- Genie2000, Canberra (2000)
2.3.2. Hieu chinh sir ty hap thu gamma trong mau
Trong phuong phap kich boat neu sii dung cac mau day va do cac tia gamma nang lugng thap se gap sai so do hieu ling tu hap thụ He so tu hap thu c6 the xac dinh dugc tu bieu thiic sau:
F = — - (2.23) lud lud
trong do, \x la he s6 hap thu tuyen tinh phu thuoc vao nang lugng tia gamma
•\ f r \ 1%^
va thanh phan chat hap thu, d be day mau kich boat.
Doi voi cac mau biet ro thanh phan va ham lugng ciia cac nguyen to
1 f r r r
hoan toan c6 the tinh dugc he so tu hap thụ Mat kliac he so tu hap thu cung c6 the xac dinh bang thuc nghiem su dung phuong phap do biic xa gamma truyen qua:
ln(///o) (2.24) trong do, I va IQ la cuong do ciia tia gamma dugc do trong tarong hgp giua
nguon va detecto co va khong co mau [1,3].
2.3.3.Hieu ling thoi gian ch^t va chong chap xung
Hieu Ling thai gian chat (dead time) \a chong chap xung (pile-up) gay len su mdt s6 dam trong ghi nhan pho gammạ Cac hieu ung na\ dac biet quan trgng trong truo-ng hop toe dp dem Ion.
Thai gian ch6t la thai gian ADC ban xii ly xung va kh6ng th6 ti§p nhan them mot xung khac. Hau bet cac may phan tich bien do nhieu kenh hien nay
dhu CO dong ho do thai gian thuc Tc (true time hoac clock time) va thai gian
lam viec Ti (live time): Tc = T| +2^ ^' , trong d6 TJ la thai gian chet cua tiing xung dugc ghi nhan. Doi voi m6i phep do ADC thuong dua ra mot thoi gian
f »
chet trung binh (%). Bang viec su dung thai gian lam viec trong cac phep do pho gamma ta se loai bo dugc hieu ling thoi gian chet. Tuy nhien neu toe do dem thay doi nhieu trong khoang thai gian do thi phuang phap nay chua dii do chinh xac.
•> •> f f
Do phan giai thai gian ciia he ph6 ke co gioi ban, khi t6c do dem Ion hai
r t \ f
xung lien tiep c6 the ch6ng chap len nhau va tao ra mot xung bi bien dang 6 16i ra ciia khuyech daị Hien tugng nay ggi la hieu ling chong chap xung. Cac xung do ch6ng chap rong ban va co dang khac cac xung khong bj ch6ng chap. Do do khi t6c do dem cao ph6 thong thuong bi bien dang.
Co bai phuong phap chinh thuong dugc sir dung de hieu chinh sir mat so dim do hieu ling cb6ng chap xung do la diing may phat xung chuan hoac sir dung cac giai phap pbSn cung nhu bo loai tni chong chap xung (pile-up rejecter).
May phat xung chuan co tha sii dung ^Q hieu chinh d6ng thoi su mat s6
dam do thoi gian chat va ch6ng chap xung. Gia sii ty le s6 xung bi mat o cac dinh ph6 va dinh xung chuSn la nhu nhau, s6 dam ciia cac dinh ph6 co tha duoc hiau chinh theo ha so:
^ = ^ , / A ' ;
trong do, ^r va ^" la s6 dam ciia dinh xung chuan khi co va kh6ng co maụ
Tran ph6 gamma dinh xung chuSn thuong co dang hep hon cac dinh gamma do do khi phan tich ph6 cSn luu y viec tinh dien tich dinh xung chuSn. Ngoai ra cung co tha sii dung nguon phong xa gamma don nang de
2.3.4. Hieu chinh can nhiiu phong xa
Can nhieu phong xa la mot van da phuc tap, anh huong true tilp toi do
chinh xac ciia cac kit qua xac dinh suk lugng phan ling. Trong cac nghien
cihi thuc nghiem su dung phuong phap kich hoat cac can nhieu phong xa chii yeu bao gom [3,14]:
- Can nhiau do cac d6ng vi phong xa phat ra cac tia gamma co nang
y f ,
lugng gan giong nhau hoac hoan toan gi6ng nhau, cac truong hgp thuong gap trong nghian cuu nhu:
^°^Bi(n,3n)^°'Bi, Ey = 569,7keV va '°'Bi(n,7n)'°'Bi, Ey = 569,4keV. '^^Au(y,2n)'^^Au, Ey = 98,88keV va '^'Au(y,pn)'^^Pt, Ey = 98,88keV...
' ^ f
- Can nhien gay boi cac phan ung hat nhan canh tranh. Vi du d6i voi hat nhan bia la Au, In, Cu dugc kich boat truang biic xa hon hgp photon-natron tii cac may gia t6c electron co cac phan ling canh tranh sau:
''^Au(y,n)'^^Au va "'^u(n,2n)'^^Au; "^In(y,2n)"'ln va "^In(y,4n)'"ln; ^^Cu(y,n)^''Cu va "Cu(n,y)^'Cụ..
Ngoai cac can nhiau tran con co cac can nhiau gay nan boi hieu ung nan, do cac mau co thanh phan phiic tap hoac ngu6n biic xa kich boat co cuong do va nang lugng lan, khi do se c6 nhieu loai phan ling ciing xay ra lam cho ph6ng phong xa cao anh huong den chat lugng cua pho gamma [1,3,14].
Viec hieu chinh cac can nhieu tren co the dua vao phuang phap ty s6 dien tich dinh, ap dung quy luat phan ra phong xa theo ham e-mu hoac sii dung cac phan ling hat nhan khac lam monitọ
D6i voi cac can nhilu gay boi cac phan ling hat nhan canh tranh co the khac phuc bang each sii dung ky thuat kich boat nhieu lan. lira ebon nang lugng kich boat, uu tien tiing phan ling ket hgp \'6i tinh toan thoi gian kich boat, thai gian do mau hgp lỵ Viec sir dung mau kich boat la dong vi sach cung la bien phap loai trii cac phan ling can nhieu rat co hieu quạ De giam
bat hieu ling nen ta c6 the lira chgn nang lugng, cuong do ciia chum biic xa kich boat ciing nhu tinh toan thai gian kich boat thich hgp.