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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%


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


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

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

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



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 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 (k

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) (k

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)

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

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

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.