Nghiên cứu một số đặc trưng của tương tác bức xạ gamma mềm với vật chất và ứng dụng trong phân tích nguyên tố bằng đêtectơ nhấp nháy

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BO ClAO DUC VA DAO I'AO
DAI HOC g u d c CIA MA NOI
TRirONCi DAI HOC KHOA H(.)C TU NHIEN

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NGHIEN CUrU MOT SO DAG TRUNG CUA TlTONG TAG
BUfG XA GAMMA MEM VQfl VAT GHAT VA UfNG DUNG TRONG
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PHAN TiGH NGUYEN TO B A N G DETEGT0 NHAP NHAY

Chuyen ngdnh : Vat ly hat nhan nguyen tu
Ma so

: 01.02.03

LUAN AN PHO TIEN SI KHOA HQC TO AN LY

NgUcri Huong dan khoa hoc :
P(JS. FTS. Fham Quoc Himj^

Ha Noi - 19%

2
MUCLUC
Tran

Mddau

4

Chuotig 1 Tuong tac ciia hire xa gamma mdm vdi vat chat

13

1.1. Hap thu va tan xa

13

1.2. Ht sd suy giam va nguyen tu sd hi6u dung ciia moi trmVng
phuc tap

IV

1.3. Nghi^n ciiu xac dinh hing thuc nghiem ht sd
suy giam ciia cac chat ddi vdi hue xa gamma m6m
Ket luan chuong 1

24
35

Chuong 2. Kdt qua thuc nghiem nghi(^n cuu mot sd dac trung

CO ban cua tuong tac gamma m6m vdi vat chal
2.1. Hi6u ling tu hap thu cua mSu

36
36

2.2. Xac dinh bang thuc nghiem nguyen tu sdhi6u dung
cua moi trudng vat chat phuc tap

39

2.3. Nghi^n cuu xac dinh thanh ph^n dong gop ciia thanh phan
tan xa Rayleigh trong phd gamma tan xa
Kdt luan chuong 2

47
61

Chuong 3. Mot sd van A6 co ban ciia phuong phap phan tich
ham lugng nguyen id thco hue xa huynh quang X
dac trung kich thich hdi gamma
3.1. Ca so" ciia phuang phap

62
62

3.2. Hicu ung chal n6n irong phan lich ham Immg thco phd
hue xa huvnh quang X dac iruni:

hS

3.3. Nghi6n cuu thuc nghi6m cac ydu td anh huctng
d^'n k6t qua phan tich ham luc;mg nguy6n td theo
dinh cua vach dac trung
K6t luan chuong 3

79
^^

Chirong 4. Sir dung hue xa tan xa lam chua'n trong va phucmg
phap ty sdphd
4.1. Phuong phap ty sdphd

^^
XX

4.2. Nghien cihi thuc nghiem chgn nang lugng biic xa tan xa
lam chu^n trong
4.3. Bie^u thiic tinh ham lugng theo phuong phap ty sd phd

9'*
93

4.4. \Su Aiim va pham vi tog dung cua phuang phap
ty sdphd
4.5. Nghi6n cthi anh hudng cua cac tap chat nang

96
102

4.6. Nghi6n curu thuc nghi6m khdc phuc anh hudng ciia
cac hue xa nhiSu co nang lugng gSn vdi nang lugng
vach phan tich

lOX

K6t luan chuong 4

109

Phan ke't luan

116

Tai lieu tham khao

119


M0DAU
Nghi^n cufu cac dac trung ciia tuong tac btrc xa hat nhan vdi vat
chat la CO sd vat ly Ai ap dung c6 hi6u qua ky thuat hat nhan trong cac
linh vuc khac nhau. Trong ph^n tich ham lugng nguyen td theo hue xa
huynh quang X dac trung, kich thich bdi gamma phat ra tir cac ngudn
ddng vi phdng xa, ca hire xa dac trung dugc chgn phan tich va hue xa
gamma kich thich A6u co nang lugng thap. Nghi^n cuu cac dac trung
cua tuong tac hire xa gamma m6m vdi vat chat la can thi^'t d^ nang cao
chat lugng ciia phep phan tich.
Khai ni6m hire xa gamma m6m su dung trong luan an nay dugc

hi6^u la cac biic xa gamma c6 nang lugng nam trong vimg nang lugng
ion hoa cua cac nguyen td co trong Bang he thdng tu^n hoan. Nang
lugng ciia cac birc xa dac trung cua cac nguyen td co trong tu nhi^n va
ngay ca cac nguyen td nhan tao d cudi Bang he thdng tuin hoan d6u nhd
hon 150 keV. Vi vay d^^ kich thich c6 hi6u qua cac hue xa dac trung ciia
cac nguyen td c^n phan tich, trong thuc t^' ngudi ta sir dung hue xa
gamma m6m.
Cac hire xa gamma m6m khi tuong tac vdi vat chat thd hicn
nhiJng tinh chat ri^ng hi6t ciia minh: Rat nhay ddi vdi su thay ddi cua
thanh phan boa hoc ciia moi trmVng, ht sd suy giam co linh nhay hac khi
nang luang cua hue xa gamma di qua hi^n hap thu ciia mdi Idp vd ciia
nguyen tu. Dt nang cao do chinh xac ciia phep phan tich. vice nghicn


cuu cac dac trung tuong tac ciia hue xa gamma nang lucmg thap vdi val
chat la can thi^'t.
V6 mat ly thuydt, tuong tac ciia hue xa gamma vdi vat chat dugc
dac trung hang iiti Aitn tuong tac ciia hue xa gamma tr^n mgl nguyen
tu. Trong thuc nghiem, he sd suy giam la mot trong nhirng dai lU(tng dac
trung cho tuong tac ciia hire xa gamma vdi vat chat.
Cac hire xa dac trung do cac nguyen td cd trong mau phat ra tham
gia vao tuong tac vdi cac nguyen td cd trong mSu trudc khi no di ve
detecto. Chinh mSu da "tu" hap thu cac hire xa dac trung do no phat ra.
Su tu hap thu ciia mSu phu thugc vao b6 day khdi ciia mSu va he sd suy
giam cua mSu ddi vdi hiic xa can phan tich.
Khi phan tich ham lugng ciia cac nguyen td theo hue xa dac trung
nang lugng thap theo phuong phap tuong ddi, su sai khac nhau ve thanh
phan hoa hoc cung nhu hi day ciia mSu phan tich va mSu chuan se gay
nen mot sai sd Idn.Trong cac hai toan phan tich c6 yeu cau do chinh xac
cao, can phai hieu chinh su sai khac tren. De^ tie'n hanh cac hieu chinh

tren, can phai hie't dugc he sd suy giam ciia mlu chuan va mSu phan tich
ddi vdi hire xa can phan tich.
Ngoai ra trong qui trinh phan tich cac mau day ciia cac phuong
phap phan tich khac chang ban nhu phuong phap PIXE, de hieu chmh
su tu hap thu ciia mau can phai hie't su phu thuoc ciia he sd suy giam
vao nang lugng ciia hire xa gamma va nguyen tu sd ciia moi trucVngI 151.
Mac du su phu thugc ciia he sd suy giam vao nang lucrng ciia hire xa
gamma mdm ra'l phirc tap, nhung nhidu tac gia da dua ra cong thirc [x{z)


mo ta su phu thugc vao nang lugng cua he sd suy giam ddi vdi moi
nguyen td. Cac thdng sd trong c6ng thirc |a(e) dugc chgn hang each lam
khdp c6ng thiirc nay vdi cac sd lieu thuc nghiem ve he sd suy giam ciia
mdi nguyen td ddi vdi cac hire xa gamma nang lugng xac dinh. D6 lam
dugc dieu tren ddi vdi mdi nguyen td can phai cd nhieu sd lieu thuc
nghiem ve he sd suy giam khdi cua no ddi vdi hire xa gamma nang
lugng khac nhau.
Sd lieu thuc nghiem ve dac trung suy giam ciia cac chal ddi vdi
cac gamma mem con it. Viec xac dinh he sd suy giam ciia cac nguyen td
ddi vdi cac gamma mem cd y ngbla trong qua trinh lam khdp tren cung
nhu trong qua trinh xac dinh hieu sua't ghi ciia detecto han dSn Si(li) d
vung gamma nang lugng thap.
Cung vdi su tie'n ho cua khoa hoc c5ng nghe, cac thie't hi va
phuang phap phan tich phd huynh quang X dac trung ngay cang dugc
hoan thien. Ngay nay phuang phap phan tich phd huynh quang X dac
trung kh6ng nhung chi cd ung dung trong phan tich thanh phan nguyen
td ma ngay cang cd nhirng ung dung trong cac ITnh vuc khac nhau:
Trong viec kie^m tra cha't lugng san pham, trong y te' [18, 20,- 22], trong
linh vuc kie^m tra do nhiem ban [13, 42,45] cua moi trudng.
Su tie'n ho ciia ky thuat do ludng cac hire xa hat nhan da cho phep

sir dung cac detecto han dSn cd kha nang phan giai nang lucrng cao, kich
thich hue xa dac trung hang cac hat tich dien gia tdc hoac hang tia X tao
ra tir cac thiet hi dien tu. Cho ddn nay da xua'l hicn cac detecto han dan
loai Si (Li) khong cd cua sd. Vice xua't hien cac detecto nay da cho phep
met rong viing phan lich ve phia cac nguyen id nhc. Ngoai ra |14| da


xual hien nhung loai detecto han dan cd kich thudc nhc\ cd do phan giai
nang lugng cao, cd kha nang lam viec d nhiet do phdng. Vice xual hicn
loai detecto loai nay cho phep tie'n hanh phan tich ham luc^ng nguyen id
theo phuang phap huynh quang X dac trung, ngay ngoai hicn truong,
vdi do chinh xac cao. Phuang phap phan tich thco hue xa huynh quang
X dac trung ngay cang hoan thien va dugc sir dung rgng rai de phan tich
ham lugng ciia nguyen td trong cac mSu vat cac loai khac nhau. Cung
vdi su phat tri^n cua cac thie't hi kich thich va detecto ghi nhan hire xa
dac trung, cac chuong trinh xir ly phd gamma va tia X ngay cang hoan
thien. Trong cac he phan tich deu cai dat cac chuong trinh cho phep xac
dinh ham lugng cac nguyen td trong cac mau vat dang khac nhau vdi do
chinh xac cao.
Tuy detecto nhap nhay cd do phan giai nang lugng khong cao
hang detecto ban dSn, nhung nd cd im viet hon so vdi detecto han dSn
dd la: Hieu sua't ghi cao, cd the^ lam viec ngay ca vdi nhiet do ngoai trdi,
thuan tien trong qua trinh van chuyen khi phan tich da ngoai. Chinh vi
nhung uu diem tren, hien nay tren the'gidi detecto nhap nhay van dugc
sir dung rgng rai nha't la trong ITnh vuc dia vat ly hat nhan.
Cung vdi cac ddng vi phdng xa, vice sir dung cac phd ke gamma
nhap nhay van cho phep giai quye't cd hieu qua mot sd yeu cau ciia hai
loan thuc te dal ra: Phan tich ham lugng nguyen td nang tren ntn cac
cha't nhe, tim kie'm tham dd mo khoang san tai hicn trucmg. xac dinh
nhanh ham lugng ciia nguyen td irong cac qua irinh cong nghc. Trong

cac phep phan tich ham lugng cac nguyen id thco phd hire xa dac irung
X vdi vice sir dung cac ddng vi phdng xa \'a deiccur nhap nha)'. ihic'l hi


gon, don gian khong can phai lam viec d nhiei do Nito long, thai gian
can cho mgt phep do nhd, gia thanh phan tich re.
Trong phan tich ham lugng cac nguyen td thco phd huynh quang
X dac trung kich thich bdi gamma, ke'l qua phan tich chiu anh hmrng
ciia nhieu ye'u td nhu su thay ddi thanh phan boa hoc ciia chat nen, su
thay ddi cua mat A6 mau, su khong dn dinh cua hinh bgc va thiet hi do...
De khac phuc cac ye'u td anh hudng tren va nang cao do chinh xac ciia
phep phan tich, nhieu tac gia da dua ra [16, 31, 33, 34, 44] phuang phap
ty sd phd. Phuong phap ty sd phd da dugc su dung nhi6u trong cac hai
toan khac nhau nhu: Trong phuong phap gamma - gamma mat do, trong
phuong phap gamma - gamma Igc lua va trong phuong phap phan tich
ham lugng nguyen td theo phd huynh quang X dac trung. Thuc cha't ciia
phuang phap ty sd phd la su dung chinh cac hire xa tan xa tir mSu lam
chuan trong de^ han che' anh hudng cua cac ye'u td den ke't qua phan tich.
Cac phuong phap chuan trong thong thudng dugc su dung
[8,11,31] de^ hieu chinh anh hudng cua cac ye'u td de'n ke't qua phan tich
hang each dua them vao mSu mot lugng xac dinh ddng vi phat ra hue xa
cd nang lugng gan vdi nang lugng cua nguyen td can phan tich. Can cir
vao dien tich hai dinh dac trung ciia nguyen td can phan tich va nguyen
tddugc dua them vao hie't dugc ham lugng cha't can phan tich.
Trong phuong phap ty sd phd, ham lucrng ciia nguyen td can phan
tich dugc xac dinh thong qua ty sd ciia sd de'm trong vimg nang luctng
dac trung cho nguyen tdcan phan tich va sdde'm trong viing nang lucmg
ung vcri cac hue xa tan xa tir mau. Su thay ddi ciia cac ydu td ke' ircn sc

anh hudng c6 dang nhu nhau len toan ho phd ciia hue xa thir cap. vi \'ay
ma dai luctng ty si) phd sc hi anh hudng il di.
Ap dung phucmg phap ty sd phd trong phan tich ham lucmg thco
phd huynh quang X dac trung, nghien ciru xac dinh hang thuc nghiem
mot sd dac trung cua tuong tac hire xa gamma mtm vdi val chal da
dugc tie'n hanh tai ho m5n Vat ly Hat nhan trucVng Dai h()c Khoa hoc Tu
nhien thugc Dai hoc Qudc gia Ha Ngi.
Ke't qua nghien cuu theo cac hudng tren da cong hd tren mgl so
Tap chi va Hoi nghi chuyen nganh.
Ban luan an vdi ten ggi: "Nghien cihi mot so dac trung ciia
tuong tdc bite xa gamma mem v&i vat chat vd ijcng dung trong phan
tich nguyen to bdng detectanhdp nhdy*' la cong trinh he thdng va phat
trien cac ke't qua tren trong thai gian tir 1982 de'n nay.
Muc dich cua luan an:
Ban luan an cd muc dich nghien cu"u va xac dinh hang thuc
nghiem mgt sd dac trung co han tuong tac cua hire xa gamma mem vcVi
vat cha't, ap dung phuong phap ty sd phd de^ nang cao cha't lucTng trong
phan tich ham lugng nguyen td theo phd hue xa huynh quang X dac
trung tren phd ke'gamma nha'p nhay.
Cac nhiem vu chinh ciia ban luan an la:
1 - Xac dinh hang thuc nghiem moi sd dac trung quan irgng ciia
lucrng tac hire xa gamma mdm vcri vat chal tren phd ke' gamma nhap
nhay:


0

- Xac dinh he si) suy giam cua mgl sd chat dc')i vc'ri gamma m6m.
- Kie'm chirng hang thuc nghiem tinh nhay trong tucmg tac ciia
gamma mdm vcri val chat, dc'ii vcri su thay ddi thanh phan hoa hoc cua

val cha'l va tinh chal phu thugc nhay hac cua he sc')' suy giam kh(')i vao
nang lucrng cua hue xa gamma va nguyen lu sc')' hieu dung cua mc')i
trucrng.
2- Nghien cu*u hang thuc nghiem nhung ye'u td anh hudng de'n ke't
qua xac dinh ham hxgng nguyen td dua vao dien tich vach dac trung cua
nguyen tdcan phan tich.
3- Nghien ciru va ap dung phuong phap ty sd phd trong phan tich
ham lugng nguyen td theo phd huynh quang X dac trung tren phd ke'
gamma nha'p nhay, dap irng dugc yeu cau ciia mot sd hai toan thuc te'
dal ra.
Tinh khoa hoc va mdi cua luan an:
1- Da nghien cuu mot each cd he thdng cac va'n de cd lien quan
tdi phuong phap phan tich nguyen td theo hire xa huynh quang X dac
trung va dua ra dugc nhiing hien phap nham nang cao chat lucrng phan
tich nguyen tdhang detecto nhap nhay.
2- Da de xual mot sd phuang phap ngoai suy khi xac dinh he sd
suy giam ciia mot so chat dc'ii vc'ri hue xa gamma mem dua vao quy lual
ham e mu mo ta su suy giam cudng do hire xa gamma trong val chal va
phucrng phap danh gia thanh phan tan xa Rayleigh irong phd lan xa cua
gamma mdm.


11

Cac ke't qua thuc nghiem thu Auac Irong han luan an nay gdp
phan lam rd hem co che'tucrng tac ciia hire xa gamma mtm vc'ri \'at chal,
hd sung them cac sd lieu thuc nghiem ve he sd suy giam dc')i vcri gamma
5.9 keV.
Tinh thuc tien ciia luan an:
Da ap dung phuang phap ty sd phd de xac dinh nhanh ham lucrng

cua nguyen td theo phd hue xa huynh quang X dac trung vcri do chinh
xac tdt tren phd ke'gamma nha'p nhay, he thie't hi khong qua dai lidn cd
th^ trang hi trong cac phdng thi nghiem phan tich thong thucrng. Ke'l qua
cua han luan an nay da gop phan giai quye't hai toan xac dinh hang thuc
nghiem nguyen tir sd hieu dung ciia cac mSu chuan phdng xa QucVc gia.
Cling nhu cac yeu cau phan tich nhanh trong tham dd va trong qui trinh
cong nghe lam giau tdng oxit da't hie'm dugc dat ra tai mot sc") ca scV san
xuat.
Ban luan an dai 124 trang gdm co 29 hinh ve va dd thi. 29 hang
hieu va 47 tai lieu tham khao.
Ngoai phan md dau va phdn ke't luan. han luan an ducrc chia lam 4
chucmg. Trong chuong 1 va chucmg 2 trinh hay mgl sd ke't qua nghien
ciru thuc nghiem dac trung tuong tac cua hire xa gamma mem vc'ri val
cha't. Hai chuong cudi trinh hay cac ke'l qua nghien ciru irng dung
phuong phap phan tich ham lugng nguyen td thco phd huynh quang X
dac trung kich thich hdi tia gamma.
Ban luan an ducrc thuc hicn tai Bg mc)n Val 1\' hai nhan ihuc
nghiem khoa Val ly iruirng Dai hc)c Tdng hcrp Ha Ngi iruc'rc da\. na\' hi
irucrng Dai hc)c Khoa hoc Tu nhien Ihugc Dai hoc Qudc i:ia Ha Noi. Ban


12
luan an la ket qua qua trinh tac gia tham gia de tai cap nha nuc'rc mang
ma sd KT-04-3. 2.1, de tai cap nha nude ma sd 52-02-06-04 va de lai
ca'p Dai hc)c Quc')'c gia mang ma sd QG 95-07.
Trong qua trinh thuc hien va hoan thanh luan an tac gia nhan
dugc su iing ho ve thie't hi va thai gian cua Bo mc)n Val ly Hat nhan thuc
nghiem, tac gia xin chan thanh cam cm Chii nhiem Bg mc)n,
PCS. PTS. Nguyen Trieu Tu va toan the can ho trong ho mon ve su ung
ho nay.

Trong sudt thai gian thuc hien va hoan thanh luan an nay, tac gia
da dugc PCS. PTS. Pham Qudc Hung hudng dSn tan tinh va chu dao va
gop nhieu y kie'n ve ngi dung va hinh thuc ciia han luan an. Tac gia xin
hay td long hie't cm sau sac tdi PCS. PTS.
Tac gia xin chan thanh cam cm PGS. PTS, Dang Huy Uyen va
PCS. PTS. Trucmg Bien da tao dieu kien cho tac gia tham gia cd hieu
qua cac cong viec trong de tai do cac PGS, PTS chu tri.
Tac gia xin chan thanh cam on Phd chu nhiem khoa Val ly
PGS. PTS. Bach Thanh Cong, Trudng phdng Dao tao Trudng Dai hoc
Khoa bgc Tu nhien, PTS,.Bui Duy Cam da tao dieu kien de' tac gia hoan
thanh luan an nay.


13

Chuons 1
TirONG TAC CUA BUG XA GAMMA MEM
VOl VAT CHAT

1.1. Hap thu va tan xa
Khi m6t chum hire xa gamma di trong vat cha't, cudng do cua nd
hi suy giam do ke't qua tuong tac ciia nd vdi cac nguyen tir ciia vat chat.
Cac hue xa gamma nang lugng thap va trung hinh khi di qua vat cha't
chii ye'u tuong tac vdi cac dien tir ciia moi trudng vi vay nang lucmg cua
biic xa gamma thudng dugc tinh trong don vi nang lugng nghi cua dien
tu. Khi dd nang lugng ciia hue xa gamma, ky hieu la k, la mot dai lucmg
khong CO thir nguyen va dugc xac dinh theo cong thirc:
k = s/moc2,

(1.1)

trong dd : mQC- la nang lugng nghi ciia dien tir bang 511 keV, con s la
nang lugng ciia hue xa gamma dugc tinh ra keV.
Khi di qua vat cha't. xac sua't xay ra mot qua trinh tucmg tac nao
dd phu thuc)c vao nans: luanz cua birc xa gamma va niiuven tir sd ciia
mc)i trucmg. Dc)'i vdi birc xa gamma mem (kirudng thcmg qua hicn tucmg hap thu quang dien va lan xa.


14

LU.

Hap thu quang dien

Hicn tmmg hap thu quang dien la qua Irinh hap thu lucmg lU
gamma hcri cac nguyen tir cua val chat. Trong qua Irinh hap thu quang
dien. loan ho nang lucrng cua lugng tu gamma ducrc iruydn true liep cho
moi dien tu nao dd ciia nguyen tu va dien tir hay ra khc)i nguyen lir vcVj
dong nang hang hieu nang lugng cua hue xa gamma va nang lucmg lien
ke't cua dien tir trong nguyen tir. Dien tu tham gia vao qua trinh ha'p thu
quang dien dugc ggi la quang dien tu. De xay ra hien tuc^ng hap thu
quang dien ddi vdi mCt Idp dien tu thir i nao dd ciia nguyen tu, nang
lugng ciia hire xa gamma can phai Idn hon the'nang ion hc)a ciia krp dd.
Khi nang lugng cua hue xa gamma Ion ban the' nang ion hoa ciia krp K
thi xac sua't xay ra hieu u*ng quang dien d krp K la Idn nha't. Xac sua'l
xay ra hieu irng quang dien dugc dac trung hang tie't dien hap thu quang
dien tren mgt nguyen tir, dugc ky hieu hang Ta . cd thir nguyen la cm-.
He sd suy giam khdi do hap thu quang dien \i^ cd thir nguyen la cm-/g,
dugc tinh theo cong thuc sau :

Ac
\^o,=^^a

(1.2)

trong dd A la nguyen tir gam cua cha't hap thu; ccm AQ la sd Avogadro.
He sd ha'p thu quang dien tren mot nguyen tu la phu thugc vao
nang lucrng va nguyen tir sdcua moi trudng [34] theo ccmg thuc sau:
Ta = 5.01 10---^ Z-^>^W khi S>IK

(1.3)

Ta= 1,62 10-^4 Z4.3()/£3 khi 1K>S>IL
irong dd Z la nguyen tu sd ciia moi truimg; ccm I|^ va 1} lan lucri \i\ Uie'
nang ion hda krp K va krp L cua nguyen lu.


15

Dc'ii vc'ri mdi krp dien lir thir i ciia nguyen lu, tai hien hap ihu cd
hai gia tri ciia Ta. Gia tri cua Ta vt phia nang lucmg Ic'rn ky hicu hi Ta'
cc)n ve phia nang lucrng thap ky hicu la Ta". Ti sd giua hai gia iri do ggi
la he sd nhay hac ciia Idp thir i va ky hicu hang ki (ki = Ta'/Ta"). Nhu
vay he sdsuy giam khcii do hap thu quang dien thay doi nhay hac khi
nang lugng hue xa gamma di
qua cac gia tri hien hap thu ciia
cac lap dien tir ciia nguyen tir.

u

IG

Hinh 1.1 mo ta sir phu
thugc vao nang Ixxgng cua he sd
ha'p thu quang dien tren m6t

C

OH

nguyen tir ciia chi [15, 21]. Tir

cong thuc (1.3) nhan tha'y rang
hien tugng quang dien xay ra
manh ddi vdi cac cha't nang va
nang lucrng cua hue xa gamma
nhd. Khi nang lugng cua hue xa

10
100
UH^tl
Nang lucmg hue xa gamma (keV)

gamma tang, tie't dien ha'p thu
Hinh 1,1: Su phu thuoc vao nang h(o'w^
quang dien giam nhanh theo cua he sol.up ihu quang dien ircn mot
ham l / s ' .

nguyen tu ciia chi.

1J.2. Tdn xa
].I.2.a. Tan xg Compton
Tan xa Compton la qua trinh lucrng lac khc)ng dan hdi ciia hire xa
gamma vc'ri cac dien tu tu do bay dien lu lien kei yeu cua cac nguyen lu.
Trong qua trinh tan xa Compton, hire xa gamma iruydn mgl phan nang
lucmg ciia minh cho dien tu va hi tan xa thco huc'mg 9; 6 ducrc goi la got


16

lan xa. Ke'l qua cua qua trinh tan xa Compton la dien lii' nhan dugc mgl
nang lucmg giai lui va nang lucmg ciia hire xa gamma tan xa giam so voi
nang lucmg cua hire xa gamma Ic'ri. Dien tu tham gia vao qua irinh lan
xa ggi la dien tu Compton. Tan xa Compton xay ra manh khi nang
lucmg cua hire xa gamma km hern nhieu so vc'ri nang lucmg lien kcl cua
dien tu. Khi nang lucmg ciia hire xa gamma tang, cac dien tu hay ihco
hudng uu tien ve phia trudc. Nang lugng cua hire xa gamma tan xa phu
thugc vao gdc tan xa 0 va nang lugng ciia hue xa gamma tdi [30, 31, 38]
theo cong thirc sau:
Ss- .

8
J
• . /

1 + k(1-cose)

(1-4)

trong dd sj va £§ la nang lugng ciia hire xa gamma tcri va gamma tan xa.
Nang lugng ciia hire xa gamma tan xa Compton khong phu
thugc vao cha't tan xa ma chi phu thugc vao gdc tan xa. Trong qua trinh
tan xa Compton hue xa gamma tan xa cd the hay theo hucmg tan xa ha'l
ky, tuy nhien xac sua't tan xa theo mot phuong 0 nao dd phu thugc vao
nang lucmg ciia hue xa gamma tdi va gdc tan xa 0. Xac suat tan xa
Compton tren mot dien tir trong mot don vi gdc khdi theo hudng tan xa
0, ky hieu la dae/dQ , dugc xac dinh [34] theo cc)ng thirc sau:
p2

= ^(ss/sj)2[(ss/sj) + (si/ss)-9n2e]

(1.5)

trong dd TQ la han kinh cd die'n cua dien lu va hang 2.82.10"'3cm.
Dc)'i vdi nang lucmg cua hire xa gamma nhc) (kciia cac hire xa gamma tan xa cd linh dc')i xirng qua gc')c tan xa 90^*.


17

M^'^lAvA.l

Nang lucmg cua hire xa gamma cang tang, cac hire xa gamma lan
xa Compton cd xu hudng hay ve phia truirc. Hinh 1.2 cho hie'l dang
phan

hd

gc)c cua

hire

xa

gamma tan xa [19,44] phu
thugc vao nang lucmg tucrng
dc'ii k ciia hire xa gamma. Tich
phan dae l^^eo toan ho gdc
kh()i Q, thu dugc xac sua't tan

^^

v^

xa Compton
theo moi
„^ i 1,2:
i i Phan
n/ - bo
; --w
- cua
' gamma
. „^,^ ,
^
* hudng Hinh
theo gov
tren mgt dien tir:
^^^ ^.^

ae=27c r^[

k^+k^+8k+2
k —2k—2
^—1n(1+2k)+

(1.6)

k2(1+2k)2
trong dd Qe la xac suat tan xa Compton theo moi hudng tren mdt dien
tu.
Vdi cac hire xa gamma mem ( k « l ) , hd qua vo cung be hac 2 se
thu dugc cong thirc sau:
Ge = at / (l+2k) « ai (l-2k),

(1.7)

trong dd at la tie't dien tan xa Thomson.
Khi nang lugng ciia hire xa gamma nhd hem 100 keV gia tri ciia 0^
tinh thco cong thuc (1.6) va (1.7) sai khac nhau khimg qua 3%. Trong
nguyen tir cd Z dien tir, tie't dien tan xa Complon tren mgl nguyen lu QJJ
hang Zac- Tie'l dien tan xa Compton tren mot nguyen tu ciia cac nguyen
Ic')' dc")i vc'ri cac hire xa gamma nang lucmg khac nhau ducrc cho Irong
|23, 25J. He sc'i suy giam khc'ii ciia qua trinh tan xa Compton ducrc tinh
thco c(')ng thirc sau:

\x^^ = ( A Q / A ) a-^ = Ao(Z/A)ac

(1.8)

18

trong dd )i^ la he sd suy giam ciia qua trinh tan xa Complon: Z va A hi
nguyen tu sd va nguyen tir lucmg ciia chat tan xa, ccm A^ la sc')
Avog'dAro.
Trir nguyen tdhydri) ccm dc")'i vcri da sd cac cha't nhe va trung hinh
ty sd Z/A - 0,5. He sd suy giam khc'ii ciia tan xa Compton hau nhu
kbcmg phu thugc vao nguyen tu sd cua mc')i trudng va giam khi nang
lucmg ciia hire xa gamma tang.
1.1.2.b. Tdn xg Rayleigh
Tan xa Rayleigh la qua trinh tan xa dan hdi cua cac hire xa
gamma tren cac dien tir lien ke't ciia nguyen tu. Tan xa Rayleigh xay ra
manh khi ma nang lugng ciia hue xa gamma nhd cd nang lugng lien ket
ciia dien tu trong nguyen tir, khi dd tan xa Compton va ha'p thu quang
dien kh6ng xay ra. Trong tan xa Rayleigh, nang luc;mg ciia hue xa
gamma khc)ng thay ddi va hang nang lucrng ciia hire xa gamma scr cap.
Xac sua't xay ra tan xa Rayleigh tren mot nguyen tir trong mgl dan vi
gdc khdi theo hudng 0 dugc xac dinh [34, 35, 44] theo cong thirc:

—^=2,387ur^-^(D(e)
dQ

(1.9a).

£^

trong dd 8 la nang lugng ciia hire xa gamma tdi, tinh ra keV; O(0)la
ham phu thugc gdc tan xa.

$(0) ducrc xac dinh theo ccmg thirc:
^Q)=

1+COS?0
^

(1.9h)

sin^0/2
Tan xa Rayleigh xay ra manh thco huc'mg uu lien ve phia iruc'rc,
ung vc'ri gdc tan xa nhc) va tren cac nguyen id nang. Tan xa Ravicigh
chicm uu the khi nang lucmg ciia hire xa gamma nhc) \'a giam nhanh khi


19

nang lucmg hire xa gamma tang thco ham l / £ \ Tich phan hieu thiic
(1.9) thco loan ho gc)c khcii Q thu ducrc lie'l dien lan xa Rayleigh c,- iren
mgl nguyen tu thco mgi phucmg. He sd suy giam khc")i \Ay do lan xa
Rayleigh dugc tinh thco ccmg thirc:

1.2. He so suy giam va nguyen tii so hieu dung
7,2.7. He so suy giam khdi ciia moi tru&ng vat chat
Tucmg tac cua hire xa gamma vdi vat chat dugc mo ta hang tie't
dien tuong tac tdng cong a ciia hue xa gamma tren mot nguyen tu. Dc*)'i
vdi cac hue xa gamma mem, tie't dien tuong tac tdng cc)ng cua nd tren
mot nguyen tir a [15, 34] hang tdng cac tie't dien tuong tac thanh phan:
Tie't dien hap thu quang dien, tie't dien tan xa Compton va tie'l dien tan
xa Rayleigh tren mot nguyen tir. Tie't dien tuong tac tdng cong a ciia
gamma mem tren mgt nguyen tu dugc xac dinh theo cong thirc sau:

a = Ta + ac + aj.

(1.11)

He so suy giam khc)'i loan phan p cua mc')i irudng vat cha't dc')i vc'ri
hii'c xaV- uamma
mem cd dan^i:
&•
\x = (Ao/A)a = (Ao/A)Ta + (Ao/A)ac + iAo/A)G, = |^^+^^ +^r

(1-12)

irong dd cac he sd suy giam khc')i thanh phan |j.q. )a(_. va \i^ ducrc xac dinh
thco cc)ng thirc (1.2). (1.8) va (1.10).
D(')i vc'ri hire xa gamma mem. hicn lUcmg hap thu quang dien ddng
vai ird chu yeu trong tucmg tac cua nd vc'ri val chat. Tie'l dien ciia hicn
lUcmg ha'p thu quang dien Ic'rn hern nhidu |27| so vdi idng lici dien cua ca


20

hai qua Irinh tan xa. He sc')' suy giam khc')'i phu thu()c vao nang lucmg \'a
nguyen tu sc*) ciia moi trucmg. Do be sc') suy giam khc)'i cua hicn lUcmg
hap thu quang dien cd tinh cha'l nhay hac moi khi nang lucmg di qua
hicn hap thu cua cac Idp dien tir, do dd be sc'i suy giam khc')'i loan phan
cung cd tinh chat nhay hac. Tinh chat nhay hac cua he sc') suy giam khoi,
tai cac gia tri hien ha'p thu cua nguyen tu, la mot trong nhung dac didm
quan trcmg nha't khi xcl tucmg tac cua hue xa gamma mtm vc'ri val cha'l.
Vdi moi trudng tuong tac xac dinh (Z = const), trcmg [ 15J da dua
ra hie'u thirc mc) ta su phu thuoc cua he sd suy giam khdi cua moi truimg

vao nang lucmig hire xa gamma:
'mac

._.

'^nficK

.

.

^{s)=exp[ S Aj(logs)' ']+exp[ Z B^dogs)^"^
i=1

(1.13)

k=1

trong dd cac he sd A{ va B^ dugc xac dinh tir thuc nghiem; imax < 5 va
kmax < 3 tuy thugc vao tirng nguyen td.
Sd hang thu* nha't trong hie'u thirc (1.13) dac trung cho hicn
tugng ha'p thu quang dien con sd hang thir hai dac trung cho tan xa
Compton. Cac he sd Ai va Bj^ dugc xac dinh hang each lam khcVp hie'u
thirc (1.13) vdi cac gia tri cua [x dugc xac dinh tu' thuc nghiem irng vc'ri
cac nang lucnrig gamma khac nhau.
Trong trudng hgp mc')i trudng phuc tap chua nhieu nguyen tc*)', he
sd suy giam khdi ciia nd dugc tinh theo cc')ng thirc:
M=2:q,.)i.

(1.14a)

trong dd qj la ham lucmg tuong dc'ii ciia nguyen id ihii' i irong moi
irucmg: con ji; la he sd suy giam kht')i cua nguyen id ihii' i ddi \di hue xa
gamma ducrc nghien ciru.


21
Xcl moi irucmg val chal chi cd hai thanh phan, chang han chal
nang trong ndn nhc. Ggi ham lucmg tmmg dc')'i cua chal nang la q, ham
lucmg lucrng dc')i ciia chal nhc (chal nen) la l-q. he sd suy giam khc')i
lucrng ung ciia cha'l nen va cha'l nang la |a| va jio. Thco (1.14a) ccmg
thirc tinh he sd suy giam khiii ciia moi IrucVng val chal dd la:
|i = (1 -q).pi +q.|J2 = lai+q.dn

(1.14h)

trong dd dp=)i2-lAiDdi vdi hire xa gamma mdm, he sd suy giam ciia nd phu thuc)c
manh vao nguyen tir sd, do dd d\i Idn. Theo (1.14h) he sd suy giam cua
moi trudng tang nhanh khi ham lugng cua nguyen td nang tang. Can cir
vao su thay ddi cua he sd suy giam khdi cd the xac dinh dugc ham
lucmg q cua nguyen td nang.
7.2.2. Nguyen tit so hieu dung cua moi tru&ng vat chat phd'c tap
Do tinh nhay hac trong su phu thugc vao nang lugng cua he sc")
suy giam cua gamma trong cac cha't, cac nhay hac nay xay ra Irong
viing nang lugng ciia gamma nhd hem 150 keV; do moi trudng val cha'l
chiu tuong tac, trong trudng hc^p chung, la nhung hgp cha't nhieu thanh
phan, ngudi ta da dua vao khai niem nguyen tii* si) hicu dung Z^jj.
Dc)'i vdi cac hire xa gamma cd nang lugng thap va trung hinh
(s<0.511 kcV) trong [24, 27, 32, 34, 40. 44] dua ra cc)ng thirc linh
nguyen tu si) hicu dung Z^iy cua moi truimg phirc lap ddi vc'ri hii'c xa

gamma cd nang lucmg km hon the nang ion hda cua cac nguyen ic)
Ihanh phan:
3;v^ _ 73
Zeff = ^'2.qim|Zj/m

(i.i5a


22
trong dd q; la ham lucmg tucrng di)'i ciia nguyen id thir i; Zj la nguyen tii'
sd cua nguyen td thu i; m^ = Zj /Aj; m = Emj,
Trcmg [42] cung dua ra cc)ng thuc tinh nguyen tu sd cua mc')i
trucmg phirc tap cua cha't dc)'i vcri cac hire xa gamma nang lucmg nho hern
3MeV.

Zeff=^'^IqimjZP/m

(l.l5h)

Cac Zeff tinh theo hai cong thirc tren chi khac nhau khc')ng qua
2%. Vi vay nguyen tir sd hieu dung ciia cac mc)i trucmg vat cha't phirc
tap Zeff cd the^ tinh theo mot trong hai c6ng thirc tren.
Ddi vdi mgt nguyen td, khi xet tucmg tac vdi gamma mem,
nguyen tir sd Z ciia nd cung dugc xac dinh tiiy theo nang lugng ciia
gamma. Khi nang lugng ciia hire xa gamma Idn hem hien ha'p thu Idp K
cua nguyen td thir i, gia tri Z^ chinh hang nguyen tir sd cua nguyen id
thu" i. Ne'u nang lugng s ciia hire xa gamma nam giua hai hien ha'p thu
ciia Idp K va L cua nguyen td thir i nao dd thi Z^ trong cc)ng thirc (1.15a)
dugc thay hang Z^j^ theo cong thiic:
Zij, = 0,3112Zii-i^ « Zi/2

(1.15c)

Con khi nang lucmg ciia hire xa gamma nam trong khoang gia tri
hien ha'p thu cua hai Idp L va M cua nguyen tcr thu i thi Zj ducrc thay
hang Zij:
Zil =0,288Zi«Zi/3,48.

(1.15d).

Vc'ri nang lucTng ciia gamma cho Iruc'rc. he sc'i suy giam khc")i ciia
moi trucmg val chat phirc tap phu thugc dcm tri vao nguyen lu sd hicu
dung ciia nd. Can cu vao cac cc)ng thirc (1.15) cd the tinh ducrc nguyen
lu sc")'hicu dung ciia mc)i trucmg val chat phirc lap dc')'i vc'ri hire xa gamma


23
cho Iruc'rc ne'u hie't dugc ham lugng ciia cac nguyen Ic')' thanh plian cd
trong dd. Tuy nhien dc)i vdi men trucmg vat chal ha'l ky, vice xac dinh
ham lucmg cua tat ca cac nguyen td thanh phan cd trong nd khc')ng phai
luc nao cung lam dugc. Trong thuc te', nguyen tu sc')' hicu dung cua mc')i
trucmg vat cha't phirc tap dugc xac dinh hang thuc nghiem dua vao dd
thi mo ta su phu thugc vao nguyen tu si) hieu dung ciia mc')i trucrng val
cha't ciia he sd suy giam khdi ddi vdi hire xa gamma nang lucmg xac
dinh. Mot trong nhirng phucmg phap thuc nghiem phd hie'n de xac dinh
he sd suy giam khdi la dua vao dd thi md ta su suy giam ciia cucmg do
chum hire xa gamma trong vat chat.
1.2,3. Qui ludt suy giam todn phan cua chiim bite xa gamma di
trong vat chat
Khi chie'u chum hire xa gamma don nang, manh va song song tc'ri

vuong gdc vcri be mat cua cha't ha'p thu thi cudng do ciia chiim hue xa
gamma di trong moi trudng [44] suy giam theo ham e mi: :
I = Ioe-^^

(1.16)

trong dd I^^, I la cudng do cua chiim hire xa gamma tai he mat (d=()) va
tai die^m each he mat mot he day d; d la he day khdi cua cha't ha'p thu; \i
la he sd suy giam khdi cua moi trucmg ddi vdi hue xa gamma dd.
Dai lucmg e'^ chinh la xac sua't de cho hue xa gamma di qua Ic'rp
hap thu d ma khcmg hi tuong tac vdi men trudng. Khi dan ic'ri cong
thirc (1.16) ngucri ta da gia dinh rang cac gamma ndu da tham gia mc")l
tucmg tac nao dd thi di ra khdi chiim hue xa han dau. Dieu nay chi ihuc
hicn ducrc khi he day ta'm ha'p thu nhd ban quang chay tu do trung hinh
l/H cua hue xa gamma trong mc)i trudng va hinh hoc cua phep do hao
dam sao cho chum gamma la manh va song song. Trong iruimu hcrp he


24
day ciia ta'm ha'p thu Ic'rn hern nhieu so vcri quang cha\- tu do ciia hue xa
gamma trong mc)i trucmg va dieu kien ve chum lia manh. song song
khc')ng ducrc thuc hien. ccmg thirc (1.16) khc')ng ccm dung. Khi do ccmg
ihirc (1.16) can phai nhan them he sd B (£. d) dac trung cho d(')ng gdp
cua cac hire xa gamma tan xa vao chiim gamma han dau. He sc'> B (£. d)
la mot ham phu thugc vao: Nang lucmg ciia hire xa gamma, han chal va
he day ciia ta'm ha'p thu, hinh hgc ciia phep do.
Trong thuc nghiem de^ tao ra chiim tia hire xa manh va song song,
ta dung cac dng chuan true dat giii'a ngudn va detecto. Ngoai ra, c')'ng
chuan true con cd tac dung lam giam anh hucmg cua cac hire xa gamma
tan xa nhieu lan.

1.3. Ngliien cufu xac dinh bang thuc nghiem he so suy
giam ciia cac chat doi voi buc xa gamma m e m
Trong cac hai toan phan tich ham lucmg ciia cac nguyen tc') theo
phuong phap huynh quang dac trung X va phan tich cac nguyen tc")
phdng xa dua vao cac hire xa dac trung nang lucmg tha'p,viec xac dinh
he sd suy giam [15,42] cua cac chat ddi vdi hire xa gamma nang lucmg
tha'p la can thie't.
Trong thuc nghiem de xac dinh he sd suy giam cira cac chat ngucri
ta dua vao quy luat ham e mu.mc) ta su suy giam cua cuimg do chiim
hire xa gamma di trong vat cha't. Khi qui lual ham sd mu ducrc thuc hien.
dd thi mi) ta sir phu thugc ciia logaril cua sc*) dem ma deicclcr ghi nhan
ducrc \'ao he day ciia cha'l hap thu |6,44| la ducmg thang. Do Aoc cua dd
thi cho hiei he si) suy giam cua cha'l ha'p ihu dc'ii vc'ri hire xa gamma. De
lao ra chiim hire xa manh va song song, irong thuc nghicmi ngudi ui hd


25
Iri nguim nhc')ng xa va deicctcr du xa nhau va cliing cac i')ng chuan Iruc
dai giira chung.

_^

6

De khao sal thuc nghiem
cac dieu kien gia dinh ciia ham

7

c mu, da tie'n hanh xay dung

dd ihi mo ta su suy giam cua
chiim hire xa gamma mem di
qua vat cha't trong cac dieu

r'^-'-'-'/A

f/AW^

kien hinh hoc khac nhau. So dd
thuc nghiem dugc mo ta trong
Hinh 1.3.

Hinh 1.3: So' do thuc nghiem xac- dinh he so
suy giam cua cdc chal doi vm gamma mem.
Detecto, cac dng chuan
I- Ngudn phdng xg: 2-Bng chuan true: S- Moi
true va ngudn d u g c dat tren he trudng hap thu: 4- Tinh the nhap nhdy
Nal(Tl): 5-dng nhan quang dien: 6- He khuech
dugc gia cong co khi chinh xac
dai: 7- Mdy phan rich bien do 1024 kenh:

de dam hao su dons true. Cac 8-Gid da
dng chuan true cd dudng kinh thay ddi tu 1 de'n 10 mm va chieu day
thay ddi tir 5 de'n 10cm. Da sir dung cac ngudn ddng vi phi)ng xa : Fc-'''^,
Snii^rn, Am-41, Co-'^'7 cd boat do phdng xa tir 2 mCi trd len. Cac chat hap
thu dugc sir dung la nhom, dong hoac chi tuy theo nang lugng ciia hire
xa gamma. Ke't qua thuc nghiem cho tha'y : dc'ii vc'ri cac hire xa gamma
24,8 keV, 60 keV va 122 keV khi su dung cac cmg chua'n true cd duimg
kinh tir 5mm irer xuc'ing dal trudc nguim va khoang each tir ngudn ddn
detcctcr km hon 60 cm khi he day ha'p thu nhc) hern 3 lan quang chay lu

do, qui lual ham sc') mu van ducrc nghiem diing.Gia tri cac he sc) su\
giam nhan ducrc khc')ng phu lhui)c \'ao kich ihuc'rc cua c')ng chuan true \'a
khoang each tir nguiin de'n detccur.