Buc xa gamma nang lugng cao tuang tac vai vat chat se gay ra cac phan ling quang hat nhan. Phan ling quang hat nhan la phan ling hat nhan
duai tac dung ciia cac biic xa gammạ Phan ling hat nhan /+ Â B + bAvong
do biic xa gamma bay vao viing tuang tac ciia hat nhan bia A tao thanh hat nhan B va hat b bay ra, dugc goi la phan ling quang hat nhan. Biic xa gamma CO the lay tu nguon phong xa, Biic xa gamma phat ra tu cac dong vi phong xa CO pho gian doan va hoan toan xac dinh, nhung co nhuge diem thong lugng ciia chum biic xa nho, nang lugng nho, khong vugt qua lOMeV. De c6 chum biic xa gamma thong lugng lan, trong thuc te nguai ta thuang sii dung chiim biic xa ham sinh ra khi cac electron phat ra tu may gia toe tuang tac vai cac hat nhan nang.
Dac diem ciia chiim biic xa ham la pho biic xa ham la pho lien tuc, thong lugng Ion. Nang lugng cue dai cua biic xa ham bang nang lugng ciia chum hat tich dien dugc gia toe, vi vay co the tao ra chiim biic xa ham co
thong lugng 16n va nang lugng co the len t6i hang tram GeV.
De giai thich phan ling quang hat nhan ta dua vao mau hap thu photon. Mau phan ling quang hat nhan de cap den ca che phan ling hat nhan khac nhau bao gom qua trinh kich thich quang hat nhan ban dau, va ca phan ra tiep theo ciia hat nhan kich thich bang each phat ra cac hat va tia gammạ
Tijy theo nang lugng gamma toi phan ling quang hat nhan dan tai viec phot xa ra coc hat khoc nhau nhu phat xa natron, proton hoac cac loai hat khac, do la cac phan ling (Y,n), (Y,2n), (y,xn), (y,p), (y,pn) (y,f). Day la cac phan ling nguang vi vay can phai co cac chiim photon co nang lugng du lan de gay phan ling.
Cung nhu cac phan ling hat nhan duai tac dung ciia cac hat tich dien va ncrtron, phan ling quang hat nhan phu thugc manh vao nang lugng cua chiim lugng tir gamma taị Doi vai cac phan ling quang hat nhan, duai tac dung cua
chiim lugng tu gamma co nang lugng duai 30MeV, co dac diem sau:
Tuang tac dien tii yeu ban tuang tac hat nhan ca 3 bac.
Cac qua trinh tuang tac giira lugng tir gamma va hat nhan co cuong do nho ban cac loai tuang tac hat nhan khac.
V6i cung nang lugng, budc song ciia lugng tir gamma lan han nhieu bu6c song ciia nucleon. Buac song ciia lugng tii gamma va cua nucleon dugc xac dinh theo cong thiic sau:
K^ = (cm)
X^=^:±lL-ian) (1.22)
trong d6 X^X^^ỆÊ lan lifdt la bu6c song cua lUdng tii gamma,
bifdc song cua nucleon, nang lifdng cua lUdng tu gamma va nang IiJdng cua nucleon tUdng ling.
Tii bieu thiic (1.22) nhan thay: de co buac song nho ca kich thuac hat nhan, cac nucleon co co nang lugng IMeV, con lugng tii gamma phai co nang lugng 30MeV. Cac phan ling quang hat nhan (y,xn) tao ra nguon natron quan trgng. Phan ling quang hat nhan la sir hap thu photon din den hinh thanh trang thai hat nhan hgp phan, cac hat nhan nay c6 the phan ra theo nhieu each, ngoai natron con co the la proton hoac cac loai hat khac, thi du nhu cac phan ling sau day: (y,n), (y,2n), (y,p), (ỵf), ••• Day la cac phan ling nguang vi vay chi co cac chiim photon co nang lugng va cuong do Ion.
E,(MeV)
(ỵy); (yY) (yj): (y^') (ỵn); (Ỵ2n); (ỵp)... (y^n) (ỵy); (yj)
Hinh L8.. Suphu thugc cua tiet dien phan iajg quang hgt nhan vao nang luang photon
Thong thuang cac chum photon nhu vay dugc tao ra tu may gia toẹ Hinh 1.8 bieu dien moi lien he giua tiet dien phan ling quang hat nhan va nang lugng cua photon [3,7,11,20]. Trong viing I nang lugng ciia photon duai nguang ciia phan ling (y,n) do do chi say ra cac tan xa dan hoi va khong dan hoi ciia photon, duong cong tiet dien doi khi eo mot so cue dai la do sir djch chuyen giira cac miic cua hat nhan biạ Trong vung II cac miic nang
<Nr \ 1 r r f
lugng van con tach roi nhau dieu nay the hien trong eau tnic tinh te ciia tiet dien. Viing III tuang ling voi su chong chap cac miic ciia bat nhan hgp phan. Trong viing nay tiet dien phan ling quang hat nhan dat cue dai va co dang hinh gauss dugc ggi la cong huang khong 16 vai nang lugng photon trong khoang tu 5 MeV den 35 MeV. Sir hap thu photon dan den hinh thanh trang thai hat nhan hgp phan, cac hat nhan nay co the phan ra theo nhieu each vi du nhu phat xa ncrtron, proton, biic xa gammạ..Cac natron dugc sinh ra chu yeu
trong vung nang lugng naỵ Viing IV vai photon nang lugng 35 ^ 140 MeV
la viing xay ra hieu ling gia dotron (quasi-deuteron). Viing V vai nang lugng photon >140 MeV la viing xay ra phat xa pion, va cac hat co ban khac [14,20].
Hien tugng cong huang khong 16 dugc giai thich theo cac quan diem sau: Goldhaber va Teller gia thiet natron va proton ciia hat nhan nhu la hai chat long rieng biet, hat nhan nhan nang lugng do hap thu cac photon tao ra sir dao dong ciia hai loai chat long naỵ Hien tugng cong huong kh6ng 16 tuong ling vai tan so cue dai cua dao dong. Sau do Wikinson xem cong huang khong 16 nhu la sieu vi tri (super position) do sir dong gop cua tat ca cac nucleon rieng lẹ M6i mot nucleon nhan mot phan nang lugng tu sir hap
t y •> f
thu photon. Cong huong khong 16 la tong tat ca cac cong huang nho do [3]. Tiet dien phan ung cua cong huong khong 16 thuong dugfc bieu dien gan dung bang duong cong Lorent [19]:
trong do EQ la nang lugng cong huong, T la do rong cong huong, QQ gia tri
tiet dien cue daị
Sir tach cua cong huong khong 16 the hien cac hat nhan bi bien dang tuong ung voi su kich thich doc va ngang qua true d6i xung cua hat nhan. Cac tinh toan ly thuylt v6 su dich chuyen luong cue ciia cac hat nhan chi ra rang tiat dien tich phan ciia phan ung quang hat nhan thoa man he thuc Levinger - Bethe:
00
a , = \c7{E)dE = 0,06NZ/A
/.: (1.24) trong do N la s6 hat nhan trong bia, Z va A la ngu>en tu s6 va s6 kh6i ciia
Bang sir so sanh tiet dien hap thu quang hat nhan toan phan quan sat dugc va cac tien doan ly thuyet da cho thay sir hap thu luang cue dong vai tro chinh trong vung cong huang kh6ng 16.
Hinh 1.9 bieu dien tiet dien bat natron ciia phan ling ^^^Pb(y,xn) [9].- Cac loai phan ling quang hat nhan sinh natron do cac biic xa gamma so cap gay ra:
Y + A - > ( A - l ) + n; Y + A - > (A-2) + 2n, ...
y + A ^ ( A - l ) * + n ; ( A - l ) * ^ ( A - l ) + y, ...
- Tuong tac ciia cac tia gamma thii cap (yeu ban) voi cac hat nhan sinh natron: / + A ^ ( A - l ) + n; / + A - > (A-2) + 2n, ... Y + ( A - l ) ^ ( A - 2 ) + n, ... soo 700 600 - 500 7" 5 400 i 300 200 100 0 Veyiuere(19"0A0 93 •—^—« Young(1972i '"LANL I •» • - n '?b(Ỵ xn Hiyi^'^t-nHiMTi^ 10 1: 2>'S- 40 E, (Me\- 208
7.2.2. Bac trirng ciia quang natron tie may gia tdc electron
Nang lugng ciia cac natron sinh ra tu cac phan ling quang hat nhan phu thugc chii yeu vao nang lugng ciia biic xa ham va so khoi ciia hat nhan biạ F.Jallu et al dua ra c6ng thiic tinh gan diing nang lugng cua notron Ê nhu sau [7,15]:
" A ^'-"^ 1862(^-1)' ^ 93M^ "' (1.25)
trong do, k la nang lugng ciia photon (MeV), A la so khoi ciia hat nhan bia, Es(y,n) la nang lugng nguong cua phan ling (MeV), 0 la g6e phat xa eiia cac natron so voi true ciia chiim electron toị
Tu phuong trinh tren ta thay nang lugng cua notron thay doi cham theo goc phat xa, dac biet la voi cac nguyen to nang. Cac vat lieu co so Z Ion do nang lugng lien ket notron thap nen thuong cho suat lugng phat natron Ion.
f r
Trir truang hgp ciia D va Be, mac dii so Z nho nhung nang lugng lien ket notron ciia chiing thap mot each bat thuong nen cung thuong dugc dung lam bia de tao nguon notron. De tao notron nguoi ta thuong ban pha true tiep cac bia bai cac electron nhanh hoac dung chiim biic xa ham ban pha vao mot bia thii cap a sat bia tao biic xa ham. Suat lugng phat notron (n/s) c6 the nhan dugc tu bieu thiie:
M
(1.26)
trong dc), M, p, va d la khoi lugng nguyen tiị mat do va be day cua bia, NQ la s6 Avogadro, (j)c la thong lugng chum dien tu toi (dien tir/s). ar la tiet dien toan phSn bao gom tong tiet dien ciia tat ca cac qua trinh dan to*! phai xa notron.
"* t r
Pho quang natron c6 dang tuong tu nhu pho phan bach va co phan bo g6c gan nhu dang huang, dieu nay co nghTa la cac phan ling hat nhan th6ng qua giai doan tao thanh cac hat nhan hgp phan hay co che bay hoi notron. Mgt each gan diing ph6 quang notron co the m6 ta boi c6ng thiic d6i voi mau bay hoi natron [3]:
f(E)=kxExe tU (1.27)
trong d6, E la nang lugng notron, k va T la cac he s6.
Hinh 1.10 bieu dien sir phu thugc ciia suat lugng natron vao be day bia Ta dugc ban bai chiim electron 100 MeV [3]:
n/electron ) CO O o -^ 200 c lugn g % 100 CO n 1 • 1 • 1 ' 1 ' r = 5 c m ^ /^l--^^^^ y\ý^'^^^^^'^ "1 cm X^^^ r = 0,5 cm Z^^^'^^^^^ r = 0,25 cm / ^