Gio trnh x l bc x v c s ca cng ngh bc x

NXB i hc quc gia H Ni 2007.

Tr 8 23. T kho: Bc x, c im ca bc x, ngun bc x.

Ti liu trong Th vin in t H Khoa hc T nhin c th c s dng cho mc

ch hc tp v nghin cu c nhn. Nghim cm mi hnh thc sao chp, in n phc

v cc mc ch khc nu khng c s chp thun ca nh xut bn v tc gi.

Mc lc Chng 1 Cc c trng ca bc x v ngun bc x ....................................................... 2

1.1 Cc c trng ca bc x ........................................................................................... 2 1.1.1 Tnh cht sng v ht ca bc x ....................................................................... 2 1.1.2 Phn loi bc x theo nng lng v bc sng................................................ 2 1.1.3 Tnh phng x v tc truyn nng lng ca bc x .................................... 3

1.2 Cc c trng tng tc ca bc x vi vt cht ....................................................... 5 1.2.1 c im tng tc ca bc x vi vt cht ...................................................... 5 1.2.2 Tng tc ca ht nng mang in vi vt cht................................................. 5 1.2.3 Tng tc ca bc x bta vi vt cht.............................................................. 6

Chng 1. Cc c trng ca bc x v cng ngh bc x

Trn i Nghip

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2

Chng 1

Cc c trng ca bc x v ngun bc x

1.1 Cc c trng ca bc x

Bc x ion ho nng lng cao c s dng to ra cc bin i mc nguyn t v phn t l cc loi bc x alpha, bta, gamma, tia X, ntron, electron v ion. Trong s ny bc x gamma v electron thng c s dng nhiu hn c so vi cc loi bc x khc.

Tuy khng c xp vo loi bc x ion ho nng lng cao, song gn y cc tia cc tm (UV) cng c s dng trong cc quy trnh x l mng mng v x l b mt vt liu.

1.1.1 Tnh cht sng v ht ca bc x

Bc x l nhng dng nng lng pht ra trong qu trnh vn ng v bin i ca vt cht. V mt vt l n c th hin di dng sng, ht, hoc sng ht. Mi dng bc x c c trng bng mt di nng lng hay tng ng vi n, mt di bc sng xc nh. Mi tng quan gia nng lng E v bc sng ca bc x c m t bng biu thc (1.1)

v= = =c

E h ,2

(1.1)

trong , h = 6.626075(40)x10-34Js l hng s Planck; c = 299 792 458 m.s-1 l vn tc nh sng trong chn khng.

Bng 1.2. Phn loi bc x theo nng lng v bc sng

Dng bc x Nng lng in hnh

Bc sng in hnh, m

Sng raio Bc x nhit Tia hng ngoi nh sng , tia t ngoi Tia X:

Tia :

- - - -

100eV 1keV

10keV 100keV

1MeV 10MeV

100MeV

102 - 10-4 10-5 10-6 10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

1.1.2 Phn loi bc x theo nng lng v bc sng

Tt c cc dng bc x c th phn loi theo nng lng v bc sng (Bng1.1).

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1.1.3 Tnh phng x v tc truyn nng lng ca bc x

1.1.3.1 Tnh phng x

- Hng s phn r, chu k bn r v thi gian sng ca ng v phng x

Bc x c th do mt cht phng x pht ra. Khi xem xt mt cht phng x ta thy khng phi tt c cc ht nhn ca chng phn r cng lc. Ti thi im t s ht nhn phn r l N(t), trong sut khong thi gian dt ch c dN(t) ht b phn r. Xc sut phn r trong mt n v thi gian c xc nh bng biu thc:

d (t)dt(t)

= (1.2)

i vi mi cht phng x, l mt i lng khng i, c trng cho cht phng x v cn c gi l hng s phn r. Ly tch phn ca phng trnh (1.2) vi iu kin N(t=0) = N0 ta c:

toN(t) N e

= (1.3) y l nh lut phn r phng x. Theo nh lut ny, xc sut ht nhn khng phn r

phng x thi im t s l:

t

o

(t) e =

Nu coi T1/2 l khong thi gian s lng ht nhn phng x gim i mt na, ta c:

( )1/ 20

N T 1N 2

= ,

1/ 2T

0

N 1eN 2

= =

hay ln2 = T1/2 hoc T1/2 = 0,693/. T1/2 gi l chu k bn r. Nu xc sut phn r trong mt n v thi gian l th tng xc sut phn r ca ht nhn trong sut thi gian sng ca n s bng 1:

0

dt 1 =

1 = Nh vy, thi gian sng ca mt cht phng x c xc nh bng cng thc:

1 = (1.4) - Hot phng x

Hot hay phng x A ca mt cht phng x c xc nh bng s ht nhn phn r trong mt n v thi gian.

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4

dNA N,dt

= = (1.5)

trong , N l s ht nhn c tnh phng x.

- n v o hot phng x

n v o hot phng x l Becquerel (vit tt l Bq)

1 Bq = 1 phn r/giy

n v ngoi h l Curi (Ci)

1Ci = 3,7 1010 phn r/giy = 3.7 1010 Bq

Hot ring ca mt cht phng x c xc nh bng hot ca mt n v khi lng.

v vm

a

NA AAAm NM M

= = = (1.6)

trong , M l Phn t lng ca cht phng x, AV l s Avogadro (AV = 6.02 1023hn/mol)

1.1.3.2 Tc truyn nng lng ca bc x

Tc truyn nng lng hay nng lng truyn tuyn tnh (LET) l nng lng m cc loi bc x ion ho nng lng cao truyn cho vt cht.

Nng lng ny dn n nhng bin i ho l trong vt liu chiu x.

Gi tr ca tc truyn nng lng nm trong khong 0.2keV.m-1 i vi bc x nng lng thp (tia gamma v electron nhanh), v khong 4050 keV.m-1 hoc cao hn i vi cc ion dng gia tc, c th lit k theo th t mc gia tng LET ca cc loi bc x theo s di y:

Nhn chung, kh nng m xuyn ca bc x t l ngc vi gi tr LET.

Nng lng ca bc x thng o bng n v ngoi h electron-Volt, vit tt l eV. N c xc nh bng ng nng ca mt electron c th nhn c khi i qua in trng c hiu in th 1V. Bi s ca eV l keV (103 eV), MeV (106 eV)...

n v nng lng trong h SI l Jun (J) 186.24 10 eV= 1J

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1.2 Cc c trng tng tc ca bc x vi vt cht

1.2.1 c im tng tc ca bc x vi vt cht

Tng tc ca bc x vi vt cht mang tnh cht tc ng qua li:

- Vt cht lm suy gim cng v nng lng ca bc x; - Bc x lm thay i cu trc ca vt cht, gy ra cc bin i vt l, ho hc, sinh

hc,... v cc bin i ny ph thuc rt mnh vo nng lng v dng bc x.

Trong chng ny chng ta ch xem xt tng tc ca bc x ion ho l nhng dng bc x c nng lng ln c th lm bt cc electron ra khi qu o thng trc ca chng trong nguyn t.

1.2.2 Tng tc ca ht nng mang in vi vt cht

Nhng ht mang in tch v c khi lng ln gp nhiu ln khi lng ca eletron c gi l ht nng mang in. Qu trnh tng tc chnh ca chng vi vt cht l va chm n tnh v va chm khng n tnh vi electron qu o. Kt qu ca qu trnh va chm khng n tnh l nguyn t b kch thch (chuyn ln mc nng lng cao hn) hoc b ion ho (electron bt ra khi qu o).

Khi n gn electron in tch e khong cch r, ht nng mang in tch Ze tc dng vi electron bng lc Coulomb:

2

2 2

e e eF ~r r

= (1.7)

S tng tc lm ht mt nng lng. Nng lng mt mt trn mt n v qung ng dE/dx t l vi Z2, mt electron ne v t l nghch vi nng lng ca ht (hoc t l nghch vi bnh phng vn tc v ca ht). Ht chuyn ng cng nhanh, thi gian tng tc cng nh, do nng lng mt mt cng t.

2

e2

nd ~dx

(1.8)

Bc x gamma v electron nhanh Tia X nng lng thp v tia bta Proton Chiu tng ca LET tron

Ht alpha Ion nng Mnh phn hch

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Do mt mt nng lng, ht mang in chuyn ng chm dn, v khi xc sut tng tc ca ht tng ln.

Qung ng t khi ht bay vo vt cht ti khi n b hp th ph thuc vo: in tch, nng lng v mt electron ca vt cht.

Nng lng do ht mt i cn c th truyn cho c nguyn t ni chung. Kt qu l nguyn t v do c phn t m n nm trong, s dch chuyn khi v tr c, ng thi chng nhn mt ng nng no . Trng hp ny gi l va chm n tnh.

Cc qu trnh trn (ion ho, va chm n tnh v va chm khng n tnh) thng din ra ng thi nhng vi nhng xc sut khc nhau.

Khi mt 34 eV trong khng kh, ht nng ch dng 15 eV cho ion ho cn 19 eV cho va chm n tnh v khng n tnh.

1.2.3 Tng tc ca bc x bta vi vt cht

Ging nh cc ht mang in, khi i vo vt cht, ht bta (electron, positron) tham gia vo cc qu trnh sau y:

- Va chm khng n tnh: Kch thch v ion ho; - Hu cp (i vi positron); - Chuyn ng chm dn trong trng ht nhn, dn ti qu trnh pht bc x hm.

Trong trng hp , nng lng b mt t l vi gia tc a ca ht 2~ a (1.9)

Theo nh lut Newton F ma= (1.10)

do 2

2

F~m

(1.11)

Nh vy, nng lng b mt t l nghch vi khi lng ca ht mang in. Trong trng hp cc ht nng, chng hn proton vi khi lng mP = 1,007u=1836me, nng lng mt mt ca n nh hn ca electron hng triu ln. V vy to ra bc x hm, khng th s dng proton hoc cc ht nng mang in khc.

1.2.4 Tng tc ca ntron vi vt cht

Tuy khng phi l ht mang in, nhng ntron vn tng tc vi electron thng qua tng tc gia cc moment t ca chng. S mt nng lng ca qu trnh ny khng ng k. Qu trnh mt nng lng ch yu khi ntron tng tc vi ht nhn, ph thuc vo ntron c va chm trc tip vi ht nhn hay khng. Ngi ta chia tng tc ca ntron thnh mt s loi:

Tn x n tnh: Trong qu trnh ny ntron khng trc tip va chm vi ht nhn. N b mt nng lng v lch hng do lc ht nhn. V phn mnh, ht nhn nhn mt nng lng

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no . Tn x n tnh c th xy ra trong qu trnh lm chm ntron. mt nng lng logarit trung bnh cho mt va chm c xc nh bng cng thc:

1

2

TlnT

= , (1.12)

trong , T1 v T2 tng ng l ng nng ca n trc v sau va chm, hoc: 2(A 1) A 11 ln

2A A 1 = + + (1.13)

trong , A l khi s ca ht nhn b va chm

Tn x khng n tnh: Trong qu trnh ny ntron b mt nng lng v thay i hng chuyn ng, ht nhn trng thi kch thch.

Qu trnh bt ntron: l qu trnh dn ti cc phn ng ht nhn do ntron va chm trc tip vi ht nhn gy ra nh trong cc phn ng sau:

(n,) , (n,p) , (n,2n) , (n,) , (n,f)... (1.14) 1.2.5 Tng tc ca bc x gamma vi vt cht

Tia gamma thuc loi bc x c tnh thm nhp cao i vi vt cht. Chng c th tng tc vi ht nhn, e- v nguyn t ni chung v do nng lng ca chng b suy gim.

S yu dn ca chm tia gamma theo lut hm m v ph thuc vo: mt vt cht, s Z v nng lng ca photon gamma E .

Ngoi cc phn ng ht nhn, i vi tia gamma nng lng cao, s yu i ca tia gamma ch yu do cc qu trnh sau y gy ra:

1.2.5.1 Hiu ng quang in

Hiu ng quang in c nhng nt c trng sau y:

- L s tng tc ca lng t gamma vi nguyn t. - Ton b nng lng ca photon gamma h b mt i do hp th, trong c nng

lng tiu tn cho vic bt e ra khi qu o Eb v nng lng chuyn thnh ng nng Ee cho e :

Ee = h - Eb (1.15) - c trng ca hiu ng quang in: Ch xy ra khi

e > . Electron bn ra thng c phng vung gc vi phng truyn tia gamma.

- Hiu ng xy ra cng mnh khi lin kt ca e cng bn vng. Hiu ng hu nh khng xy ra vi e

c lin kt yu, c bit l khi nng lng lin kt

e lket

8

8

+ i vi mi lp electron, khuynh hng chung

ph 3

1~E

(1.16) + i vi K >

ph 72

1~E

(1.17)

+ i vi K >>

ph1~E

(1.18)

Hnh 1.1 S ph thuc ca tit din hiu ng quang in vo nng lng ca photon gamma

1.2.5.2 Hiu ng Compton

Hiu ng Compton c nhng nt c trng sau y:

- L hin tng tn x ca vi e c lin kt yu trong nguyn t. - Hiu ng ging nh s va chm n tnh gia 2 vin bi: truyn bt nng lng

cho electron v bay lch hng c, e- nhn mt ng nng mi. - Tn x Compton ph thuc vo mt electron trong nguyn t. Mt e cng

ln, cng tn x cng mnh. - Cng tn x ph thuc vo nng lng ca photon gamma E. Mi tng

quan gia nng lng ban u h, nng lng tn x h ca photon gamma v gc tn x c biu th bng cng thc:

2

o

hh ,h1 (1 cos )m c

= + (1.19)

trong , moc2 l nng lng ngh ca electron (0.511MeV)

- Tit din tn x Compton comp ph thuc vo nng lng nh sau (Hnh 12): + nh:

comp 0~ (1 ) (1.20)

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9

+ ln:

comp ~

(1.21)

Hnh 1.2 S ph thuc tit din tn x Compton vo nng lng ca photon gamma

1.2.5.3 Hiu ng to cp

Hiu ng to cp c nhng nt c trng sau y:

- Hiu ng ch xy ra khi E >1.02 MeV (nng lng ngh ca e v e+); - Hiu ng ch xy ra trong trng ht nhn; - Trong trng Coulomb, hiu ng ch xy ra khi E < 2.04 MeV; - (Do s chi phi ca nh lut bo ton nng lng v xung lng). - Tit din to cp ph thuc vo s Z v nng lng ca photon gamma (Hnh 1.3):

2Pair ~ Z ln E (1.22)

Hnh 1.3 S ph thuc ca tit din to cp Pair vo nng lng ca photon gamma

Tit din tng hp ca c ba qu trnh c biu din trn Hnh 1.4:

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Hnh 1.4 S ph thuc ca tit din tng tc ton phn vo nng lng ca photon gamma E

1.2.5.4 S suy gim ca bc x gamma khi i qua vt cht S suy gim bc x ca chm gamma hp

Khi chm bc x gamma hp truyn vung gc vi lp vt cht b dy dx, s suy gim ca cng bc x dI c biu th bng cng thc:

dI(x) = - I(x)dx (1.23) hoc di dng tch phn:

I(x) = Ioe-x (1.24)

trong Io v I(x) l cng bc x gamma trc v sau lp vt cht b dy x; - h s suy gim tuyn tnh ph thuc vo bn cht ca lp vt liu.

Trong trng hp chm tia hp, ng gp ca cc tia tn x khng ng k, hoc c th b qua.

S suy gim bc x ca chm gamma rng

Khi lng t gamma i qua vt cht di dng mt chm bc x rng, trong thnh phn ca chm ngoi cc tia i thng, cn c thnh phn tn x.

Cng ca chm bc x rng c m t bng cng thc

I(x) = Io e-x BE(h, Z, x) (1.25) trong - h s suy gim tuyn tnh ca chm hp; BE(h, Z, x) - h s tch lu nng

lng c tnh ti ng gp ca bc x tn x. i vi chm hp BE(h, Z, x) = 1, khi ta c:

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I(x) = Ioe-x (1.26)

i vi chm bc x rng, BE > 1 v n ph thuc vo nng lng tia gamma h, nguyn t s Z v b dy x ca vt liu.

Do nng lng hp th khng hon ton t l vi tc ng sinh hc nn ngi ta phn bit h s tch lu nng lng v h s tch lu liu lng BD(h, Z, x). Khi ta c biu thc tng t i vi liu lng:

D=Doe-x BD(h, Z, x) (1.27) Cc gi tr s ca h s tch lu c th thu c t vic gii phng trnh truyn vi tch

phn i vi ngun im ng hng v ngun phng n hng trong mi trng v hn ng nht vi cc tham s h, Z, x khc nhau. Trong thc tin cc gi tr B c xc nh bng thc nghim.

1.3 Cc c trng ch yu ca qu trnh truyn nng lng

1.3.1 Cc c trng ca qu trnh truyn nng lng

H s truyn nng lng tuyn tnh:

- H s truyn nng lng tuyn tnh L (Linear energy transfer- LET) ca ht mang in trong mi trng vt cht c xc nh bng cng thc:

dELdl

= (1.28)

trong dE - tn hao nng lng trung bnh ca ht mang in trn qung ng dl. Ni chung, nng lng ca ht c tiu tn cho qu trnh ion ha v kch thch cc nguyn t ca vt cht, phn khc tiu tn cho qu trnh pht ra bc x hm. Cc in t pht ra trong qu trnh ion ha, c th c nng lng gy ra qu trnh ion ha tip theo; kt qu l trn ng i ca ht mang in xut hin cc vt ca s ion ha tng. Cc in t th cp c th gy ra hin tng ion ha tip theo c gi l cc in t .

- H s truyn nng lng tuyn tnh ph thuc vo ng nng ca ht s cp v qung chy tuyn tnh ca ht trong vt cht.

Liu hp th:

Liu hp th D ca mt cht c khi lng dm c xc nh bng t s gia nng lng dE c cht hp th v khi lng ca cht :

dE dEDdm dV

= = (1.29)

trong - mt vt cht, dV - th tch n v. n v ca liu hp th l gray, vit tt l Gy:

1Gy = 1J kg-1

n v ngoi h SI l rad

1Gy = 100 rad = 104 erg/g.

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Sut liu hp th:

Sut liu hp th c xc nh bng cng thc:

dDD'dt

= (1.30)

Sut liu hp th D' c coi l liu hp th trong mt n v thi gian.

n v ca sut liu l Gy s-1

1Gy.s-1 = 1J.s-1.kg-1=1Wkg-1 .

Kerma (Kinetic energy released in material) K - ng nng gii phng trong vt cht.

Kerma l tng ng nng ban u dEk ca cc ht mang in gii phng ra trong mt n v khi lng vt cht:

k kdE dEdm dV

= = K (1.31)

- Sut Kerma: Sut Kerma c xc nh bng cng thc:

dKK 'dt

= (1.32)

n v o ca Kerma v sut Kerma tng ng ging nh n v o ca liu v sut liu.

- Dng r nng lng:

Dng r nng lng l nng lng b tht thot khi b mt ca mt n v th tch xem xt. Dng r nng lng c xc nh bng biu thc J/, trong J l vect mt dng.

- Phng trnh cn bng liu:

Phng trnh cn bng liu c vit nh sau:

bk dEdEdE Jdm dm dm

= (1.33)

trong dEk - nng lng tiu hao cho qu trnh hm ca cc ht mang in.

- Liu chiu:

Liu chiu P c xc nh bng s n v in tch sinh ra iu kin chun khi b chiu x.

dQ dQPdm dV

= = (1.34)

n v liu chiu l C kg-1, n v ngoi h l Rontgen, 1R = 2,58 10-4C kg-1

1.3.2 Phn b liu theo chiu su

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xuyn su ca nng lng bc x vo vt liu c th c m t bng ng phn b theo chiu su, trong liu hp th tng i ti cc im o c v theo khong cch hay su tnh t b mt vt liu chiu x. Dng ca ng phn b liu - su ph thuc vo bn cht ca bc x, nng lng ca bc x hoc chm ht, cu hnh ca ngun v mu. Dng in hnh ca ng cong liu- su trong nc i vi bc x c gii thiu trn Hnh 1.5 (i vi bc x gamma v tia X) v Hnh 1.6 (i vi chm electron nhanh)

Qua cc th trn ta thy kh nng m xuyn ca tia X ln hn ca electron. Nng lng lng ca bc x ny cng cao th kh nng m xuyn cng ln. i vi bc x gamma v tia X (Hnh 1.5) quan st thy cc nh cc i ti su 0,12; 0,5 v 1,0 cm. i vi bc x electron (Hnh 1.6) nng lng 1,8; 4,7 v 10,6 MeV cc nh cc i tng ng nm ti 0,25; 0,95 v 1,9 cm. V tr ca cc nh cc i ph thuc vo kch thc chm bc x v c lin quan ti i lng truyn nng lng tuyn tnh cng nh hiu ng electron th cp gy ra.

th Hnh 1.5 v Hnh 1.6 m t ng phn b liu - su trong php chiu mt pha i vi mi trng nc v hn. Trong cng ngh bc x, cc sn phm chiu x thng c kch thc hu hn. m bo tnh ng u tng i ca liu chiu trong sn phm, ngi ta thng chiu sn phm t hai pha. T s R gia liu chiu cc i Dmax v cc tiu Dmin c gi l t s ng u liu:

maxmin

DRD

= (1.35)

Hnh 1.5 ng cong liu - su tnh theo phn trm i vi php chiu trong nc A: bc x gamma ca ngun 137Cs; B: bc x gamma ca ngun 60Co; C: bc x tia X4 MeV

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Hnh 1.6 ng cong liu- su tnh theo phn trm i vi php chiu x electron nhanh trong nc A- electron 1,8 MeV; B-electron 4,7 MeV; C- electron 10,6 MeV

T s R ph thuc vo kch thc ca mu, mt ca vt cht trong mu v nng lng ca bc x.

Hnh 1.7 gii thiu phn b liu theo su chiu t 2 pha i vi lp nc c b dy 20 cm.

Hnh 1.7 Phn b liu trong lp nc dy 20 cm (Chiu x t hai pha) A-Bc x gamma ca ngun 137Cs; B-Bc x gamma ca ngun 60Co; C-Bc x tia X 4 MeV.

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Hnh 1.8 Phn b liu theo b dy vt liu chiu t hai pha i vi electron 5 MeV

1.3.3 Hiu ng bc x th cp

Khi b hp th trong vt cht, bc x in t c th to ra cc electron th cp. Ti cc im nm cch b mt cht hp th mt khong cch ln hn qung chy ln nht ca electron th cp, mt n v th tch nhn c electron tn x t mi pha. Tuy nhin cng gn b mt, s lng electron th cp m mt n v th tch vt liu nhn c cng gim, do mt phn cc cc electron th cp thot ra khi b mt. Do , phn b liu theo su ca bc x ion ha tng dn theo b mt v t ti gi tr cc i khong cch bng qung chy ln nht ca electron th cp. cc su ln hn electron suy gim theo quy lut hm m nh bc x s cp.

1.3.4 Cu trc vt ca ht

Khi mt ht mang in p vo vt cht, n mt nng lng, chuyn ng chm dn, to ra mt vt cc nguyn t, phn t b kch thch v b ion ha dc theo tuyn ng i ca ht. Electron v positron l nhng sn phm ca qu trnh hp th nng lng; chng c linh ng rt cao, v c th to ra cc vt nhnh dc theo qung ng i ca ht.

Ni chung, qu trnh hp th mt bc x ion ha bt k u to ra cc vt sn phm ion ha v kch thch. Cc sn phm ny c bn l ging nhau, c bit l trong vt rn.

Tuy nhin cc dng bc x khc nhau vi nng lng khc nhau, s c tc mt nng lng khc nhau, dng ca vt do cng khc nhau. Chng hn, chng c mt dy c hn hoc phn tn hn; cc nhnh cng c kch thc to nh hoc di ngn khc nhau. S khc nhau cn quan st thy v mt hiu ng ha hc, v s lng cng nh t l ca cc sn phm c to ra, v kch thc ca vt gc ban u.v.v... Do , i lng truyn nng lng tuyn tnh (LET) c ngha quan trng trong vic nh gi mt cch tng th cc hiu ng ha hc...

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Ngi ta c th tnh c s lng vt, chng hn trong mt th tch dng hnh tr dc theo vt. Samuel v Magee [6] thng tnh cc vt khong cch 1m v ng knh ban u khong 2m i vi cc electron th cp do photon gamma to ra trong nc hoc cc cht hu c th lng.

L thuyt cu trc vt ca Katz v cng s [7] xem xt mi tng quan gia s lng vt do ht to ra vi nng lng hp th trong vt cht.

1.3.5 Hiu sut ho bc x G v xc sut to phn t kch hot

Hai i lng quan trng trong qu trnh x l bc x l liu lng hp th v hiu sut ho bc x. Liu lng hp th c th o bng cc n v eV.g-1, eV.cm-3, rad v sau ny c thay th bng n v h quc t SI l gray (1 Gy = 1Jkg-1 = 100 rad).

Hiu sut ho bc x hay gi tr G l s phn t kch hot c to ra khi vt cht hp th nng lng 100 eV. Trong h SI, G c o bng n v molJ-1 hoc molJ-1. M 100 (G(Phn /100eV)

N W= t (1.36)

trong , M l s phn t b bin i di tc dng ca bc x cn N l s cp ion c to ra t cc phn t bin i ; W l nng lng trung bnh to ra mt cp ion trong vt liu b chiu x.

T s M/N c gi l hiu sut to cp ion. Thot u c coi nh l hiu sut ho hc ca mt h kh song v sau cng c p dng cho mt h cht lng, mc du i vi cht lng kh o trc tip c i lng hiu sut to cp ion.

i vi a s cc cht, W xp x bng 30 eV, do , gi tr G xp x bng 3 ln hiu sut to cp ion.

Mi tng quan gia liu hp th, hiu sut ho hc v hiu sut ho bc x ca sn phm trong h SI c biu din nh sau:

6 )[Phn 100eV] 9,648 10= -1Hiu sut ha (mol.kg

G t/Liu hp th (Gy)

(1.37)

y cn lu rng, 1eV = 1,602 106 J v s phn t trong mt phn t gam (s Avogadro) bng 6,022 1023 mol-1.

Trong cc l thuyt truyn nng lng hin i, thay cho gi tr G, ngi ta dng khi nim xc sut to ra mt phn t kch hot t mt phn t nhy bc x k [Gy-1] hay 1/D37 [Gy-1], trong , D37 l liu trung bnh cc phn t nhy bc x nhn c trong mt ln va chm. Mi tng quan gia cc i lng ny nh sau:

37D1k = (1.38)

photon gamma photon gamma photon gamma

Hnh 7.9 2 Hnh 7.9 2