IL NUOVO CIMENT0 Vet.. 36 B, N. 2 11 Dicembre 1976

Observation of Free Fluorine ~-Nueleonie Atom.

V. G. VARLAMOV, B. A. DOLGOSHEIN~ YU. P. DOBRF.TSOV~ V. G. KIRILL0v-U6RYU~mV, P. L. NEVSKY, A. M. ROaOZHIN and V. P. S~tlL~A

_4cadency o] Sciences o] the USSR P. N. Lebedev Physical Institute - 117924 Moscow

(ricevuto il 23 0ttobre 1974; manoseritto revisionato ricevuto il 18 Febbraio 1976)

Summary. ~ The existence of the free wnueleonic atom has been experi- mentally shown by measuring the Larmor precession frequencies of the total moment for an atom in magnetic-field intensities 1.1 and 2.1 G. The precession frequencies have been obtained when negative muons stopped and decayed in Ne gas at a pressure of 42 atm.

1 . - I n t r o d u c t i o n .

I n terms of the conventional terminology, the system (Z nucleus-~-~--meson) is a muonic a t o m w i t h Z atomic number. I n this paper, according to (1), we call

the system Z nucleus with a negative rouen on the K-shell plus an electron shell

with Z - - 1 electrons a wnucleonic a tom with Z - - 1 atomic number. The ~-nucleonic-atom formation is usually preceded by a heavy destruction

of the electron shell of the parent a tom in the process of cascade transit ion of a negative rouen to the K-shell. A new electron shell is formed through the interaction of a ~-nucleonic a tom or ion with surrounding atoms and the prod-

ucts of medium radiolysis a~rising from rouen stops and their cascade tran- sitions; and the characteristic time needed for a wnucleonic a tom to create a bound state in a normal condensed medium is evaluated to be ~ 10 -1~ s (~).

A chemically inert medium, e.g. the atmosphere of a noble gas, is considered

(1) V.N. GORELKIN and V. P. SMILGA-" ~trn. Eksp. Teor. _Fiz., 66, 1201 (1974). (*) A. A. DzHUI~A~.V, V. S. EVSEEV, Yu. V. OBUKHOV and V. S. ROGANOV: ~urn. J~ksp. Teor. Fiz., 62, 2210 (1972).

131

132 v . G-. V A R L A M O V , B. A. DOLG.OSHEI lq , Y U . P . D O B R E T S O V , E T C .

to be the mos t suitable one for a search of a free t~-nucleonie a tom, since it mini- mizes or even eliminates the probabi l i ty of the interact ion of a chemically act ive single a tom. Thus, when negat ive mouns come to rest in a gas t a rge t filled with noble gases (He~ 57e, Ar, Kr , Xe), ~-nucleonic a toms analogues of hydrogen or of a re levant halogen (F, C1, Br, I ) m a y be formed.

Given the lack on exper imenta l evidence on the existence of ~-nueleonie a toms, we a t t e m p t e d to observe free fluorine wnueleonic a toms in gaseous neon (3).

Fo r a full electron shell of fluorine a toms to be produced, the main gas ha~ to contain a chemically inact ive admix ture wi th an ionization potent ia l lower than the binding energy of the last electron in a ~-nueleonie a tom. Fo r this purpose xenon has been chosen, whose ionization potent ia l is 12.08 eV.

Charge exchange of ~-nucleonic nucleus [~-,*~Ne] +9 results in the neut ra l 2O a tom s h e l l , F. The ground s ta te of a ~-nueleonic fluorine electron shell is 3p!

and the neares t metas tab le level 2pi is 0.05 eV higher. Precession frequen- cies of the to ta l m o m e n t of wnucleenic fluorine in a weak magnet ic field are

s for (~% is the precession fre- expected to be equal to �89 for 2P t and oJ~, ~o~ 2p! quency of t r iplet muon ium in a magnet ic field of the same intensity) (see appendix). A related contr ibut ion of ampl i tudes corresponding to different precession frequencies to the t ime distr ibution of decay electrons is discussed in the appendix.

The method of to ta l m o m e n t precession of an a t o m in the magnet ic field was used to observe free ~t-nueleonic fluorine a toms. The a s y m m e t r y p a r a m e t e r of the electron angular distr ibution due to rouen decay was exper imenta l ly measured.

2. - E x p e r i m e n t a l apparatus.

A special gas ta rge t (*) was used to register rouen stops in a noble gas. I t is shown schematical ly in fig. 1. The cylindrical shell of the ta rge t is made of stainless steel, with the d iameter of its central pa r t being 10 em and the thickness of the wall 0.2 era, filled wi th the invest igated gas mix tu re a t abou t 45 a tm. A copper shell, 0.05 cm thick, with a light-reflecting coating of CsI and qua terphenyl l ight-convert ing layer sput te red on it increased the l ight ou tpu t due to scintillations in the gas target . The ta rge t vo lume was viewed b y a photomult ipl ier . The electrode concentr ic with the body of the t a rge t divides it into the cent ra l pa r t , where muon stops are registered, and the gas

(3) V. G. VARLAMOV, YU. P. DOBR~TSOV, B. A. DOLOOS~N and V. G. KIRILLOV- UORYU~OV: ~ r n . Eksp. Teor. Fiz. t~is. Red., 17, 186 (1973). (4) V . G . V~LAMOV, YU. P. DOBRV.TSOV, B. A. DOLC, OSHm~r and A. It. ROGOZmN: Proceedings o] the I r~ternatio~al Conlerence on Apparatus in High-Energy Physics (Dubna, 1970), p. 800.

OBSERVATION OF FR.F~ :FLUORINE [J.-NUCL:EONIC ATOM 1 3 3

layer per iphera l to the t a rge t wall. The gap between the electrode and the t a rge t wall is 1 cm. The electrode is a nonmagne t ic nichrome net . The size of a uni t cell is 0.3 cm, the wire is 0.02 cm in diameter . A d.c. potent ia l of

1 kV is applied to the electrode to remove free electrons f rom the gap. A high- vol tage pulse wi th an ampl i tude ~ 10 kV was applied to the electrode when the part icle ar r ived a t the target . The pulse durat ion a t half-pulse height was equal to 100 ns.

Fig. 1. - The gas target: 1) high-voltage lead-in, 2) shell of the target, 3) electrode, 4) window, 5) photomultiplier.

I n case a par t ic le crossed the gas layer a t the t a rge t wall, a l ight signal was registered due to the pulse electroluminesccnce in gas (s). This light signal was synchronous with a high-vol tage pulse.

The absence of the signal G can be in terpre ted as the part icle being s topped ei ther in the gas or in the electrode. The pa r t of the gap where the beam entered the t a rge t was filled with an insulator allowing a particle to enter the t a rge t wi thout a n y anticoineidence signal G being registered.

(5) V.G. VA~A~OV, B. A. DoL6osIrl~I~ and A. M. ROG0ZHI~: Elementarnye chastitsy i kosmic]~skie luchi (The Elementary Particles and Cosmic Rays) (Moscow, 1967). p. 84.

134 v . G . ~rARLA-MOVj B. A. DOLGOSHEIlq~ YU. P. DOBR:ETSOV, ]~TC.

To determine the exclusion efficiency for the particles crossing the target , i t was turned about with respect to the direction of the rouen beam. Therefore~ whenever the part icle hi t the target , i t mus t be accompanied by an electro- luminescent anticoincidence pulse G. The exclusion efficiency measured for the part icle crossing the t a rge t layer was not less than 99.6 %.

The mix tu re of noble gases in the ta rge t continuously circulated th rough a metal l ic calcium furnace. The t empera tu re of the furnace was kep t a t (600 - - 650) ~ C.

The ta rge t was placed in a magnet ic field produced b y a solenoid, 15 cm in d iameter and 30 cm long. The ta rge t and the solenoid were shielded b y me:~ns of a sandwich magnet ic screen (25 cm in diameter~ ]00 cm long) to p ro tec t t h e m f rom the scat tered field (.~ 5 G) of the accelerator. I n the area occupied by the t a rge t the inhomogenei ty of the field did not exceed 3 %.

I t was found t h a t uncontrollable var ia t ions of the magnetic-field s trength, A H ~ • 0.1 G, migh t occur in the process of measurements due to a var ia t ion in the residual scat ter ing fields. This fact was t aken into account in the da ta processing.

The lay-out of the exper imenta l a r rangement is given in fig. 2. The 160 MeV/c rouen beam arr ived f rom the meson channel of the synchrocyclot ron of the J I N R . The polarizat ion of bo th negat ive- and posi t ive-rouen beams was within 0.75 • 0.05.

Pb 5hieLd.~

Fig. 2. - The experimental lay-out and simplified electronic logics: 1), 2), 3), 4), 5) plas- tic scintillation counters; GC gas target; CC1, 2, 3, 4, coincidence circuits; ID integral discriminator.

OBSERVATIO]~ r OF FR]~F~ FLUORINE pt-NUCLEOI~IC ATOM 135

A pulse corresponding to a muon being s topped in the gas was formed b y coincidence pulses of the scintillation counters 1) ( ( 2 0 x 2 0 • s) and 2) ((10 • • cm 3) with scintillation pulse G in the ta rge t gas. I f the anti- coincidence pulse G f rom the gas t a rge t was absent, the t ime analyser s tored the t ime interval between a muon stop in the t a rge t and its decay electron. Elec- t rons were registered b y coincidence of telescope consisting of three plast ic scintillation counters 3), 4), 5) ( ( 3 5 • 2 1 5 s each).

Aluminium plates, 6 m m thick, were inserted between the counters to reduce the efficiency of g a m m a - r a y registration. The t ime analyser was pro- vided with (, g u a r d , logics guaranteeing the t ime independence of the r andom coincidence background within the measur ing t ime interval .

The t ime spectr~ were analysed within a t ime in terval of 1 ~s to 5 ~ts af ter the muon stopped. The 1 ~s shift of the analysed t ime in terval excluded decay electrons coming f rom negat ive muons a t rest in the ta rge t electrode and f rom xenon muonic a toms, since the l ifetime of negat ive muons in bo th cases is less than 0.2 ~s.

3 . - T e s t e x p e r i m e n t s .

The following tes t exper iments were per formed: a) the measu remen t of the residual polar izat ion of negat ive muons in a carbon rod placed in the centre gas of the target , b) the observat ion of the precession of the t r iplet muon ium formed both in mol ten quar tz and in a neon-xenon mix tu re in a positive- muon beam.

The results of the above measurements are summar ized in table I . I t shows

TABLE ]. -- The results o] control experiraeats (,).

Beam Target Mag- A s y m m e t r y Precess ion E x p e c t e d value net ic p a r a m e t e r f r equency of precession field (rad/~s) f r equency (G) (rad/~s)

~t- C in t he gas t a rge t 61.2 0 . 0 5 2 i 0 . 0 0 6 5.32-i-0.08 5 .23• (a)

t~ + SiOz (quarz) 1.1 0.14 • 9.55:]:0.22 9.7 • (*)

~+ Si02 (quarz) 2.1 0.12 • 18.20• 18.5 • (o)

i~ + 4 2 a t m N e - ~ l a t m X e 2.1 0.07 :~0.01 19.5 • 18.5 -4-0.9 (c)

(a) Here and in table II the error in the calculated frequency value shows possible variations in the magnetic-field intensity. (b) Corresponds to t h e frec-muon precession frequency. (r) Corresponds to the triplet-muonium precession frequency.

the presence of the m u o n precession in the carbon rod wi th the precession f requency expected for a free muon. The pos i t ive-muon precession f requency

136 v . o . VARLAMOV, B. A. DOLGOSHEIIq, YU. P. DOBRETSOV~ ETC.

in the muon-xenon mix tu re can be seen to conform with t h e value expected for the muonium, and the value of the a s y m m e t r y p a r a m e t e r is consistent with a ~ 100 ~ probabi l i ty of muon ium format ion in the gas mix tu re (ap- p rox imate ly one-ha]f of the muon stops mus t occur in the electrode of the

gas target}.

4. - Observation of free fluorine ~-nucleonie atom.

Fluorine F-nucleonic a toms were observed in t h e neon-xenon mix tu re (42 a t m Ne and 1 a rm Xe) b y measur ing the to ta l m o m e n t precession of an a tom in magnet ic fields of q 1.1 and 2.1 G.

As has been ment ioned above, the F-nucleonic fluorine to ta l m o m e n t pre- cession should be character ized by three frequencies (see appendix, formula (A.3)). I n fig. 3 curve A) represents the results of the harmonic analysis of the

Loo ~- ~ ~ A)

0.50 (y

0 o

CL - -050

o 50 E c~

o L

L/:

B)

- 0 , 5 0

k J L L [ L 0 2 4 6 8 1[0 lt2 14 116 1L8 20

process~on frequency ( racL /~s)

Fig. 3. - T h e results of the harmonic analysis of the time distribution for decay electrons: A) experimental curve, B) calculated curve.

decay electron t ime distr ibution measured in the field of 1.1 G and the curve B) was especially calculated for the exper imenta l conditions when precession takes place a t three frequencies wi th weights corresponding to the stast is t ical popu-

OBSERVATI017 OF FREE FLUORIN~ ~.-NUCLEONIC ATOI~ 137

lat ion of mul t ip le ts (see appendix) . The harmonic analysis was made with the me thod described in (B). The comparison of the calculated curve wi th the exper imenta l one shows the presence of the precession frequencies expected for the case of fo rmat ion of a free ~-nueleonic fluorine a tom. Table I I presents the frequencies, the a s y m m e t r y pa ramete r s and the Z ~ values obtained f rom the processing of the exper imenta l da ta by the least-square analysis using the funct ion (A.3).

TABLE II. - The results o] measurements o/ residual negative-rouen polarization in the ]luartine muonic atom.

Target Mag- Asymmetry Value of Expected Val- Number netic parameter precession value of ueof of field frequency precession X Z degrees (G) (rad/~ts) frequency of

(rad/~s) freedom

20 xoNe 1.1 0.115 ~0.031 9.95-4-0.66 9.7 ~0.9 332 335

42atmNe-~- 2.1 0.128~0.032 19.0 • 18.5 • 188 199

+1 a t m X e 61.2 0.007 ~0.004 - - 5.23 ~0.09 85 91

The mos t impor t an t results presented in table I I and in fig. 3 indicate t ha t the precession f requency values obta ined in bo th cases (1.1 and 2.1 G) are in good agreement wi th the calculated ones obta ined in te rms of the a l ready known magnetic-field intensities. The a s y m m e t r y fac tor value is to be regarded as the m i n i m u m value of the actual a s y m m e t r y factor, since there is a possibili ty of muon depolarizat ion because of chemical reactions or of electron shell mo- m e n t re laxat ion occurring in a tom collisions wi th impurit ies. The a s y m m e t r y coefficient for the negat ive muons measured with the precession f requency of the free rouen in the same gas mix tu re in the magnet ic field 61.2 G is also given in table I I . As should be expected, the a s y m m e t r y coefficient in this case is

close to zero.

5 . - C o n c l u s i o n .

The existence of a free a tomic sys tem such as the fluorine tz-nucleonic a t o m has been shown. The s tudy of negat ive-rouen precession in pa ramagne t i c ~-nucleonie a toms seems promising for invest igat ing various propert ies of gases

and condensed media (7).

(6) V . W . HUGH~S, D. W. McCoLN, K. Z~OCK and R. PR~POST: Phys. ~ev. A, 1, 595 (1970). (7) V. G. VARLAMOV, B. k. DOLG-OSHEIN, YU. P. DOBR•TSOV, V. G. KIRILOV- UGRYUMOV, P. L. NEVSKY, A. M. ROGOZHI~ and V. P. SMrLGA: Yad. Fiz., 21, 120 (1975).

138 v . G . VARLAMOV, B. A. DOLGOSHEIN, YU. P. DOBRETSOV, ETC.

With the aid of the ~-nucleonic a toms one can obtain da ta on chemical reac- t ion rates of single a toms (as in the case of muonium) (e). This especially con- cerns halogen a toms formed in noble gases.

The s tudy of nega t ive-muon precession in [~-nucleonic a toms in high-intensi ty magnet ic fields m a y prove valuable for invest igat ing the hyperf ine s t ructure of ordinary a toms (~).

Measurements on hyperf ine s t ruc ture split t ing in ~-nucleonic hydrogen m a y be a valuable supplement to the conventional exper imenta l de terminat ion of the hyperf ine-spl i t t ing cons tant in muon ium (9).

We a t t e m p t e d to observe free hydrogen ~L-nucleonic a toms (3). However , inadequate stat is t ical a~eura~y of the exper iment did not allow us to d raw any definite conclusion (').

The authors are ve ry grateful to Prof. S. S. GERSHTEIN for useful discussions which have considerably influenced this invest igat ion; and also to Prof. I . I . GUREVITCH for interest and encouragement . The authors are indebted to Profs. V. P. DZHELEP0V and L. I . LAPIDUS for the possibili ty of carrying out this exper iment a t the sinchrocyclotron of the J I N R ; as well as to V. S. ROaA- NOV, YU. M. GRASHIN, ~T. Z. ANISIMOV, A. M. KONSTANTINOV, V. A. DVOR- ETSKY and V. A. BOITS0V for their invaluable assistance.

A P P E N D I X

wnueleonic spin precession in weak magnetic fields.

Let us consider the case when the spin of the pa ren t nucleus, upon cap- tur ing a negat ive muon, is zero. The nucleus of the result ing bL-nucleonie a t o m acquires the spin and the magnet ic m o m e n t of the negat ive muon. Str ic t ly speaking, the value of the magnet ic m o m e n t of a bound muon somewhat differs f rom tha t of a free muon, the correction being of the order of 10 -3 to 10- ' .

I n a magnet ic field normal to the direction of the initial polarization, the muon spin precesses, the na ture of the precession for pa ramagne t i c wnueleonic

(8) R.M. MOBLEY, D. M. BAILEY, W. E. CLELAND, V. W. HUGHES and J. E. ROTHBERG: Journ. Chem. Phys., 44, 4354 (1966). (9) W . E . CLELAND, J. M. BAILEY, M. ECKHAUSE, V. W. HUGHES, R. PR~EPOST, J. E. ROTHBERG and R. M. MOBLEY: Phys. Rev. A, 5, 2338 (1972). (*) In the course of the publication of this paper results have appeared on the observa- tion of ~-nueleonic hydrogen (10). (,o) p.A. SOUI)~R, I). E. CASPERSON, T. W. CRANE, V. W. HUGHES, D. C. LU, H. ORTH, H. W. R~IST, M. H. YAM and G. zu PUTT.XZ: Phys. Rev. Left., 34, 1417 (1975).

OBSERVATION" OF FREE FLUORINE ~ -NUCLEONIC ATOM 1 ~

atoms being governed by the binding of the muon magnetic moment and by the a tom electron shell. Since fields of the order of (10 ~ -10 6) G are needed to break a tom LS binding, J (the to ta l electron shell moment) m ay be considered a good quan tum number for fields less than 10 4 G and it is the transit ions between various terms of multiplets F ~ J =t= �89 where J is the electron shell moment , tha t are responsible for the nature of the precession. As follows from paper (1), which presents a comprehensive analysis of the theory of the pheno- menon concerned, the spin precession frequencies must satisfy the selection rule AM = -4-I, since the selection rule corresponds to the muon spin-flip.

Since most of the frequencies (as a rule, all those corresponding to transi- tions between the terms of various multiplets F + �89 ~ - F - - � 8 9 are high, they cannot be experimental ly resolved. The observed precession is averaged over these frequencies.

I t should be noted tha t the precession frequencies themselves do not depend upon the populat ion of the multiplets F. The populat ion does affect the ampl i tude of precession only. Corresponding equations were derived in paper (1~) for a statistical populat ion of states with different values, in fact stating tha t different projections upon the muon initial polarization axis are of equal proba- bility.

In ve ry weak magnet ic fields (much weaker than the characterist ic field of hyperfine splitting), the precession pa t t e rn is much more simplified. All the frequencies corresponding to the transitions between different states inside mult iplets F turn out to be equal (polarization is averaged for high frequencies characterist ic of transit ions between multiplets) and the precession is charac- ter ized by the following two frequencies co. only:

(A.1) 2 J + 1 4- i 21M~-[]

~eo~ = H - g(L, S, J ) M , ::t= , ~" J ( 2 J + 1) 2 J + lJ

where g(S, L, J ) is Land~'s factor of the electron shell, Mr is the Bohr magneton, [M~ - I is the negat ive-muon magnetic moment . Therefore, the polarization aver-

aged over high frequencies can be wri t ten as

(A.2) <P( t )> - - P(o)

3(2J + 1) ~ [ ( J + 1)(2J ~- 3) cos w+t ~- J ( 2 J + 1) cos co_t],

where P(0) is the ~--meson polarization with which a muon arrives at the K- shell. Natural ly, af ter averaging, <P(0 )>~P(0 ) .

A halogen a tom may be formed in bo th the ground state J = ~ and the exci ted one J ~ �89 As can be seen from formula (A.1), only one precession f requency is left in the excited states and two in the ground state.

The probabi l i ty of the product ion of a wnucleonic fluorine a tom cannot be theoret ical ly calculated with adequate aceuracyy bu t can be easily found from the exper imenta l da ta in terms of the ampli tude of the corresponding frequency.

The scheme of ~-nucleonie fluorine levels and the table of q u an tu m num- bers characterist ic of every s ta te are given in fig. 4. The amplitudes of various

(11) H. t~]BERALL: Phys. Rev., 114, 1640 (1959).

140 v . G . VARLAMOV, B. A. DOLGOSIIEIN, YU. P. DOBRZT$OV, ETC.

I F J I L S 9j 9~ J a,

0 1 r 2 -- 0

t

7 "3 1 I

2p I , -J T • I I - ~ p~ ~ 2 2 5 1

Y 5 2 1 24

[

Fig. 4. - D i a g r a m of t h e s t a t e s a n d t h e i r q u a n t u m n u m b e r s for t he ~-nucleonic f luor ine a t o m . 2 ' = t h e t o t a l m o m e n t of a n a t o m , J = t h e t o t a l m o m e n t of a n a t o m shel l , / = t h e m o m e n t of a nucleus , L , S = t h e o r b i t a l a n d spin m o m e n t s of t h e a t o m shel l , g z = L a n d 6 ' s f ac to r of t h e e l ec t ron shel l , gw= L a n d C s f ac to r for t h e h.f . s t r u c t u r e , ~ = t h e a m p l i t u d e of t h e precess ion f r equency .

frequencies were calculated by assuming that the states 'Pt and 2p! are formed proportionally to their statistical weight 2J + 1. In view of this, the decay electron angular distribuzion takes the form

(A.3) _,v(t) = No exp [ - - ,~t].

�9 {1 + a[& cos (�89 + ~) + § cos (~t + q) + ~& cos (~cot + ~) ]} ,

where a is the a symmet ry factor, ;L -1 is the muon lifetime in a muonic neon atom, o4 is the t r iple t -muonium precession frequency, ~ the initial precession phase.

�9 R I A S S U N T O (*)

L ' e s i s t e n z a d e l l ' a t o m o l ibero L~-nueleonico ~ s t a t a d i m o s t r a t a s p e r i m e n t a l m e n t e misu- r a n d o le f r equenze del la preeess ione di L a r m o r del m e m e n t o t o t a l e pe r u n a t o m o in i n t e n s i t ~ di l . l e 2.1 G del e a m p o magne t i eo . Le f r equenzc di p recess ione sono s t a t e o t t e n u t e q u a n d o i m u o n i n e g a t i v i si f e r m a v a n o e d e c a d e v a n o nel N e gassoso ad u n a press ione di 42 a rm.

(') Traduzione a e~ra della Redazione.

Pe31oMe. - - CyLtICCTBOBaHHe cao60~3loro M~OHyXnOHHOrO aToMa ~ T o p a 6 ~ o 06Ha- pyN(eHO 3KcnepHMeHTanbHO n p n H3MepeHHH ~aCTOTH napMopoBol~ npeueccHH He/]HerO MOMeHTa aTOMa B MRrHHTHI~IX HOYIRX 1.1 r c a 2.1 rc. qaCTOTl~I IIpeLIeccHH 6birth HO-

Jly~eHhl IlpH OCTaHOBKe H pacrm~e OTpHI~aTeYlb}tbIX MIOOHOB B ra3oo6pa3HOM HeOHe n p ~ ~an~eHHu 42 arM.