 38 nsec. and
34 nsec., respectively.
2.2.3 Accidenta1 Coinc1dences
For fairly short half-life nuclides, such as 27Mg»
and 28A1, accidental coincidence events can be calculated
as follows. The windows of one single-channel analyser are
set to accept pulses in the 0.84 MeV gamma region, and those
in the other single-channel analyser are set to accept
pulses in the 1.0 MeV gamma region.
300
200
100
= 38 nsec.
2 T
S o u r c e u s e ci: 2 2 N a
0
1 . 72 1. 84 .. 96 2 . 08 2.20 x 0.1 usee.
R e 1 a t i v e D e 1 a y S e 11 i n g
Fig. 2.2
Reso1ving~Time Determination
0d•HdPrO 'O •
oCO 0P COdd oo ca
o P
0 dO Hd0
•HOd
•Ho
o
o00
OOlO
C•rH
+-C0
w
0O£0
HO£
•rHoo
rH0$-P
0To•HOO
300
200
100
0
5 10 15
n - 2T nin 2cl
Slope=34 nsec.
n i x n2
20x106 sec2
Fig. 2.3
Resolving -Time Determination
( i ) Acci dental__coinc i dences duo to pure aluminium
From equations (2 - 1), (2 - 2), and (2 - 3):
where 0.72 and 0.28 are the 2 7Mg branching ratios
(Lederer and. Shirley, 1978) for the 0.844 MeV and
1.014 MeV gammas of 27Mg, respectively. The total
number of accidental coincidences measured in the
counting period is:
2 t ( 0 . 28 ) ( 0 . 72 ) e i c 2 n o 2 dt , ( 2- 5 )
where t is decay time = 20 sec., and tc is counting
time = 2000 sec.
The total counts recorded by the single-channel
analyser set for the 0.844 MeV region is:
(2-6)
Similarly, for the single-channel analyser set for the
1.0 MeV region:
n o e 2 ( 0 . 2 8 ) clt (2 - 7)
Combining (2-5), (2-6), and (2 - 7), we obtain:
N = 2t x 7.25 x 10 4 N2N9a
(ii) Accidental coincidences due to pure silicon
From (2 - 1), (2 - 2), and (2 - 3):
n i = n o e i p i
n 2. - n o e 2 P 2
n = 2 t n ! n 2cl
(2-8)
(2-9)
where pi and p2 are the probabilities that a 1.78 MeV
gamma-ray photon will scatter to an energy of 0.84 MeV
and of 1.01 MeV, respectively.
Total accidental coincidence counts:
1
If n i . 7 b is the coun t ing rate registerecl by a single-channeI
an a1ys er at 1.78 MeV in detecto r 1, wit h counting e f ficien cy
iei.78; then:
i
H I . 7 8 = n o C 1 . 7 8 (2 - 10)
Then, dividing (2 - 8) by (2 - 10), we obtain:
i
Taking Ni.7 8 as the total number of counts registered by
the single-channel analyser at 1.78 MeV in detector 1, we
have :
and
Similarly, if Nj.78 is the total number of counts registered
by the single-channel analyser at 1.78 MeV in detector 2,
2
with counting efficiency e!.7 8,then:
N? N iThe values of (—7 ) and (—7 —) can be found from
N 1 . 7 8 N 1 • 7 8
experimental spectra, and both were found to be approximately
equal to 0.4.
For a typical spectrum of aluminium, the total number of
accidental coincidence counts calculated was 0.23, for a mass
of aluminium = 0.37 g.
For a typical spectrum of silicon, the total number of
accidental coincidence counts calculated is 0.11, for a mass
of silicon = 0.17 g.
2.2.4 Coincidence Measurements
By measuring the coincidences of the two gammas of
27Mg in jadeite samples, it was found that the coincidence
counting rate was directly proportional to the 0.84 MeV
single-channel analyser counting rate, and to the 1.01 MeV
single-channel analyser counting rate, respectively (Fig. 2.4). i
These coincidences may first be considered as true coincidences
Then, when plotted against the product of the two single-
channel analyser counting rates, a. parabolic curve is obtained
o0CO
OOoCnI
C•H
130U20
0
0
£33O
O
0O£0
13•HOt—t
Ho
o
8C
6C
40
20
0
4 8 12 16 20 24 28 32 x 10
Single-Channel Analyser Countingat 0.84 MeV
Fig. 2.4
Coincidence Counts Versus Single-Channel Counts
N1XN2
Fig 2.5
plot of conincidence counting versus product
of Sca 'S Counting
(Fig. 2.5). However, if the gammas are in true con evidence,
the Ni obtained by calculation was much larger than that
found by expertment. Instead of j ade it e , pure a. 1 urnin ium
and silicon were then used for similar coincidence countings,
with the single-channel, analyser windows again at 0.84 MeV
and 1.01 MeV, respectively. An activated sample of silicon
was sandwiched between a pair of thin (1.2 mm thick)
Australian jades before counting, so that the overall
density approached that of aluminium. The result obtained
is indicated in Fig. 2.6. The slopes of the two graphs
are 1.98 x 10 and 1.29 x 10 , respectively. However,
the single-channel analyser countings in Fig. 2.6(a)
include the photopeak area, N, and the Compton continum, B.
If the coincidences are plotted against B, then the slope
of the graph becomes 1.33 x 10 3, which is similar to that
of Fig. 2.6(b).
The results thus show that coincidences in activated
a 1 urniniurn arise not from from its ? 7Mg photopeaks , but
rather from gamma-ray seattering. Simi1ar phenomena occur
with jadeite samples, the coincidences recorded being due
to scattered 1.78 MeV gamma-rays of 28A1. This peculiar
coincidence happens as the two detectors are placed very
close to each other and in a face-to-face geometry.
(Fig. 2.7).
2 . 3 Internal Con vers 1 o n C o e ff 1. c i e n t s
To check on the correctness of the coincidence
measurements made in this study of the decay scheme of 27Mg,
the internal conversions of its 0.844 and 1.014 MeV gamma
rays were estimated by extrapo1ation. In returning to the
ground-state configuration, excited states of radionuclides
usually decay either by a radiative process (emitting a
gamma-ray photon) or by a non-radiative process, which in
most cases is the emission of an internal conversion
electron.
The total internal conversion coefficient is
defined as the ratio of the probability for internal
conversion to that for gamma emission. In this work, the
conversion coefficients of the two gamma rays from 2 7Ivlg
were obtained by extrapolating values for various atomic
numbers greater than 30 (Lederer, 1978) to Z=12. By means
of a least-squares fit, plotting the logarithm of the
conversion coefficient against the logarithm of the atomic
number, the internal conversion coefficients for the gammas
i n 2 7 M g were f o u n d 1; o b e
Energy in MeV :iK:Li
aLII Hi
0.844
1. 014
2.97 x 105
3.97 x 105
6.50 x 10 6
7.02 x 106
2 . 20 x 10 '8
1.58 x 108
3.95 x 10 8
1.83 x 10~9
Hence, ar = 3.63 x 10 2 (100% E2) for the 0.844 MeV gamma,
• and = 4.67 x 10 5 (90% Ml + 10% E2) for the 1.014 MeV.
These calculations thus show that internal conversion in
27Mg can be neglected, since the internal conversion
coefficients are extremely small for both energies.
w-P5=1Go
o
CDOGCD
-d•Ho
•Ho
o
50
4(
30
20
10
S1 o p e = 1. 3 3 x 10
against B
1 IN
J3l.
-- a g a i n s t (N+B)
Slope=l.98x10 '
0 2 4 6 8 10 12 14 16 18xl0,(
SCA Counting at 0.84 MeV
( a) A1 urninum Samp 1 e
w+-Gpo
o
ooG0)
•Ho—
oo
5C
4C
3C
2C
1C
0 4 8 12 16 20 24 28x103
Slope=l.29x10
SCA Counting at 0.84 MeV
(b) Silicon S amp1e
TT-i cr 9. I
C oin cidences v ersu s S C A Co unting
i'Yir ( 'A A I nnii mini 1b .9 1 1 i cnn
h v
i nc ident
gamma photon
f ree
e iectron
scat tered
gamma photon
hi;'
scattered
eiectron
Na I (T I)
c rystaI
i
Vsample
Na I ( TI )
crystaI
2
Fig. 2.7
Compton Scat, t e r i n g
Chanter III
Ex per linen Lai
3 • 1 14 MeV Neutron Generator
The neutrons for the present research were produced
by a 14 MeV neutron generator (Kaman type 711A) (Kaman,
1973) via the nuclear reaction:
This generator was provided by the International Atomic
Energy Agency (IAEA) in 1974, and is of the sealed-tube
type, It is installed at the Science Centre of The
Chinese University o f 11 ong Ko n g. As the ins t a11a tio n sit e
is quite close to classrooms, the radiations generated are
heavily shielded to at least a safety factor of 10 (Chuang,
1975), in addition to the allowed safety level suggested
by the manufacturer. Operation of the generator is achieved
through a control console, which is situated in a control
room separated from the generator room by a thick concrete
wall.
Common to all sealed-tube accelerators, this
generator tube consists of an ion source, an accelerating
structure, a target section, and a replenisher. The ion
source is of the Penning Ion Gauge (PIG) type, in which
deuterons and tritons are produced with atomic-to-molecular
ion ratio of about 0.05 (Wood, 1972). Situated behind the
ion source, the resplenisher unit is designed to adjust the
hydrogen gas pressure in the acce 1 erator tube . I t cons:i st s
of two gas occluding elements made of titanium wrapped on
tungsten heating wire, and will absorb or release hydrogen
isotopes depending on the titanium temperature. The target
of the accelerator tube is of the gas-in-me 1:a 1 type , with
copper as the backing plate. Deuterium and tritium gas
are impregnated into a t; itanium matr ix .
As the accelerator employs a high voltage power
supply (160 KV), pressurized sulphur hexafluoride is used
for insulation. In a closed-loop heat-exchange system,
Freon-113 is used to cool the ion source, and in a similar
closed-loop system clean de-ionized, water is used to cool
the target assembly.
The spectrum of the neutrons emitted from the 14 MeV
neutron generator is shown in Fig. 3.1 (Chuang, 1979). The
energy of the neutrons centers at about 14 MeV, and in fact
depends on several factors, including the energy and the
atomicity of the incident ions, whether the reaction is
deuterium-upon-tritiurn ortritium-upon-deuterium , a.nd
the angle of emission (Chuang, 1979), (Stark, 1971),
(Fig. 3.2).
3.2 Neutron Flux Measurements
The absolute 14 MeV neutron output rate from the
generator was estimated by the Texas Convention Method
{Caman, 1969). This technique is based on activation
analysis and the nuclear reaction involved is the
4-I—U.T
ai -~r~c
H—
aP
r~4-K
arr
1C
p
f
2
2 l t 8 1( 12 14 M
ang leof
4 observatior
- c:
- 90(
- 150
Ne111 ron E ri p r q- v ( Mp V
Fie. 3.:
Neutron Energy Spectra at 0°, 90° and 150C
Incident particle energy ~ 0.16 MeV
(1) T + D
(2) D + T
4 He + n
4 He + n
Incident particle energy = 0.08 MeV
(3) T + D
(4) D + T
4 He + n
4 He + n0
bn£0£w
£O£XJ£0
15- 4
15-0
14-6
14-2
13-8
13-4
13-0
(i )
(2 )
(3 )
(4 )
0 20 4 0 60 80 IOC 120 140 160 180
(4)
'3 )(2)
!1)
A n g1e o f Ob s e r v alio n
Fig. 3.2
Variation of Neutron Energy as a Function of the
Ang1e of Observation for Different_ Mode1s of the
Production of the Neutrons
r _ c OCu(n,2n) ' ' C u r o a. c' t i o n . A co p p e r i o i 1 i s i. r r a d 1 a ted a. t
20 cm from the neutron target, and the 0.511 MeV gammas
r ofrom positron annihilation, due to the induced Cu
activity, were counted for one minute. The position of
the foil during co u n ting was 2.7 cm f r om a 3-inc h x 3-inc h
Nal(Tl) scintillation detector. The neutron yield at 2.5 rnA
beam current and 160 KV acceleration voltage was found to
be about 3.3. x 1010 nsec. (Table 3.1).
The Pneumatic Transfer System
For rapid transport of sample to the irradiation
position, and back to the counting station, a pneumatic
system was developed (Wong, 1975). It is a single-tube
system, with a gas inlet connected to the gas supply unit
and an outlet connected to the exhaust gas pipe line of the
building. Transit time is approximately 3 seconds and of
good reproducibility. The transfer gas used in this system
is air, but can be replaced by other gases, e.g., nitrogen
gas .
The transfer system (Fig. 3.3) consists of (i) an
adjustable constant-pressure gas supply unit, which is a
gas cylinder modified to act as a constant-pressure gas
reservoir connected to an air compressor; (ii) a programmable
electronic timer, to control the opening of the various valves
for blowing the sample towards the irradiation station and
backwards, and also the time for irradiation; (iii) an
irradiation station, which consists of a sample rotation
Beam Current, in ti iA Foil Mass . in
i
! Uncorrectey n min t c mi n
Background
C orrect ed
fnn n t~ q rn i
NeutronT ' 1.3 ' ,
l.
1.'
1. '
1.:
2 . ,
1 . 2 3 7 (
1 . 310
1. 240(
1 . 146
1.241!
708'
1055!!
1139!
1171r,
1618!
582!
800!
8 76 (
950
1 o n o
1.49 x 101
1.93 x 101
2.23 x 101
2.62 x 101
q o -i v i n1
Table 3.:
14 MeV Neutron Yield Measurements by the Texas Convention
direction of gas
flow when blowing in
direction of gaso
flow when blowing back
SLl
inletout I et
SL3
i rrad iat ionstat ion
SL 2 SL 4
count i ngsample in
station
SL 5
sample
discharge
Fig. 3.3
Schematic Diagram of the Pneumatic Transfer System
mechanism rotating at a rate of 170 rpm; and. (iv) a sample
loading station from which the sample is sent in for
irradiation .
The operation of the pneumatic transfer system is
as follows: the gas supply unit is connected to the inlet
of the pneumatic control panel, and the outlet of the
pneumatic control panel is connected to the exhaust gas
pipe line of the building. The outlet of the air compressor
is connected to a gas cylinder, and the air compressor is
? 7turned on until the pressure gauge reading reaches 5.5 Kgcm,
which is the optimum gas pressure for a sample of mass 8 g.
The capsule containing a sample is placed in the
loading station. When a steady production of neutrons is
achieved (a bo u t 3 0 s ec. a f ter t ur nin g o n the n eu11 o n
generation), the capsule can then be sent to the neutron
irradiation station. The distance from the middle of the
capsule to the generator target is 6 cm, and the axis of
the capsule is par• a11e1 to the target surface.
The whole control process is passed to the programm¬
able electronic timer once the start button is pressed.
At first, it activiates the solenoid valves SL2 and SL3,
so that they open for 3 seconds, letting the gas blow the
sample from the loading station to the irradiation station.
Then it starts to count the irradiation time, and when the
preset time is up, the solenoid valves SL1 and SL4 open
for 3 seconds, letting the gas blow the sample from the
irradiation station back to the counting stationdischarging
the sample by opening the solenoid, valve SL5 for 1 second.
3.4 Sample Container
Most of the samples undergoing analysis in this study
are quite expensive, special care had to be taken to prevent
them from being damaged, in the process of transporting them
from one station to another station. Sample containers were
constructed to fulfill this task. They are machined from
low-oxygen polyethene rods. Each capsule is of length
3.0 cm, with a cap which can be screwed tightly onto the
body. The outer diameter of the container is 3.2 cm,
whereas the inner diameter of the pneumatic tube is 3.5 cm.
Soft cylindrical tissue, having the same dimensions as the
container, is split into two halves. The sample to be
analysed is then sandwiched between the tissue before
being put into the container (Fig. 3.4). This is to ensure
that there is no damage to the sample during transportation,
and to fix the sample at the centre of the capsule. After
irradiation, the sample is taken out and put at the assigned
position for counting. This is to prevent unnecessary
contributions from induced activities of the capsule and
tissue,, and to provide the shortest source-to-detector
dist ance.
Low-oxygen polyethene was chosen for capsule making
because it only introduces a low background activity and it
is inexpensive. The transfer capsules can be reused for a
number of times, but it must be ascertained that the cap
screws tightly onto the capsule each time. Otherwise, the cap
may slip off the capsule, resulting in a loss of sample.
i I
s a mt
Fig. 3.b
Schematic Diagram of Detector System
42
3.5 Gamma-Ray Spectrometry
3.5.1 Detectors
Common to many research laboratories, solid sodium
iodide (thallium-activated) scintillation detectors were
used in this study for the detection of the gamma radiations
emitted by the radionuclides formed. In the experiments, the
crystal size employed was 2-inches x 2 inches. The detectors
(Canberra, 802-2) were positioned 2.2 cm apart, without lead
shielding, the arrangement of the detectors being shown in
Fig. 3.5. The scintillation detectors were each connected
to their respective preamplifiers (Canberra, 2005), and each
coupled to a double delay-line amplifier (Canberra, 1411).
A T-connector was used to provide two outputs from the
amplifier. One output was led to a single-channel analyser
(Canberra, 2036A), and the-other to a multichannel analyser
(Canberra, Omega 1, 1024 channels). One single-channel
analyser was adjusted to accept only in the region of 0.84
MeV, and was connected to a logic shaper and delay unit
(Canberra, 2055), where pulses were delayed before they were
delivered to a coincidence analyser (Canberra, 2040). The
other single-channel analyser was adjusted to accept only pulses
in the region of 1.0 N.TeV, and had its output directly led
into the coincidence analyser. This coincidence analyser
gives one count when one pulse from each detector arrives
within the resolving time of the circuit. Two face-to--face
detectors were installed, so that coincidence counting
and gamma-ray activity measurement could be carried out at
the same time.
With fine acljustment of an adjustable voltage gain
at the end of each photomultiplier tube base, the two
scintillation detectors could be made to match each other.
The optimum working voltage for the detector system was
found to be 1100 volts (Fig. 3.6), which was supplied by a
regulated high voltage power supply (Nuclear-Chicago Corp.).
The resolutions of the detector system were measured at the
0.662 MeV photopeak of 1 3 7Cs and at the 1.33 3 MeV photopeak
of 6°Co as 6.1% and 7.6%, respectively. Due to the
instability of the electronic components, three standard
point sources, 13 7Cs (0.6616 MeV), 6 °Co (1.1732 and 1.3325
MeV), and z Na. (0.5110 and 1.2745 MeV) were used in
calibrating the spectrometry system from time to time.
The photopeak counting efficiencies of the system,(Fig. 3.7),
were measured at these energies with standard sources
(Gunnink, 1961), and an absolute appi oach to do tennine the
intrinsic photopeak efficiencies was carried out by the
method of Lazar (1956).
One of the major factors that affect the precisions
attainable in 14 MeV activation analysis is the monitoring
of the neutron output rate, which can be clone by means of
an internal standard (Santos, 1968) or by means of an external
method (Mott, 1965). In our case, a neutron detector of
scintillation type (Nuclear Enterprises Corp., NE 422),
containing lithium in a matrix of zinc sulphide, was used.
As neutrons cannot produce scintillations directly, for
the detection of slow neutrons it is necessary to incorporate
some material, for example, lithium,into the phosphor, in
order to produce ionizing radiation on the passage of
(DPd
Pr~r—r-yPoo
0)
•H;
Co;—1
r-P
510
410
10-'750 850 9 5 0 1050 1150 1250 volts
opt imum
vol tage
A p p 1 led V o 11 a, g e
Fig. 3.6
Opt i mil rri V or king Vo 11 a ge _ Determin a t i_o n
de t e c t or 1
detector 2
G•H
Ocs
•rHo
I•r-1
Cl—w
CuCDQo-Po
£
5-
2
!
05
0-2 0-5 i 2
Gamma Energy, in MeV
P i g 3 . 7
Photopeak Efficiencies of the Two 2-inch x 2-inch
Nal(Tl) Scintil1ation Detectors
neutrons t hrough 111 o mi x111ro . W i t h t h i s dc) t oc tor , nou t runs
are detected in l.lie crystals by seinti 1.1 at ions arising i'rorn
the p r o d u c t s o f 11i e r e a c t i o n :
The signals generated are transmitted through a preamplifier
(Canberra, 2005), an amplifier (Canberra, 2012), and then
to a ratemeter (Ortec, 449), which is connected to a chart
recorder (Hewlett packard, 680). Even rapid changes of the
neutron flux can be read immediately from the chart recorder,
as the time constant of the ratemeter is preset at 0.1 sec.
The counting efficiency per unit volume of the phosphor
is 50% for thermal neutrons (Nuclear Enterprises, 1975).
In some cases, the efficiency of such a crystal may reach
100% (Nicholson and Sue1ling, 1955) (Fig. 3.8).
3.5.2 Electronic Relay Unit
A simple electronic relay was constructed (Motorola,
1976) to initiate the automatic'start of the single-
channel analysers, the scalers, and the multichannel
analyser. Adjustment of delay time (which equals 1.1 x
time constant of the RC circuit) can be done easily through
the change of a resistor andor a capacitor. This is a
monostable type relay, and the single-channel analysers,
scalers, and multichannel analyser will continue counting
once they are started until their preset time is up. When
the irradiation time registered by the programmable electronic
time is up, this relay starts its function. The accuracy
of this relay depends on the electronic stability of the
resistor FL and capacitor Ck , and is about ± 0.03 sec. A
block diagram of the relay circuit is shown in Fig. 3.9,
and the response c 1 iarac terist ics of 111e re 1 ay is ske 1:ched
in Fig. 3.10.
3.6 St andard Samp1es
In these experiments, the major-element composition
of jades was determined by using the comparator method,
which is often employed in activation analysis. In order
to obtain accurate results, the standard should have the
same geometry and have the same matrix as the sample. Under
these conditions, both the standard and the sample will
have the same neutron flux depression and gamma-ray
attenuation, and the same counting efficiencies.
Since standards of the mentioned properties were
not available, we had to prepare the matrices. For this
study, they needed to be of oval shape, of high density(S.G.
2.7 v 3.3), and contribute no interfering effects to the
measurement of the activated elements of interest. Many
materials and chemicals were tried, for example, polvethene,
calcium sulphate, etc. They either had too low a density
or they did not form a hard body that would resist deformation
during analysis. We then tried to use chemical mixtures,
and calcium sulphate mixed with lead oxalate turned outV
T h i c k n e s s o f C r v s t a 1'
F i s;. 3.8V.-1
Calculat eci SIow-Neut ron Detect ion Ef f ic .iency of Lil(Eu)
Crysta.I _as a Function of the Thickness of the
Crystal
- f
r- .
1 f H O A 1Rch If
Y(
I|500
Y()1(
3V rela
to controlswitch
Fig. 3.
M on net ah 1 e Re lav Circuit Diagrai
00 0 00 0 he
rH bD d dO bD 0 0I—I H Co i—!
0 a oFh 0 b
0C)
i—! 0d 0 fcuC
rH 0 0 d(J -H Cb 0I—I bD 0 i—I
•H 0 OQ O
00 O 00 0 CD
CO tfj d dCJ aD 0 0I—I i—i d i—I
0 0. OEH r 0
Or ')
rH (Id 0 h.
C3 0 0 KO H pH I-H bD 0 r-
•H 0 CQQ
0of.
d drH 0
« o
5
5
5
t i m e
t i rn e
t i m e
20
'U-0 Pi sec
t i m e
r:
-1 imr
0 P) sec
Fig. 3.10
response Characteristics of the IC ' s and Re 1 aj
to be the best matrix. Alter adding the desired elements,
for example, si I icon or aluminium powder, and an adequate
amonut of water to them, we could make standards of any
shape we desired. The mixture becomes hard as it dries.
No interference reactions occur, and the specific gravity
of this standard ranges from 2.3 to 2.9, depending on the
mixing ratio of the chemicals and the amount of water added
to it. The mass of the desired element, M, is calculated
f r om the equation:
where M is amount of element added, M is the mass of thee m
matrix, and M is the mass of the standard after drying.
This relation holds when the mixture is homogeneous, and
the water of crystallization of CaSCR is taken into account.
Repeated experiments showed the existence of large
fluctuations in M (greater than 25%), probably due to non-
homogeneity of the mixture.
Shifting to another method, we used pure silicon
wafers, pure aluminium (99.9%), and pure magnesium (99.5%)
metal for the standards. They were made to have a shape
and thickness similar to the jade samples. Since iron was
only available in powder form, special capsules of length
2 cm and diameter 4 mm were made, to hold the iron powder.
This standard was made in such a way that the length of the
iron column inside the capsule did not exceed 5 mm. Results
showed that these standards worked very well.
R(3su 11s and Discuss i on
Over 100 different specimens of jades were analysed,
in this study, via 14 MeV neutron activation.. The results
are tabulated in Tables 4.1, 4.2, 4.3, and 4.4t and
typical Nal(Tl) pulse-height spectra for the various kinds
of jades are shown in Fig. 4.1 to Fig. 4.8. Since all
these jades were on loan from a jewellery, there was just
enough time for each piece of jade to be analysed twice.
No large fluctuations in the percentages of the major
constituents (for example, silicon, aluminium, magnesium,
and iron) for different jades of the same species can be
observed from the data. In total, 24 jadeites were analysed,
resulting in standard deviations of the means for aluminium
and silicon of ± 9.1% and 7.6%, respectively (Table 4.1).
Of course, jades from different places may contain slightly
different concentrations of impurities, and hence exhibit
slightly different chemical compositions. Together with
experimental error, these differences in chemical compositions
lead to the deviations observed. The main experimental
error lies in positioning samples at the designed place
for counting. Repeated tests showed that this variability
ranged from ± 3% to ± 7%.
Pure jadeite crystal has the chemical formula „
4 aA 1 ( S iO 3 ) 2 . Ow i n g to interferences contributed b y the
intense 1.779 MeV photopeak of 28A1 from silicon, the short-
Table 4.1
Results for the Activation Analysis of Jadeites
Sample
I Mass, in 2 iN'O.[Mass of Si . in 2 7 o f S i Mass of A1. in a T of A1
0.9360
0.6483
0.6084
0.3620
0.7087
0.7416
0.5941
0.5705
0,6606
0.7094
A 1
A 2
A 2
A 4
A 5
A 6
A 7
A 8
A 9
A10
0, 216
0. 193
0. 176
0. 123
0. 194
0. 22 3
0. 146
0. 171
0. 175
0. 216
0. 014
0. 011
0. 008
0. 008
0 . 012
0. 013
0. Oil
0. 012
0. 009
0. 009
23 . 04
29. 70
28. 89
OO O QO _) • O O
27. 38
30. 10
24.63
29. 97
26 . 53
30. 41
1. 45
1. 70
1.23
2 . 25
1. 64
1. 74
1.85
2 . 13
1. 40
1. 34
0. 096
0. 086
0. 074
0. 048
0. 078
0. 090
0. 056
0. 074
0. 076
0 . 090
0. 005
0. 004
0. 003
0. 003
0. 004
0. 005
0. 004
0. 005
0. 004
0 n n 0
10.25
13 . 27
12 .21
13.20
10. 96
12 . 10
9 . 45
13 . 03
11 . 53
19 7R
0. 59
0.61
0.43
0. 83
0. 52
0.63
0. 59
0. 86
0.57
0. 38
Table 4.1 (continued)
Sample
Mass, in g No.
Mass of Si, in g % of Si Mass of A1, in cJ ro% o f A1
0.7521
1.0904
1.0561
1.3816
1.0830
1.3702
0.7681
0.8640
0.9667
1.2698
0.8409
1.2164
1.6864
0.8492
All
A 1 Qr-L
A13
A14
A15
A16
A17
A18
A19
A20
A21
A22
A23
A24
0. 187
0. 326
0. 312
0. 397
0. 298
0. 399
0. 234
0. 240
0. 277
0. 334
0. 234
0. 325
0 . 478
0. 240
0. 010
0. 023
0. 018
0. 024
0. 019
0. 027
0. 014
0. 013
0. 02 0
0. 017
0. 014
0. 017
0. 040
0. 013
9 a c q
9 Q C) 9
29. 51
28 . 71
2 7 . 52
29. 11
30 . 52
28 . 29
28. 70
26 . 27
27. 8 7
26 . 71
28 . 35
28 . 29
1 O A_L . o 9
2 1 9
-i f~ —_l . b t
1 . 77
1. 78
2 . 00
1. 86
1 . 57
2 . 08
1. 36
1. 68
1.41
2 . 36
1. 57
0. 071
0 . 12 8
0. 112
0. 159
0 . 121
0. 168
0. 093
0 . 090
0. 112
0. 136
0. 097
0. 132
0. 187
0. 090
0. 003
0. 009
0. 005
0. 008
0 . 0 0r
0. 010
0 . 005
0 . 004
0 . 007
0 . (.J 0 o
1. 005
0 . 0 0 7
0. 012
0 . 004
9.45
11.75
10.59
11 . 47
11. 17
12 . 26
12 .15
10 . 59
11 . 5 7
10. 71
11 . 56
10. 88
11 . 08
10 . 5 9
0 . 36
0. 78
0.48
0 . 58
0.68
0. 70
0. 70
0 . 47
0 . 5
0 . 39
0 . 63
0. 56
0 . 69
0.47
Mean : 27.90 ± 2.12 Mean : 11.49 ± 1.05
Table 4.2
Results for the Activation Analysis of Taiwan Jades (Nephrites)
S anin 1 e M n e o r
S i . 1 id q-1 !class .in si No.
% of SiMass of
Mr, in r% of Mg
Mass of
F e, in r% of Fe
riclS S OI
A1, in g% of A1
0.28210.016
0.21010.011
0.21110.013
0.25110.018
0.217+0.017
0.21410.008
0.20210.012
0.220+0.010
0. 193+0,010
0.19 710.DQn
30.00+1.74
25.3411.34
25.4311.55
30.99+2.22
28.09±2.14
29.77H. 10
27.2111.62
27. 1H1.23
28.30il.47
9 7 9 n +1 9 q
0. 12210.006
0 . 10 310. 0 0 6
0.089-0.006
0.09910.006
0.09910.005
0. 12910.012
0.08210.005
0.10010.006
0.09610.006
0.09110.004
12.9810.65
12,4110.77
10.7U0.75
12. 1810.69
12.8310.63
17.9311.66
11.0510.67
12.3210.74
14.0810.88
12.6110.55
0.01610.010
0.01410.009
0.011+0.010
0.01610.012
0.013+0.010
0.0090.008
0.012+0.009
0.019-0.013
0.009+0.009
0.015+0.011
1.65H. 15
1.6611. 12
1.3011.17
2.0211.43
1.6411.34
1.2411.13
1.6111,22
2.3211.65
1.3811.37
2.0811.42
0.00540.0002
0
0.00210.0005
0.002=0.0003
?
0.00310.0006
0.00210.0003
0,00110.0004
0.00310.0006
0.002=0.0005
0.0510.02
o
0.27=0.06
0,2510.04
0,3710.08
0.2710.05
0,1210.05
0.4410.09
0,2810.07
Mean: | 27.9501.88 Mean: i 12.9112,00 Mean: j 1.6910.35 Mean: ! 0.26=0. 12
Table 4.3
Results for the Activation Analysis of Australian Jades
Sample
Mass, in gNo.
-Mass of Si, in g % of Si Mass of A1, in g % of A1
0.5425
0.6496
0. 6594'
0.6416
0.8377
1.2014
0.4670
0.4059
0.6499
0.8382
C 1
C 2
C 3
C 4
C 5
C 6
C 7
C 8
C 9
C10
0.247± 0.017
0.322± 0.019
0.300± 0.016
0.279± 0.016
0.379± 0.026
0.527± 0.036
0.203± 0.012
0.198± 0.010
0.318± 0.021
0.378± 0.028
45.53± 3.08
49.58± 2.99
45.50± 2.36
43.51 ±2.51
45.25± 3.08
43.83± 3.02
43.49± 2.65
48.82± 2.36
48.93± 3.21
45.10± 3.29
0.002± 0.0002
0.005± 0.0004
0.004± 0.0005
0.002± 0.0002
0.008± 0.001
0.008± 0.0006
0.002± 0.0004
0.005± 0.0006
0.007± 0.0005
0.007± 0.0009
0.33± 0.04
0.77± 0.05
0.65± 0.09
0.37± 0.03
0.93± 0.12
0.68± 0.06
0.38± 0.08
1.63± 0.20
1.02± 0.07
0.79± 0.10
Mean: 45.95± 2. 3z Mean: 0 76± 0 39
Table 4.4
Results for the Act 1vat ion An a1y sis of Agate s
Sample
Mass, in gCD SI (),Photopeak Counts of Si Mass of Si, in g % of Si
0.4641 D 1 66578 ± 3222 3.230 ± 0.014 49.49 ± 3.10
0.3477
0.6396
0.5578
0.4265
D 2
D 3
D 4
D 5
50733 ± 1936
93260 ± 3624
79284 ± 3329
63352 ± 2909
0.175 ± 0.009
0.322 ± 0.017
0.274 ± 0.015
0.219 ± 0.013
50.34 ± 2.65
50.30 ± 2.68
49.04 ± 2.76
51.24 ± 3.09
Mean 50. 08 0.85
' i j !(».I cmj ) , |); M11
Ai (n , p) 2 7Mg
0.84 MeV
1250
1000
750 l
500
250'
0 0-5 10 1-5 -20
0.511 MeV
2nd escape;
of
1-78 MeV
0,75 7 MeV
Al (n,p) Mg
1.01 MeV
- 9 c 2 9 A 1S i (n, p) A1
1st escape of
1.78 MeV
1.28 MeV
' Si(n,p)2 A1
1.78 MeV
Gamma Energy (MeV)
Fig. 4.1
Typical Spectrum of Jadeite with Delay Time 20 sec., Counting
Time 2000 sec.
T—!0Jr~I I't-00r-4
CJ
0)cx
COAJr—3O,
o
1000
750'
500
250f
i—!0Cr-1-HCu
o
r.r-1(1)rrr—-
W4-3r(3o
o
oN 1° 1-5 f 20
G amm a En e r g y (M e V)
Fig . 4,2
Typical Syectrum of Jadeite with Delay Time 45 min.,
C') u n t i n g T1 rn e 2 0 0 0 sec .
24 IJg ( n , p ) 2 4 Nil
2 7 A1 (n , a )2 'fNa
1.46 IvleV
1.37 MeV
28Si(n,p)Z0Al
1.78 MeV
Si (n, p) JA1
1st escape of 1.78 MeV
2nd escape ''Fe (n , p) 5 6Hn.of 1.78 MeVA f
0.77MeV 0.84 MeV 1.28 MeV
1250
1000-
750-
500
250
OL0-5 10 15I v... 2 0'v,
Gamma Energy (MeV)
Fig. _4. 3
Typical Spectrum of Taiwan Jacle with Delay Time 20 sec.,
i—i0)GGdr~]
o
GCD
CO)
r—r iPAMO
o
Countin g Time 2 000 sec._
7;
JJ
CI o mmo 1 ra r~ r» rv 7 ( 1;T r
T7» -4 I
Typical Spec:!, rum o f Taiwan Jade with Delay Time 45 mi
f i - n- Ti O HOH
r
f{
2nd escape oi
1. 78 Me Vi
0.77 MeV
2 9 Si (n, p) 2 9A;
1st escape of
1.78 MeV
1.28 MeV
Si (np) ~ A1
1.78 MeV
1 25C
10CX
750
500
250
0 „n. 10 1-5 _2 0
_ a...
G amm a En e r g y (M e V)
Fig. 4.5
Typical Spec trum of Austra1ian Jade, wit h Pe1ay Time
20 sec., Counting Time 2000 sec.
i—CDr~r»—iGdr~|
o
c.0)o
in+-—',—.»
oo
loooi-
750
500
250 i
ol1-0 1-5 2-0
G amm a En e r g y (M e V)
Fia. 4.6
Typica1 Spectrum of Australian Jade with Delay Time 45 min,
Coun ~l i n g Ti me 2 000 se c_._
1.46 MeV
i—!0£££
O
rrHG)
GO4-r—1r—'
oo
IQOOj-
750ii
5oo(
250
0 0-5L_
2nd escape
of
1.78 MeV
J I
,0.77 MeV
9 9 Si(n,p)9A1
1st escape
of
1.78 MeV
1.28 MeV
Si(n3 p)z'A1
1.78 MeV
1-0 1-5 20
Gamma Energy (MeV)
Fig. 4.7
Ty p i c a 1. Sp e e t r urn o f Ag ate wit h De I ay Time 20 sec.,
Coiin t i ng Time 2 000 sec.
i—I0)Gt—I
cdrCO
UCDp(
W4-r-j2O
u
1000
75'
)1
i25
0 .10
i1-E '2•C
.........-
1. 46 MeV
y vr
G 11 nun a En e r ay (MeV)
Fig. 4.8
Typical Spect rurn of_ Aga te with De1 ay Time 45 mir
Counting Time 2 000 sec.
i—10c—
d
O
U0n
0-Pr—r4—1r—'f—'Q
Lived (It = 11.0 see.) radionucli do , ' F , [X'fduccd hv«
sodium act ival ion, could no I bo del.orm i nod. The average
percent content by weight of aluminium in jadoite was found
to be 11.49% , d 1 f f'or i ng f rom the va 1 ue ca 1 cu.J ated f rom t:he
chemical formula, NaA L ( SiO 3) 2 by 14.0%, whereas the silicon
percentage found differs from the formula value by only
0.6%.
U s 11; 111 y the inone t a 1 y va J, ue o f a j ade i t e s pe c i men
is based upon its size, its transparency, and its green
colour. In this experiment, we also tried to detect one
interesting element, chromium, which is present in jades
as an impurity, and which is believed to be the origin of
the green colour (Savage, 1964), (Switzer, 1979). However
detection difficulties arise, since the photopeak of the
1.434 M e V g a m j 11 a o I 111 e r a d i o n 11 c 1 i d e , 5:' V , p r o d u c e d f r o in
activation of chromium, is situated on the Compton edge
of the 1.779 MeV gami 11 a of 2 8A1, 1;he 1 nduced radioactive
nuclide of the activated silicon. Furthermore, the actual
amount of chromium is extremely small. As a result of
these difficulties, the content of chromium could not be
determined. Methods, including peak stripping of the
spectrum by graphical and computer techniques (Kowalski,
1968), were attempted, but chromium could not be found,
with acceptable statistical accuracy. Nevertheless, from
the experimental spectra, jadeite can easily be .identified
from other jades by its aluminium peaks.
Taiwan jade is a kind o 1' nephrite, and its value
is much lower than thai ot jadeito. Noph r i I e lias the
chemical compos i 1 i on , da 7 (Mg , Fe ) 5 (Oh ) 2 ( Si 4O 1 1 ) 2 i • • , i t
is a silicate ot calcium and magnesium with the presence oi
some iron. In these experiments, 10 Taiwan jades were
analysed (see Table 4.2), an d the composition s foun d were
28.3% for silicon, 13.2% for magnesium, 1.6% for iron, and
0.2% for aluminium, all in percent by weight. The standard
deviations f o r I, he j)e 1 ce 111 con t en t o f t he var ious e 1 erne 111 s
were ± 8.5% for silicon, ± 18.8% for magnesium, ± 17.6% for
iron, and ± 57% for aluminium (Table 4.2).
The gamma-ray spectra o f activated Australian jades
and agates showed that no detectable amounts of other
elements than silicon wore present (Tables 4.3 and 4.4).
These beautifu1 green sto 11 es are usua11y sold under the
name j ade , t he i 11 cost be i 11 g much 1 ower t han j a.de i t e an d
a little higher than Taiwan jades. From the present
experiments, the percent content by weight of silicon in
Australian jade and. agate were found to be 45.95% and
50 .08%, respect i.ve 1 v . A list ing of the ex.perirnent a 1 resu 11s
for compos ition s o f j a deit e an d n e p hrit e b y o t he r wor kers
(Pearce, 1971) is given in Table 4.5 .
The overa 1. L ana. 1.ysing t ime f or one samp 1 e of j ade
was about 90 minutes. The irradiation time was 10 minutes,
so that t he long ha 1 f -1 i f e nuc 1 i des cou 1 d b(3 gener at ed
appropriately. Two decay times, 20 seconds and 45 minutes,
were employed. The counting time was chosen to be 2000
seconds, so that the counting statistics were better for
Table 4,5
Chemi cal An a 1 vs i s of Jacle si
(Percents bv Weight)
Si A1 Fe Mn«r
-g CarIV H
Nephrite
New Zealand
Switzerland
Silesia
C h i n a
A1 a s k a
B cd w e n i t e
J cXcl Gi. ~LG
Switzerland
China
Mexico
26 . 68
26 . 12
26 . 26
27.29
26 . 42
19. 89
27 . 28
27.64
27.63
0.7-
0. 1L
0.8-
0. 24
0.8-
LI. 95
L2 . 14
L3 . 17
2 . 84
4 . 44
4. 20
1. 41
5.84
0 . 9o
1.28
1. 21
0. 86
0.06
0. 66
0. 16
0. 39
13 . 47
13 . 06
14 . 79
15 . 36
12 . 71
25 .66
0.77
0.69
0. 31
9 . 60
8 . 68
5.73
9 . 25
9 . 16
0.46
2 .21
1 . 92
0.69
i
9.45
9.58
9 . 04
| 0.41
0. 26
0.23
0.39
0. 45
0. 03
0. 16
1.46
0. 02
0 . ID
Results taken from Pearce, 1971.
69
the 0.84 MeV and 1.37 MeV peaks. In fact, if the attempted
determination of chromium is omitted, the total time of
analysis can be shortened to about 30 minutes (10 minutes
irradiation, 1. minute decay, and 10 minutes counting) per
jadeite sample.
The analysed jades were chosen to have approximately
oval shapes arid similar sizes, so that the counting
efficiencies for the p ahotopeaks being measured were
essentially constant. Also, each jade sample had a small
thickness( 3 mm 0. 5 mm), and since the absorption
coefficients for the various jades do not vary much, the
total attenuation correction factor can be considered to be
5/10 for all the jades. For aluminium, the intensity of the
0. 844 MeV peak was found to be 2.4 times larger than that
of the 1. 014 MeV peak, hence only the 0. 844 MeV photopeak
was used in evaIuatitry' the alull]iniuIll coneentrations if
jades of shapes other than oval shapes are used, the
corresponding standards can be made by using the same
calcium sri].phaLe/load oxalate mixture metthod. The
accuracy of this method should be better for larger sample
sizes, as the mixture would then be more homogeneous than
for smaller samples.
Chapter V
Conclusions
From the experiments, we see that the two gamma
radiations of 2;Mg are not simultaneous events, which
agrees with that deduced from the recent decay scheme of
27Mg. The internal conversions of these gamma radiations,
0.844 and 1.014 MeV, are extremely small: 3.65 x 10 5
and 4.67 x 10 5, respectively. The accuracies of these
values depend on the accuracy of the published data used.
The ratio of the intensity of the 0.844 MeV photopeak of
27Mg to that of its 1.014 MeV photopeak was found to be
2.41 ± 0.17, which differs from the accepted value of
2.33 by 3.4%. With the information provided in this paper,
we hope that we have established a nondestructive test
for the authenticity of jades. To our regrets, however,
the differences in the greenish colour, which is considered
to be the most valuable characteristic of jade, could not
be distinguished using this method.
71
Appendix ( A )
The Characteristics of Gems
In Tables 1 to 4, the gem minerals are classified
according to crystal. system, hardness, specific gravity,
and optical PropeltieS.
Table 1
Crstal System
diamonds, gold, garnetcubic system
zircon, rutiletetragonal system
corundum, quartz, tourmaline, berylhexagonal system
topaz, enstatile, chrysoberylorthorhombic system
nephrite, jadeite, malachite, serpentine.monoclinic system
diopside
turquois, cyanite, rhodonitetriclinic system
opal, amberamorphous
72
Table 2
Hardness
HardnessHardness MineralsMinerals
1.0 6.5nephritediamond
6turquois9corundum
5.5-6.5opal8.5chrysoberyl
5-6rhodonite8topaz
5-6diopside7.5-8beryl
3.5coral7. 5zircon
3.5malachite7-7.5tourmaline
2.5-3.57 pearlquartz
2.5-4serpentine6.5-7.5qarnet
2.5-36.5-7 goldjadeit
2-2.5amber6-6.5rutile
73
Tab e 3
Spccitic Gravity
S. G,MineralsS. G.Minerals
3.2diposide4.0-4.8zircon
3.1tourmaline4.2rutile
3.0nephrite4..0corundum
2.7beryl4.0malachite
2.73. quartzchrysoberyl.
2.7turquois3.5diamond
2.6serpentine3.5topaz
2.6pearl3.4epidote
1.9-2.3opal3.3jadeite
74
Table 4
Optical Properties
BiOptical
Character relringenceIndices of RefractionMinerals
n2.417diamond
0,287£2.903w2. 616rutile
0.057El.988wl.931zircon
0.254al.655131.875malachite yl.909
0O08El.760wl.768corundum
0.051yl.780a1.729epidote 131.763
0.010al.747chrysoberyl 131.748 Y1.757
0.007al.716 y1.723epidote 131.719
yl.726 0.024a1.702diopside 131.708
0.013y1.667al.65401.659jadeite
0.016£1.670wl.654tourmaline
-yl.627 0.008al.619topaz 131.620
y1.650 0.040(1.610tourquois 131.620
yl.632 0.026(1.606nephrite 131.620
0. 008E1.590wl.598beryl
0.011Y1.557a1.546r31.550serpentine
0.009El.553w1. 544quartz
nl.460opal
75
Appendix (B)
occurrence of Jades
jadeite
nephrite
76
Appendix (C)
Colour plates of jades
A6A2
A21
A 20
AlA 13
A 22A 5
Plate 1
jadeite
77
A 24 A 23
A 17 A 18 A12 A 14
A 16A 19
A 9
A 11
A 7
A 8
A 15
A 10
A 4 A 3
Plate 2
jadeite
78
B5
B3
B4 B2
B1
B 10
B6
B8
B7 B9
Plate 3
Taiwan jade
Cl
C5
C4C2
C3
C 10
C6
C9
C8C7
Plate 4
Australian jade
D2
D5D4
D3D1
Plate 5
agate
81
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