- Genie2000, Canberra (2000)
3.1. XAC DINH DAC TRLTNG PHAN BO THEO GOC CUA BLfC XA HAM NANG LUONG 65 MEV VA 15 ME
HAM NANG LUONG 65 MEV VA 15 MEV
t t
Dac diem ph6 nang lugng ciia biic xa ham phat ra khi chiim electron
f •>
dugc gia t6c tuong tac voi bia nang la pho lien tuc, co nang lugng cue dai bang dong nang ciia hat toị Ph6 nang lugng ciia ciia biic xa ham o cac goc
f
phat xa khac nhau co dang rat khac nhaụ Cac biic xa ham phat ra co nang lugng lan co xu the uu tien ve phia truac. De xac djnh dac trung phan b6 theo goc ciia biic xa ham De tai da chgn phuong phap phan tich kich hoat. Dang duong cong m6 ta phan b6 ciia th6ng lugng cbiim biie xa phat ra theo g6c dugc xac dinh thong qua xac dinh t6c do cua phan ling '^^Au(y,n) '"^^Au tai cac
f ^
goc chieu khac nhaụ Phan thuc nghiem dugc thuc hien tren may gia toe tuyen tinh (linac) 100 MeV dat tai Trung tam gia toe Pohang, Han Quoc. Cac so lieu thire nghiem tien si Pham Diic Khue, Trung tam Vat ly Hat nhan, Vien Vat ly va Dien tii, Vien Khoa hgc Viet Nam cung cap. So do thi nghiem dugc bo tri nhu hinh 3.1.
chum é
Linac A —^^^—^
la Au
/y;
Chum biic xa ham co nang lugng cue dai 65 MeV phat ra tir bia W mong dugc thuc hi6n tren may gia toe linac 100 MeV. Cac la vang co duang
kinh 0,5 inch, bi day 0,001 inch dugc gdn tai cac goc each nhau 15'* tren mot gia đ mSu co duofng kinh 16 cm. Tam ciia mSu a vi tri 0^' thang hang vai tam
ciia bia W va triing voi true cua chum electron. Khoang each tir bia W t6i cac
mSu Au la 8 cm. Cac mSu Au dugc kich boat trong thai gian 4 gia, a dieu kien
nang lugng 65 MeV, dong 35 mẠ
Viec nhan dien dong vi va xac dinh cuong do biic xa gamma dua tren cac dinh ciia cac san pham phan ling. Hinh 3.2 la pho gamma dac trung ciia
mau Au dugc kich boat a goc 0*'.
10' 10' 10' € 10^^ •0) •a CO o 10 10 10 10 < 3: V'Vv^y^V^ ^ ^ ^ * ^ M ^ ^ | ^^^liU^^im.. 632 ,264 1896
N3ng luong gamma (keV)
2528
Hinh 3.2. Pho gamma dac trUng ciia man Au duac kich hoat tren tritdng bi(c xg photon- natron plidt ra tCrbia W vai chum electron 65 MeV.
Oe xac djnh phan bo theo goc phat xa ciia cac photon ham, trong thuc nghiem xay dung sir phu thugc cua t6c do cua phan ling '^^Au(y,n)''^'^Aụ Ẹ^ = 8,07 MeV theo goc chieu xạ Ket qua t6c do phan ling '^^Au(Y,n)'^'Au dugc hieu chinh dua vao ty so tiet dien dong phan. dinh 333,03 KeV ('^^"^Au) va 147,81 KeV ('"^^""Au). Va hieu chinh can nhi^u phong xa bang each sir dung
phan ling 2'Al(n,a)2''Na v6i nang lugng nguong E,h=3,25MeV, E,= 1368,63 KeV lam monitọ Doi voi phan ling '^^Au(Y,n)'''''Au, co phan umg gay nhilu
''''Au(n,2n)'^'^Au khi kich hoat tren truong buc xa h6n hgp photon-notron tii may gia toẹ Va hieu chinh can nhilu phong xa bang each sir dung phan ung 2'AI(n,a)^Na v6i nang lugng nguong E,h=3,25MeV, EY= 1368,63 KeV lam monitọ Trudrc het phan tich pho gamma ciia dong vi ^"Na, tinh dien tich dinh
phd 1368,6 keV. Ta biet dugc tiet dien phan ung cQa Al voi notron a =
0,687.10"^''cm^ hoan toan tinh dugc thong lugng notron (j)„. Mat khac, tiet dien
tuong tac ciia Au voi notron a = 2,27.10"^'*cm^ vay chiing ta xac dinh dugc
hoat do CLia dong vi '^^Au sinh ra tir phan urng ''^^Au(n,2n)'^^Aụ Tir day tinh toan dugc toe do ciia phan ling ''^''Au(y,n)''^^Aụ
Ket qua tinh toan suat lugng ciia phan irng '''^Au(Y,n)''"^Au theo goc chieu xa dugc dua ra trong bang 3.2.
Bang 3.2: Suát lugng phan irng ''^^Au(y,n)''^^Au theo goc chieu xạ SoTT 1 2 3 4 5 6 7 Mau AuAM AuBM AuCM AuDM AuEM AuFM AuGM Goc chieu xa -15" 0" 15" 30" 45" 60" 75"
Suat lugng phan ung (phan ung/giay) 1,79.10" 3,19.10'" 1,95.10" 1,74.10'- 9,99.10" 8,16.10-'- 7,67.10" Sai so (%) 6,81 5,68 6,62 7,83 8,27 8.01 8.23
Hinh 3.3 mo ta sir phu thugc giiJa suat lugng phan ung vao goc chieu xạThay rang khi goc chieu xa tang len thi suat lugng phan ung giam. Tu goc chieu xa 0" den goc 30", suat lugng cua phan li-ng '^^Au(7,n)''^^Au giam nhanh. Tir goc 45" den goc 75" suat lugng ciia phan ung '''"Au(y,n)''^^'Au giam cham.
Hay noi each khac phan bo thong lugng ciia photon ham giam khi goc chieu
xa tang. Di danh gia phan bo doi xiing suat lugng theo goc chieu xa, tien hanh
so sanh suát lugng ciia phan umg ''^^Au(Y,n)'^^Au tai goc 15" va -15". Xet dai lugng t, theo tieu chuan Student ta co:
I =
V^ , +d',
= 0,82 < / „ „ , = 4,3
trong do Y,, 6, la suat lugng va do lech chuan tai goc 15" va Y2, Sj la suat lugng va do lech chuan tai goc -15".
- 3 0 0 30 60
Goc phat na (dp)
Hinh 3.3: Suat luong phan i(ng phu thugc phan bo goc.
Nhu vay voi miic hgp ly 0,05 hay do tin cay 95% ket luan suat lugng phan umg tai goc 15" va -15" la bang nhau dieu do chumg to phan bo thong lugng ciia photon ham tai goc ± 15" la nhu nhaụPhan bo goc cua cac photon ham CO the dugc mo ta gan diing boi bieu thuc ban thuc nghiem sau:
f{ÔEJ = (k + Ej'j + 80 (3.4)
ih
trong do Ẹ^ nang lugng nguomg ciia phan ung (MeV); 9 (radian) la goc giua vi tri ciia mau va true ciia chiim electron; |i = 0,511; k =1 doi voi 23 MeV< E,h< 65 MeV; k = 0,45 doi voi E,h< 15 MeV.
Cac k^t qua tren Hinh 3.3 cho thay thong lugng cac photon ham giam manh theo goc phat xạ Phan bo ciia cac photon nang lugng cao chii yeu tap trung trong goc hep xung quanh true ciia chum electron. Doi voi nhom photon ham CO nang lugng E,,, > 8 MeV, gia tri ciia goc 9^^= (2,34±0,21) dọ Trong
khi do chum buc xa ham nang lugng cue dai 15MeV phat ra tu may gia tdc
electron MT-17 co do phan tan Ion hon, cu thi Q^,= (13,6±1,2) dọ Cac kk
thu dugc trong d6 tai phu hgp voi tien doan ly thuylt va cac s6 lieu thu dugc tir cac thi nghiem khac[19,20]. Sir suy giam ciia chum biic xa ham theo g6c thu dugc trong de tai c6 the dugc sii dung de hieu chinh thong lugng biic xa ham trong cac mlu co kich thuoc Ion.