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

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

^n •>"' J'J

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

BUI VAN LOAT

NGHIEN CUrU MOT SO DAG TRUNG CUA TlTONG TAG

BUfG XA GAMMA MEM VQfl VAT GHAT VA UfNG DUNG TRONG

• • •

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%

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 24

Ket luan chuong 1 35

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

CO ban cua tuong tac gamma m6m vdi vat chal 36 2.1 Hi6u ling tu hap thu cua mSu 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 47

Kdt luan chuong 2 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 62

3.1 Ca so" ciia phuang phap 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 79

K6t luan chuong 3 ^^

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 9'*

4.3 Bie^u thiic tinh ham lugng theo phuong phap ty sd phd 93

4.4 \Su Aiim va pham vi tog dung cua phuang phap

ty sdphd 96

4.5 Nghi6n cthi anh hudng cua cac tap chat nang 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

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)

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 | 1 4 | 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

Trong phan tich ham lugng cac nguyen td thco phd huynh quang

X dac trung kich thich bdi gamma, kél qua phan tich chiu anh hmrng ciia nhieu yéu td nhu su thay đi thanh phan boa hoc ciia chat nen, su

thay đi cua mat A6 mau, su khong dn dinh cua hinh bgc va thiet hi dọ

De khac phuc cac yéu 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 chát ciia phuang phap ty sd phd la su dung chinh cac hire xa tan xa tir mSu lam chuan trong dê han ché anh hudng cua cac yéu td den két qua phan tich

Cac phuong phap chuan trong thong thudng dugc su dung [8,11,31] dê hieu chinh anh hudng cua cac yéu td dén két qua phan tich hang each dua them vao mSu mot lugng xac dinh đng 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 tđugc dua them vao hiét dugc ham lugng chát 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 dém trong vimg nang luctng dac trung cho nguyen tdcan phan tich va sđém trong viing nang lucmg ung vcri cac hue xa tan xa tir maụ Su thay đi ciia cac ydu td ké 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:

- 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

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 d a \ na\' hi irucrng Dai hc)c Khoa hoc Tu nhien Ihugc Dai hoc Qudc i:ia Ha Noi Ban

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 (k<l), tucmg lac vdi moi

irudng thcmg qua hicn tucmg hap thu quang dien va lan xa

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

C

OH

<J

Hinh 1.1 mo ta sir phu

thugc vao nang Ixxgng cua he sd

ha'p thu quang dien tren m6t

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

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

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

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

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:

8

J

1 + k(1-cose)

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 d a e / d Q , dugc xac dinh [34] theo cc)ng thirc sau:

p2

= ^ ( s s / s j ) 2 [ ( s s / s j ) + ( s i / s s ) - 9 n 2 e ] (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<l) phan hd gdc

ciia 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 hudng „^ i i i n/ - ; w - ' „^,^ , ^ * Hinh 1,2: Phan bo theo gov cua gamma

tren mgt dien tir: ^^^ ^.^

k —2k—2 ae=27c r^[ ^—1n(1+2k)+ k ^ + k ^ + 8 k + 2

k2(1+2k)2

(1.6)

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 = a t / (l+2k) « a i (l-2k), (1.7)

trong dd a t 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)

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) (1.9a)

d Q £^

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:

1+COS?0

^Q)= ^ (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 xa uamma mem cd dan^i: V- & •

\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 | 2 7 | so vdi idng lici dien cua ca

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)^"^ (1.13)

i=1 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 \ d i 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-lAi-Ddi 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 ^ _ 7 3

Zeff = ^'2.qim|Zj/m (i.i5a

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

3 M e V

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 , 2 8 8 Z i « Z i / 3 , 4 8 (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

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 g a m m a 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

De khao sal thuc nghiem

cac dieu kien gia dinh ciia ham

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

kien hinh hoc khac nhau So dd

thuc nghiem dugc mo ta trong

Detecto, cac dng chuan

Hinh 1.3: So' do thuc nghiem xac- dinh he so

suy giam cua cdc chal doi vm gamma mem I- Ngudn phdng xg: 2-Bng chuan true: S- Moi

true va n g u d n 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 dai: 7- Mdy phan rich bien do 1024 kenh:

de dam hao su dons true Cac 8-Gid da

dugc gia cong co khi chinh xac

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

De minh hoa, iren

Hinh 1.4 la dd thi mo ta sir suy 8.5 -h

giam ciia chiim hire xa gamma ^

nang lugmg 60 keV qua ddng

Dung phuang phap hinh phucrng

9.0

'c 7.5

c 7.0

iie.s h tdi thieu de xac dinh cac he sc') va g^ e o -

sau dd tinh gia tri yp- irtn mdi

^-5.0

1.6

hac tu do Ket qua thu duoc gia

tri 7^2 tinh tren mdi hac tu do la Be day hap thu (g/cm')

0,7 Tuy nhien vdi hire xa Hinh 1.4: Dumg cong suy giam ciia gamma 5,9 keV, thuc nghiem chi '^^^^'^ ^<? ^'^'- -^V ^^^^na 60 keV trong

ra rang ke't qua thuc nghiem phu ^'

thugc nhieu vao dieu kien thuc nghiem nhu dudng kinh D ciia dng chuan true, khoang each do tir ngudn de'n detecto

13.1 Anh huffng ciia kich thu&c dng chuan true -Phuang phap ngoai suy ve chiim tia manh xac dinh he so suy gicim ciia khong khi doi v&i buc xq gamma 5,9 keV

Ddi vdi cac birc xa gamma mem, nang lucmg tha'p dudi chuc kcV khong the hd qua tuong tac cua nd [7,11,17 ] vdi khc)ng khi Chinh krp khc)ng khi giiJa nguc'in va detecto hap thu dang ke cac hire xa gamma di tir ngudn de'n detecto

So dd thuc nghiem xac dinh he sc')' suy giam cua khc)ng khi dc')i vc'ri hire xa gamma nang lucrng 5,9 kcV ducrc mo la |17| iren Hinh 1.5

Nguc'in ddng vi phdng xa Fc55 cd boat do phdng xa 3 mCi Cac hire xa gamma ducrc ghi hang dctectcr nhap nha\' VA-S-968 vdi linh the' Nal(Tl) mcing 2mm Do phan giai nang lucmg irng vdi hue xa iianimii

Hinh 1.5 So' do thuc nghiem xac dinh he so suy giam ciia khong

khi doi vai gamma 5,9 keV

1 - Ngudn dong vi phdng xa Fe-'"'''; 2- Ong chuan true; 3- did dd; 4- Tinh the Nal (Tl); 5- Ong nhan quang dien; 6- He khuech dai;

7 - Mdy phan tich bien do 1024 kenh

nang luong 5.9 keV la 35% Chum birc xa gamma dugc chua'n true hc'ri cac ong chuan true bang sat co ducmg kinh thay dcii tu 1 mm d^'n 5 mm

va hi day 1.5 cm Toan bo he ngudn - detecto va cac dng chua'n true dugc dat irtn thudc quang hgc Vdi kich thudc dng chua'n true cd dinh

khao sat su phu thugc cua cudng do cua chum hire xa gamma di d6n

detecto vao khoang each tu ngudn de'n detecto Do thi cua ducVng cong

suy giam Lnl ung vdi cac ducVng kinh ciia dng chua'n true khac nhau ddu

cd dang nhu nhau.Khi khoang each tir ngudn den detecto nhd gia iri lnl giam nhanh khi d tang, khi d Idn hon 30 em gia tri lnl giam cham khi d lang dd thi ehi' tuyen tinh trong khoang 10 den 25 cm He sd su\ giam luyen tinh dugc xac dinh tir dd thi dua vao cac die'm thuc nghiem ung vdi khoang each d thay ddi lir 10 de'n 25cm

Do dc'ic cua dd thi lnl mc') ta su suy giam cuimg do cua chiim hiic

xa gamma 5,9 keV trcmg khc)ng khi ducrc xac dinh hang thuc nghiem irng vc'ri duimg kinh cua chuan true khac nhau ducrc cho cr hang sd 1.1

Bam: so 1,1 : Do doc cua ducyn^ cotv^^ ha'p thu <^amma 5,9 kcV

trong khong khi phu thugc vao dudng kinh I) cua ong chuan rriXc

Y)6 thi tren

Hinh 1.7 cho hie't qui luat

su phu thugc cua dd doc "^

cua do thi Inl(d) vao ^

ducmg kinh D cua dng ^

chuan true Can cu vao '•§

cac die^m thuc nghiem '3

theo phuong phap hinh "^

phucmg tdi thie'u ngoai

suy ducrc gia Iri cua he sd

suy giam tuydn tinh irng

0.05

0.04

0.03

- 0,02

Ducmg kinh dn^ chuan iruv (i-nno vc'ri ducmg kinh c*)ng „ , , , ^

Hinh IJ: Su phu Ihwn vao didymi kinh ofli!

chuan true lien tdi 0 tire ,./,,,^„ „.^^ ,.,^^, ^^ j-^-^ j - , ,/,^ /„/ ^,^3 ,^^ ^,^,, ^,^^,,

la khi chum hue xa giam cuang do cliiim bih xa ^ammu 5.9 l,('\' -ramma ducrc coi la chum "'"'^" f^'^''"^ 1^'^'-

30

tia manh ly tucrng He sd suy giam tuye'n tinh nhan ducrc thco phucrng

phap ngoai suy tir dd thi la (0,46 ± 0,02) cm-^ Khc)'i lucmg rieng cua

khc)ng khi khC) la 0,001293g/cm\ Tliuc nghiem ducrc tie'n hanh trcmg

dieu kien do am ciia khc)ng khi 35 % Tir dd tinh dugc he sd su>' giam

khe')'! ciia khe)ng khi dc'ii vc'ri hire xa gamma 5,9 kcV la:

|i = (23,l ±1,1 ) c m - / g Ddi vdi cac hire xa gamma mem, nang lugng nhd hon 10 kcV

trong [33] da dua ra cc)ng thuc han thuc nghiem xac dinh he sd suy giam

khdi ciia cac chat theo cong thiic:

p = C;^^-D^4 (1.17)

trong dd C,D la cac hang sd chi phu thugc vao nguyen tir sd cua cac cha't

va da dugc cho trong cac hang sd; con X la hudc sdng cua birc xa

gamma, dugc xac dinh theo hieu thirc k =12,34/ s, trcmg dd s la nang

lugng cua gamma dugc tinh theo keV

Tir cac gia tri hang sd C va D ciia khong khi va nude trong hang

sd [33] thay vao cong thirc (1.17) tinh dugc he sd suy giam khc'ii cua

khong khi la 23,7 cm-/ g Nhu vay trong pham vi sai sd cd 5% ket qua

thu dugc trong luan van nay phu hcrp vdi ke't qua tinh theo cc')ng thirc

han thuc nghiem tren

De")i vdi hue xa gamma mem, do ddc dd thi lnl, mi) ta su suy giam

cuimg do chiim hue xa gamma trong khc)ng khi phu thuoc vao kich

thuirc ciia e*)'ng chuan truc.Trong thuc nghiem nhan tha'y ngay ca khi

duimg kinh iing chuan true cd I - 2mm, irong khi dd khoang each lu'

ngudn den detecto tir 5 cm trcr len, km hem il nhal 25 lan ducrng kinh

c'ing chuan true, chiim hire xa gamma van chua the ducrc coi la chiim lia

manh Vi vay dd xac dinh he sc"> suy giam cua kh(')ng khi ddi \ iri gamma

nang lucmg nhd, chang han nhu hire xa gamma nang lucmg 5.*^ kcV,can phai tie'n hanh khao sal su phu thuc)c cua do dc'ic cua dd thi lnl vao kich

thuirc c'ing chua'n true va ngoai suy ve truimg hcrp chiim tia manh \y

tucmg Phuemg phap ngoai suy dugc trinh hay trong luan an, cd the ap dung cho nhirng trucmg h<Tp,ddi vdi cac hire xa gamma mdm, kich thuc'rc ciia cmg chuan true va hinh hgc do anh huemg dang ke de'n kei qua do

7,3.2 Hieu chinh anh hu&ng cua l&p khong khi giira nguon vd

detects - Xac dinh he so suy giam cua nhom doi v&i bite xq gamma 5,9 keV

Di xac dinh he sd suy giam ciia nhom ddi vdi hue xa

gamma nang lugng 5,9 keV da sir dung nhung la nhom merng 14 p.m So

dd thuc nghiem dugc hd tri nhu Hinh 1.3 Da tie'n hanh thuc nghiem trong cac trudng hgp khoang each tu ngudn de'n detecto thay ddi tir 5 de'n 22 cm Tai cac khoang each tu ngudn de'n detecto khong ddi, da xay dung dudng cong suy giam cua cudng do chum hire xa gamma nang lucmg 5,9 keV trong nhom

vdi vi tri khac nhau cua cac la

nhom ha'p thu: ngay sat

ngudn, d giua ngudn va

detecto, d sat detecto Ke't

qua thuc nghiem chi ra rang

khi c")'ng chuan true cd kich

thuirc nhd tir 4 mm trd xuiing, ^ 6 io 12 16 18 20 22

Khoang each ti; nguon den detecto (cm)

di) di)c ciia ducmg cong suy „ , ^ , , ; - ,, - ,

^ c -^ fjifijj jfi^ Su phu thuoc vao khoang each lu

g i a m khc)ng phu thue)c v a o vi ,j^udn den deterur cuu do doc cua duon- con:^ tri cua c a c ta'm ha'p thu hap thu iron^ /thorn doi vol hue \y ^ummu

Niiom ra tai mc')i *:ia tri khc)nii • '

32

ddi ciia khoang each tir nguim de'n deteclo da lien hanh xac dinh do d(')c cua dd thi tren irng viri cac kich thuirc c*)ng chuan true khac nhau Kei qua chi ra rang, trong pham vi sai sddo dc*)'c ciia dd ihi Inl(d) kb(')ng phu thuc>c duimg kinh e)'ng chuan true Vi vay, da xay dung duimg cong Inl(d) irng vdi khoang each tii' ngucin de'n detecto do khac nhau khi dng chuad true dat trudc nguim cd duimg kinh 2 mm Ke'l qua thu ducrc lii thuc nghiem da chi ra rang do derc ciia dd thi lnl phu thuc')c vao khoang each do tir ngudn de'n detecto

Bang sd 1.2 la ke't qua xac dinh do ddc ciia dd thi lnl, mi) ta su suy giam cudng do chiim hire xa gamma 5,9 keV trong nhom tuong ung vdi khoang each tir ngudn de'n detecto trong khoang 5cm de'n 22cm Dd thi mo ta su phu thugc vao do cua do ddc dd thi lnl, mo ta su suy giam cudng do chiim hire xa gamma 5,9 keV [10] trong nhom cd dang la

•>

ducmg thang nhu Hinh 1.8

Tir hinh 1.8 nhan tha'y rang khi do 1cm, khoang each tir ngucSn den detecto Idn, ddng gdp ciia gamma tan xa tir khe)ng khi vao chiim gamma di de'n detecto tang, do ddc dd thi lnl nhd Can phai hieu chinh anh hudng cua Idp khong khi giua ngudn va detecto de'n ke't qua do Anh huemg ciia Icrp khong khi cang nhd khi khoang each tir nguim de'n detecto cang ngan He se") suy giam ciia nhi)m dc)i vcri hire xa gamma 5,9 keV dugc ngoai suy tir dd thi Hinh 1.8 chinh la giao die'm cua dd thi vdi true lung, tucmg irng vcri do di)c cua do ihi Inl(d) irng viri phep

do cd khoang each nguem - detecto hang khi)ng

Tir cac sc") lieu thuc nghiem tren dd thi Hinh 1.8 thco phucrng phap hinh phucmg tiii thieu xac dinh ducrc he sd suy giam khdi cua nhc')m ddi vc'ri gamma 5,9 kcV la ( 113,2 ± 2,2 ) cm^ / g

Bam: so 1.2: Suphu thuoc cua do doc duimg cong suy giam ciia gamma 5,9 keV trong nhom vao khoang each d^ tic nguon den detecto

Khoang each ngudn

- Detecto (cm)

5,7 10,0 11,5 16,5 18,1 20,0 22,0

Do ddc cua ducmg cong Inl(d)

(cm2/g) 107,5 ± 1,9 98,7 ±1,8 94,3 ±1,8 88,5 ±1,2 79,6 ±1,0 75,4 ±0,9 71,7 ±0,8

Tu cac gia tri cua he sd C va D irng vcri gamma 5,9 keV ddi vc'ri nhom trong hang sd [33 ] thay vao cong thuc (1.17) tinh dugc he sd suy giam khdi cua nhom la 117 cm2 /g Vdi sai sd 5 % ke't qua thuc nghiem trong bang luan an phu hgp vdi ke't qua tinh toan theo cong thuc han thuc nghiem

De' so sanh ke't qua thu dugc cua luan an vdi cac tac gia khac, Hinh 1.9 mo ta su phu thugc he sd suy giam cua nhc)m vao nang lucmg ciia hire xa gamma nang lugng dudi 10 keV, cac sd lieu lay tir 133,341 Trong pham vi sai sd gia tri he sd suy giam khc'ii cua nhc)m ducrc xac dinh thco phucmg phap ngoai su>;,ducrc irinh hay Irong luan an nay cung nam tren dd thi dd

Nang lugng hire xa gamma (keV)

Hinh L9: Suphii thugc vao nang lugng gamma cua he so suy

giam khoi cua nhom

Phucmg phap ngoai suy cho phep de' xac dinh he sd suy giam cua nhom tir dd thi mo ta su phu thugc ciia do ddc ciia dd thi lnl vao khoang each do tir ngudn tdi detecto cho phep ap dung de xac dinh chinh xac he suy giam cua cac cha't ddi vdi hire xa gamma mem Trong thuc nghiem khong the thuc hien cac phep do vdi khoang each tir ngudn de'n detecto hang khc)ng Ngoai ra de dam hao chum tia hep va song song cac phep

do thudng dugc tie'n hanh vdi khoang each tir nguem de'n detecto km Dc'ii vdi cac gamma nang lucTng 1cm, anh hucmg cua Icrp khc)ng khi giira nguiin va detecto khe)ng dang ke' va cd the hd qua, cc)n cac hire xa gamma nang lucrng tha'p, khcmg the hd qua anh hucmg nay va can phai hicu chinh su anh huemg ciia krp khc')ng khi gitra nguem \'a dcicclo Phucrng phap ngoai suy trinh hay trong han luan an nay ihue cha'l la su hicu chinh anh hucmg dd

Ket luan chuemc 1

Trong chuong nay da chi ra rang diii vc'ri hire xa gamma mdm khi tucmg tac vcri mc)i trucmg, hicn tugng hap thu quang dien \'a lan xa la hai qua trinh chu ye'u Dac trung quan trgng nha'l cua tucmg tac hiie xa gamma mem vdi vat cha't la tinh cha'l nhay diii vc'ri su thay ddi thanh phan hda hgc cua vat cha't va tinh cha't phu thuc)e nhay hac vao nang lugng ciia he sd suy giam Trong chuemg nay da trinh hay ke't qua thuc nghiem nghien ciru cac dieu kien gia dinh ciia quy luat ham e mu, mc) ta

su suy giam ciia cudng do hire xa gamma mdm trong vat cha't Ket qua thuc nghiem trong chucmg nay cdn chi ra ddi vdi hue xa gamma mdm, nang lugng 5,9 keV, lop khong khi giiia ngudn va detecto se anh hucmg de'n ke't qua do Cac phucmg phap ngoai suy trinh hay trong chuong nay cho phep hieu chinh anh hudng cua chiim tia khong manh ciing nhu anh huemg ciia Icrp khong khi giua ngudn va detecto de nang cao do chinh xac cua phep do

36

Chuong 2

KET QUA THUC NCiHIEM NGHIEN ClTU MO I SO

f)AC TRUMG CO BAN CUA TUONG TAC GAMMA M £ M VOII VAT CHAT

2.1 Hieu urng til hap thu cua mau

Ddi vdi hue xa gamma mem, hien tugng ha'p thu quang dien dc')ng vai trd chii ye'u trong qua trinh tuong tac ciia nd vdi vat chat Tiei dien cua hien tugng ha'p thu quang dien phu thuerc manh vao nguyen tu sc")

cua moi trucmg theo ham 7P (vdi n hie'n thien tir 3,5 de'n 4,5) Vi vay

tucmg tac cua hue xa gamma mem vdi vat cha't ra't nhay vdi su tha\' ddi ciia thanh phan vat cha't cua moi trudng Day chinh la co sd ciia cac phuemg phap phat hien nhiing thay ddi cua thanh phan vat chal dua vao tuong tac cua hire xa gamma mdm vdi vat cha't Tuy nhien tinh chat nay cung gay nhieu khd khan trong qua trinh phan tich ham lugng cac nguyen td dua theo cac hire xa huynh quang X dac trung Mot phan cac hire xa gamma hoac tia X sinh ra tir mSu hi chinh cac cha'l cd tron^ mau phan tich ha'p thu trudc khi nd hay ra khdi mSu di ve detecto Qua trinh

tu ha'p thu cua mSu phu thuerc vao nang lugng ciia hire xa can phan lich

va nguyen tu sc") hieu dung cua mSu, De')'i vdi mot nguyen tc')' xac dinh can phan lich nang lugng cua hue xa kich thich khcmg thay ddi su lu hap thu ciia mau phu thuc)c vao nguyen tu sd hicu dung va he da\ cua mau Kei qua la ham lucTng nhu nhau ciia chal can phan tich Irong eae

mau cd cha'l win khac nhau, do dd nguyen tu sd hicu dung khac nhau

van cho cuimg do khac nhau ciia hire xa can phan lich Nhu \'ay dien tich dinh hap thu loan phan cua hire xa dae trung cua nguvcn U'^ can