ANH HU'OfNG CUA H l f U tfSG LCfP C H E T L E N C A C D A C T R U W G CUA D E T E C T O R BAN DAN SIEU T I N H K H I E T L O A I P
N G 6 Q U A N G HUY
Tnrdng Dgi hgc Cdng nghiep TPHCM, 12 Nguyin Van Bdo, Phuang 4, Qugn Gd Vdp, TP Hd Chi Minh.
Email: [email protected]
Abstract: The present work aims at reviewing the studies of the influence of dead layer effect on the characteristics of a high purity germaniiun (HPGe) p-type detector, obtained by the author and his colleagues in the recent years. The object for study was the HPGe GC1518 detector-based gamma spectrometer of the Center for Nuclear Techniques, Ho Chi Minh City. The studying problems were: The modeling of an HPGe detector-based gamma spectrometer with using the MCNP code; the method of determining the thickness of dead layer by experimental measurements of gamma spectra and the calculations using MCNP code; the influence of material parameters and dead layer on detector efficiency; the increase of dead layer thickness over the operating time of the GC1518 detector; the influence of dead layer thickness increase on the decrease of detector efficiency; the dead layer effect for the gamma spectra measured in the GC1518 detector.
Tir khoa: Detector HPGe, lap chit, hi?u sudt detector, chuong trinh MCNP
I. GIOI THIEU
Mgt trong cac d^c diem quan trgng cda detector HPGe Igai p la cd lop tilp xiic n"^ d mat tren va mgt ben cua tinh the germanium, dimg lam cue duong ciia detector, dugc ggi la Idp chet. Ldp chet vdi be day khoang mm dugc hinh thanh do khuech tan cae nguyen tu lithiiun len tinh thi germanium. Ldp chit 1^ mgt vSt lieu chi hip thy vA tin xa tia gamma ma khdng thu nSng lugng cua cac tia gamma. Do dd nd anh hudng tryc tiep den hlgu suat do ciia detector v^ tu dd ^nh hudng dSn c^c kit qu& do dgc tren phd kl gamma.
Viec nghien ciiu anh hudng cvia be day ldp chet len cac dac tnmg cua detector HPGe dugc tien hanh tu nhung nam 1970. Tuy nhien sy phat trien manh me cac nghien cuu nay bat dSu tir khi ling dung phuong phap md phdng Monte Carlo de md hinh hoa cae pho gamma do tren cac phd ke gamma dung detector HPGe. Vin d§ la d cho, cac chuong trinh Monte Carlo nhu MCNP, CYLTRAN, DTEFF, GEANT, GESPECOR, ... mieu ta kha chinh xac qua trinh van chuyen gamma trong phd kl gamma, gdm budng chi, detector va hgp dyng mau. Tuy nhi^n hieu suat detector nhan dugc bing tinh toan theo chuong trinh Monte Carlo thudng Idn hon hieu suit thyc nghigm, cd khi din vai chyc phin tram. Dl l ^ phu hgp cac kit qua tinh tojui vdi dii li^u thyc n^igm ngudi ta dilu chinh bl day ldp chit, thudng la tang bl day ldp chet so vdi so lieu ciia nha san xuat cung cip de higu suat tinh t o ^ giam di.
Tuy nhiSn trong hang logt cac cdng trinh dang tren cac tap chi qudc tl, sy dilu chinh bl day ldp chet chi cd tinh chat ngau nhien vdi muc tieu dat dugc sy phu hgp ciia hieu suat tinh toan vol higu suit thyc nghigm. Chiing toi dat lgi vin dl nghien ciiu vigc dieu chinh bl day ldp chit mgt each cSn ban hon va cd y nghia vat ly r5 rang hon. Dd la, detector HPGe khdng thudng xuyen lam lanh bdi nito Idng trong qua trinh boat dgng cua minh. Chi do nhiet dd tao dieu kign cho ldp lithium tilp tuc khulch tdn, lam tang bl day ldp chit. Do do su tSng bl day ldp chit theo thdi gian hogt ddng ciia detector, anh hudng cua nd len sy giam hieu suat detector va cac ket qua do dgc khac trSn pho kl gamma la cac vin dl miu chot cin nghiSn c\iu.
288 Hdi nghi Khoa hgc vd Cdng nghe hgt nhdn todn qudc lin thiIX, Ninh Thugn 18-19/8/2011
II. PHO KE GAMMA DUNG DETECTOR HPGE GC1518 VA MO HINH H 6 A BANG CHLTONG TEJNH MCNP [1,2,3,4]
Doi tuong nghien ciiu cua chiing toi la h? pho kl gamma diing detector HPGe GC15I8 cua Trung tam Hat nhan TPHCM. Detector GC1518 ducjc dat trong buong chi nhu trinh bay tren hinh 1.
Hinh 1. Mgt cdt dgc ciia buong chi, kich thuac tinh bdng cm
Vonhom Tinh the germanium
Tin hieu ra
Hinh 2. a) Mdt cdt dgc ciig detector HPGe GC15I8, kich thuac dugc tinh bing mm;
b) Mo hinh detector.
Tren hinh 1, detector GC1518 la mot ong hinh tru ban kinh 3,81 cm vai chiSu cao nSm ben trong buong chi la 8,40 cm. Buong chi co dang hinh try vanh khuyen voi ban kinh ngo^
25 cm, cao 50 cm, ban kinh trong 15 cm, cao 30 cm. Bi day chi 6 cac mat tren, mat duoi va cac mat xung quanh hinh tru bang 10 cm. Ben trong bu6ng chi co lot cac lop tiuic, dong vi paraphin voi hinh hoc nhu dan ra tren hinh 1.
Detector GC 1518 hinh tru d6ng true (hinh 2) c6 cac thong s6 danh dinli nhu sau: Hieu suat tucmg doi 15% so voi detector nhSp nhay NaI(Tl) kich thuoc 3 inch x 3 inch, dp phan giai nang lugmg 1,8 keV va ti s6 dinh/Compton 45:1 tai dinh 1332 keV ciia dong vi Co.
Phan chinh ciia detector la tinh thi germanium co ducmg kinh ngoai 54 mm, chiSu cao 32 mm, d giira co mot h6c hinh tru duong kinh 7 mm, cao 17 mm. Tin hi$u lay ra til mpt cai I6i dat 6 h6c trong tinh the germanium. Mat tren va mSt ben tinh the germanium co mot lop
Hgi nghi Khoa hgc vd Cdng ngh? hgt nhdn todn qudc Idn thu IX. Ninh Thudn 18-19/8/2011 289 lithium khulch tan ngoai cung vdi bl day tuong ducmg 0,35 mm Ge, dugc ggi la ldp chit. Dd
\k ldp n^ lam dign eye duon^ nli vdi nguon dign duong. Mat trong hdc ciia tinh thi cd mgt ldp boron dugc ciy ion vdi be day tuong duong 3.10"^ mm Ge. Day la ldp p"^ lam dign cue am noi vdi ngudn dign am. Do day cac ldp lithium va boron do nha san xuit do nam 1996. Chiing c6 thi thay ddi theo thdi gian va tang len dang kl neu detector dl d nhigt dg cao mgt thdi gian dai. Cac thdng sd hinh hgc va vgt lieu khdc cua detector dugc trinh bay tren hinh 2. Viec md hinh hoa he pho kS theo chuong trinh MCNP dugc miSu ta trong cac cdng trinh [1-4], trong d6 sl lieu input dugc chgn theo cac thdng sd do nha san xuit cung cip va do dac thyc tl.
III. ANH HirdNG CUA SU* THAY DOI CAC THONG SO DETECTOR LEN HIEU SUAT CUA NO [5]
Phuong phdp Monte Carlo dugc ling dyng rgng rai trong vigc md hinh boa phd kl gamma dung detector HPGe. Khi dd cac thdng sd hinh hgc va vat lieu eiia detector dugc cung cip bdi nha san xuat. Vdi bg sd ligu do, cac cdng trinh nghien ciiu tren thi gidi diu cho thiy sy khong phu hgp tdt giiia kit qua tinh todn va thyc nghigm. Do dd cac cdng trinh nay phai higu chinh lai cac thdng sd bang each do lgi hogc biln thiSn chiing sao cho kit qua tinh toan phu hgp vdi thuc nghiem. Ddi vdi detector HPGe dgng ddng true, 8 thdng sl thudng dugc khao sat neu trong bang 1.
Tam thdng sd nay dugc chia thanh 2 nhdm. Gia tri danh dinh ciia chiing dugc neu trong bang 1 cung vdi mien bien thien. Nhdm thii nhat gdm 6 thdng sl 1-6, nhdm nay hiu nhu khdng thay ddi trong qua trinh hoat dgng ciia detector. Nhdm thii hai 7-8 cd miln biln thien khdng bi hgn che trong phgm vi sai sd ma cd the lay khoang 5 lin gia tri danh dinh cua chung, do be day ldp boron va ldp chet cd the tang dang ke trong qua trinh boat dgng cua detector.
Hieu suat detector dugc tinh theo chuong trinh MCNP ddi vdi cac ngudn chuan dang diem d§t d do cao 5 cm so vdi mat detector. Cac ngudn cd nang lugng tia gamma tu 59,5 keV den 1332 keV. Cgt cudi cung trong bang 1 trinh bay ^y thay ddi tuong ddi cua hieu suit detector khi thay ddi 8 thdng sd. Tii bang 1 thay rang khi cac thdng sd 1-6 bien thien tron^
miln dung sai ciia chiing thi higu suat detector chi thay ddi tu 0,02% den 1,92%. Ddi vdi be day ldp boron, khi thay ddi gia tri den 5 Ian thi hieu suat ciing chi thay ddi mdt lugng rat be khoang 0,002%. Dd la do ban than ldp boron rat mdng. Cdn ddi vdi ldp chet, hieu suat thay d6i kha Idn, khoang 18,7% khi be day tang len 5 lin, tir 0,35 nun len 1,50 mm. Nhu vay sy bien thien ciia 7 thdng sd dau khdng anh hudng dang ke den hieu suat detector, trong Iiic dd su tang be day ldp chet lam thay ddi dang kS hieu suat. Dya tren ket qua nay, cac cdng trinh cua chiing tdi chi xet anh hudng cua su thay ddi ldp chet len sy thay ddi hieu suat detector.
Ban^ I. DQ thay ddi tuang ddi cua hieu sudt detector khi cdc thdng sd nhdn cdc gid tri trong mien dung sai (ddi vdi 6 thdng sd 1-6) vd trong ddi den 5 Idn thong so danh dinh (ddi vai 2
thdng sd 7-8).
s6
thi tV 1 2 3 4 5 6 7 8
ThSng s6 Be diy vo nh6m cua detector
Khoang cSch giQa vo detector vi mat tren ciia tinh thi germanium
Be diy cua vo hQp chtjra tinh the gennanium Du6ng kinh tinh thS germanium Chieu cao tinh thi germanium Do sau h6c tinh thi germanium B I day I6p boron Bl day 16p chet
Kich thu6c danh djnh (mm) 1,50 5,0 1,8 54,0 32,0 17,0 0,3.10'
0,35
Dai liich thuac (mm) 1,42- 1,58
4,5-5,5 1,7-1,9 53,5-54,5 31,5-32,5 16,5-17,5 (0,3-1,5). 10'
0,35-1,50
Dp thay doi tuong dii cLta hi?u suat (%)
0,40 1,40 0,02 0,06 1,92 0,75 0,002
18,7
290 Hgi nghi Khoa hgc vd Cdng nghe hgt nhdn todn quic lin thi IX, Ninh Thudn 18-19/8/2011
IV. NGHIEN CUU SU GIAM HlgU SUAT DETECTOR VA SV TANG Bfi DAY LCIP CHET THEO THdl GIAN HOAT D Q N G CUA DETECTOR GC1518 [6,7]
Am-241; 59 keV Na-22:511keV
0.0007 0.0008
3.5 3 E S 2
I '-5
f ] 0.5
0
Na-22; 1274 kcV
0.0005 0.0006 Efficiency
d
Co-60; 1332 keV
1.39
y--5.5409Ln(x)-40.631 R ' - 0 . 9 9 9 6
\ » ^ _ . . . . - - 2 0 0 9 1.11
0.000508 1 1 0.000535 0.00O45 000055
Efficiency
0.0004 0.0005 Efficiency
Hinh 3. Phuang phdp xdc dinh bi day lap chit theo kit qud do hi$u sudt kit hgp vdi tinh todn
Sy giam hieu su4t detector GC1518 dugc xac djnh bang th\fc nghi$m va tinh toin MCNP tai_cac thai diSm 1999, 2005 va 2009. Su giam hi$u suit tir 1999 dSn 2005 dugc do nho hai mlu ZrSi04, tong do vat lieu ZrSi04 dugc nh6t bang cdch trgn vdo nhya polyester vJ
Hgi nghj Khoa hgc vd Cdng ngh? hgt nhdn tadn qudc lin thi IX, Ninh Thudn 18-19/8/2011 291 de kho cimg. Phuong phap nhot mau nay de mau trong mgt hon hgp dgng ran nen kin va khong hi ro ri khi radon ra ngoai. Sir giam hieu suat the hi^n a dg giam tuong doi ciia diSn tich cac dinh nang lugng toan phSn cita d3y U va Th do nam 2005 so v6i so lifu do nam 1999. K& qua do dac cho thSy, sau 6 nam hoat dpng (1999-2005) hi$u suit detector GC1518 giam 8%.
S\r giam hi$u suit detector GC1518 tir 2005 den 2009 dugc xac dinh bang each do su giam di?n tich cac dinh nang lugng toan phSn ciia cac nguon phong x£i i\im ^^Na (511 keV va 1274 keV), '"Cs (662 keV) va " t o (1173 keV va 1332 keV). Trong cac nam tir 2005 den 2009 hieu suit detector giam 5,5%.
Hi?u suit detector tai cdc nam 2005 va 2009 dugc so sanh vdi hi$u suit tinh toan MCNP doi voi 3 nguon phong xa diem Na, Cs va Co dat a do cao 15 cm so voi mat detector. Trong tinh toan, be day lop chet biln thien trong mien tir 0,35 nun den 3,0 nun. Lam phii hgp hieu suit tinh toan vdi hi?u suat do dac suy ra cdc gia tri be day ldp chSt bang 1,15 mm nam 2005 vJ 1,46 mm nam 2009 (Hinh 3).
Tir cac kit qua thuc nghi?m va tinh toan neu tren thiy ring sau 13 nam hogt dgng (1966-2009), hieu suit detector GC1518 gidm di khoang 18,5 %, hrong do 5% trong giai doan 1996-1999; 8% trong giai doan 1999-2005 va 5,5% tir 2005 dSn 2009. Nguyen nhan su giam hi^u suit nay la do su tang be day lop chet tir 0,35 mm nam 1996 den 0,65 ram nam 1999;
1,15 mm nam 2005 va 1,46 mm nam 2009. Mgt kSt qua quan trgng la su tang tuyIn tinh cua bl day lop chit va do giam hieu suat theo thoi gian tir 1996 den 2009 (Hinh 4a) cung nhu su tang tuyIn tinh cua bS day lop chet theo dg giam hi^u detector (Hinh 4b).
Efficiency reduction (0.1%) y-0.1405x-280.43
R^ = 0.9!
Dead layer thickness (mm^
y=0.084Sx-168.9721 R ' - 0 . 9 9 8 0
5 10 15 Efficiency reduction (%)
Hmh 4. a). Sir tdng tuyin tinh ciia bi day lap chit vd dg gidm hi$u sudt theo thai gian;
b). Sir tdng tuyin tinh cud bi day lap chit theo dg gidm hieu detector.
V. CO CHE ANH HUdNG CUA SU TANG BE DAY LCiiP CHET LEN SU GIAM HlEU SUAT DETECTOR [7]
Lop chit cua detector ddng true GC1518 loai p la lop lithium phu ca mat tren va mat bao quanh tinh thi germanium. Lop iiay chi hip thu va tan xa tia gamma ma khong thu nang lugng ciia tia gamma. Nhu v jy co thi coi lop chit nhu mgt vat can xa thuin tuy tia gamma.
S\r anh hucmg cua bl dJy lop chit len sv giam hieu suit detector co thi dugc giai thich theo hai CO chi sau day. Mgt la, khi be day lop chet tane thi tinh chit can xa tang, do do hieu suat detector giam, ta ggi la hi?u lin^ can xg cua lop chet. Thur hai, khi bl day lop chit tang thi thi tich miln hoat dgng cua tinh the germanium giam, lam giam hieu suit detector, ta ggi la hieu
292 Hdi nghi Khoa hgc vd Cdng nghe hgt nhdn todn qudc idn thu IX. Ninh Thudn 18-19/8/2011
ling the tich. Hai hieu iing can xa va the tich cung tac dOng ISn sy giam higu suit detector nhung ddng vai trd khac nhau trong cae mien nSng lugng gamma khac nhau.
Viec nghien ciiu cac co che nay tien hanh tai nSng lugng 59,5 keV cua ngudn ^""Am va mien nang lugrng 511-1332 keV ciia eac ngudn ^^Na, 'Cs va *^Co. Kit qua cho thiy, t^
vach nang lugng 59,5 keV, sy giam hieu suat chi dugc xac dinh bdi higu ting can xa con hieu ling the tich cd the bd qua. Dieu do cd the giai thich dugc do sy hap thy rit manh cua tia gamma 59,5 keV trong tmh the germanium. Khi dd hi$u suit detector dugc mieu ta bdi c6ng thiic £(x) = a exp[-n(x-xo)], trong dd X la be day ldp chet; a = 0,003731; n = 0,9881 mm'';xo
= 0,35 mm. Ddi vdi mien nang lugng gamma 511-1332 keV thi su gidm hi^u suit dugc xac dinh bdi ca hai higu iing the tich va can xa. Dd la do cac tia gamma nang lugng 511-132 keV bi hap thy yeu va cd the xuyen qua tinh the germanium n6n chiing cam nhan dugc thS tich tinh the germanium. Trong truong hgp ndy hieu suat detector dugc miSu ta bdi cong thiic e(x) = p V(x) exp[-)i(x-xo)], trong dd V(x) la the tich tinh the germanium trir ldp chit vdi hi day x; )a = 0,0293 mm"'; xo = 0,35 mm; p nhan cac gia trj 2,16.10'^ cm"^ ddi vol vach 511 keV; 1,64.10'^ cm"^ ddi vdi vach 662 keV; 9,66.10'^ cm'^ doi vdi vach 1173 keV; 8,74.10"^
em"^ ddi vdi vach 1274 keV; 8,34.10"^ cm"^ ddi vdi vach 1332 keV.
VI. HIEU UKG BE DAY LCJP CHET D 6 I Vdl CAC P H 6 GAMMA DO TREN P H 6 KE GAMMA DUNG DETECTOR GC1518 [8]
Trong mdt cdng trinh dang tren tap chi cjudc te (Pavel Dryak et al., Appl. Radiat. Isot.
68, 1451-1453,2010) cac tac gia da gia thuyet rang ldp chet hip thu nSng lugng tia gamma va phat ra tin bleu nhung eae tin hieu dd hoSe bi bit bdi cac bay, bi Idm cham hay cac hieu iing khac. Trong cdng trinh neu tren, sy ed mat ciia cac tin hieu bi Idm chSm dugc chiing minh bang cac phep do tning phung a-y ciia ngudn phdng xa Idng ^'"Am giiia mgt diu d6 nhip nhay va mgt trong hai detector germanium vdi be day ldp chet khdc nhau. Thi nghiem nay da phat hien dugc mdt lugng Idn cae tin hieu bi lam cham ddi vdi detector GC4018 vdi b^ day ldp chet Idn 0,53 mm va hau nhu khdng tim thay tin higu bi lam chdm nao ddi vdi detector BE5030 vdi be day ldp chet rat mdng 500 nm. Nhu vay ro rang cac tin higu dugc lam cham nay xuat phat tir ldp chet va sd tin hieu tang theo bl day ldp chit.
Trong nghien ciiu ciia chiing tdi, thi nghiem dugc tiln hanh ddi vdi mgt detector GC1518 nhung tai hai thdi diem khac nhau vdi bl day ldp chit khac nhau. Dd la bl day 1,15 mm nam 2005 va 1,46 mm nam 2009. Chiing tdi do cac phd gamma tir cdc ngudn phong xa chuin diem ^^Na, '^^Cs va ^^Co vao nam 2005 va 2009 vdi hinh hgc dat ngudn hoan toan gidng nhau. Cac phd gamma dugc chuan sad cho dien tich cdc dinh gidng nhau ddi vdi nam 20Q5 va 2009, trong dd dinh 662 keV ddi vdi ngudn '"Cs, dinh 1274 keV ddi vdi nguIn ^%
vd dinh 1332 keV ddi vdi ngudn ^**Co.
Cs- [37: Gamma ^>ecira measired 112005 and 2009 Cs-137; Measuremeni and calculaion; 2005 and 2009
Energy (keV) Blcrgy (keV)
Hinh 5. a). Hai phd gamma cua ngudn '^^Cs do ndm 2005 vd 2009. b). Cdc phd do dgc (M) vd tinh toan (C) ddi vdi ngudn '^^Cs ndm 2005 vd 2009.
Hgi nghi Khga hgc vd Cdng nghe bgt nhdn tgdn qudc Idn thi IX, Ninh Thudn 18-19/8/2011 293 Hinh 5a, nhu mgt vi du, trinh bay hai phi gamma cua nguIn '"Cs. Tit hinh 5a thiy rang hai pho nay khac nhau trong mien nang lugng 50-170 keV va hoan toan trung nhau trong mien nang lugng Ion ban 170 keV. Trong mien nang lugng 50-170 keV phi gamma nam 2009 Idn hon pho gamma nam 2005, iJmg vol bl day lop chet nam 2009 16n hon nam 2005.
Dieu do cho thay co hieu img be day lop chet trong miln nang lugng nay. Hinh 4b trinh bay cac phi gamma tinh toan trong miln nang lugng 50-17() keV, chimg gin nhu bing nhau va diu thip hon cac phi gairuna do dac. Nhu da bilt, cac phi gamma tinh toan theo chuong trinh MCNP gIm cac tin hieu gamma tvr cac qu4 trinh quang di$n, tan xa Compton va tao cap. HiSu sl giira pho gamma do dac va tinh toan tuong ling vdi cac tin hieu bi lam cham neu trong cong trinh noi tten cua Pavel Dryak, vdi s6 dem tang theo bl day lop chit. Ggi RA la ti s6 di?n tich pho ling vcri cac tin hi?u bj lam ch$m so vcri c^c tin hieu tir cac qua trinh vat ly tinh theo chuong trinh MCNP. Ti s6 niy bing 1.249 ± 0.035,1.270 ± 0.033 and 1.448 ± 6.036 dii vdi " ' C s , ' Co and ^^Na. Cac ti s6 do phai bang cac ti sl RE giira nang lugng cac tia gamma tirnguln phdng xa hip thu trong ldp chit trong hai truimg hgp bl day 1,46 mm va 1,15 mm.
Thjt vay, cac ti s6 RE bing 1,262; 1,264 va 1,263 dii voi "'Cs, "Co and ^^Na, trimg voi cac ti sl RA neu treii. V$y anh hudng cua hieu ilng bl day lop chit len su khac biet cua cac phi gamma trong mien nang lugng thap 50-170 keV dugc xac dinh bdi cac tin hieu hi lam cham xuat phat tir Idrp chet cua detector.
TAI LIEU THAM K H A O
[1]. Ngo Quang Huy, DoQuang Binh va Vo Xuan An. Buac dau md hinh hda phd ki gamma dimg detector bdn ddn sieu tinh khiet dung chuong trinh MCNP. Tap chi Phat triln Khoa hoc va Cong nghe, Dai hgc Quic gia TPHCM, Tap 8, Sl 8, Trang 17-25, 2005.
|2). Ngo Quan^ Huy, Vo Xuan An va D 8 Quang Binh. Mdphdng cdc phd gamma phuc tgp do tren hi phi ki gamma diing detector HPGe bing chucmg trinh MCNP. T?p chi Phat triln Khoa hgc va Cong nghe, Dai hgc Qudc gia TPHCM, TSp 9, S6 9, Trang 63-70, 2006.
[3]. Vo Xuan An, Ngo Quang Huy, Do Quang Binh. Application of the MCNP code to simulgte the gammg spectra of several radigactive sources measured by the HPGe detector spectrometry. Proceedings of the sixth national conference on nuclear science and technology. Da Lat 26-27/10/2005. Science and Technique Publishing House, pp.
376-379,2006.
[4]. Vo Xuan An, N^o Quang Huy va Do Quang Binh. Mo hinh hda bdng phuang phdp Monte Carlo h^ phd k& gamma diing detector HPGe vol hgp dung mdu dgng Marinelli. Tuyen tap cac bao cao Hgi nghi Vat \j toan quic lin thur VI, Ha Ngi, 23-25/11/2005. Nha xuit ban Khoa hpc vk Ky thu?it, trang 589-592,2006.
[5]. Ngo Quang Huy, Do Quang Binh va Vo Xuan An. Khdo sdt dnh hudng cua cdc thong sd.
vgt ly den hieu sudt cua detector bdn ddn sieu tinh khiet bdng chucmg trinh MCNP4C2.
Tap chi Phat triln Khoa hgc va Cong nghe, Dai hgc Qudc gia TPHCM, TSp 10, Sl 10, Trang 21-26,2007.
[6). N.Q. Huy, D.Q. Binh and V.X. An. Slucly on the increase of inactive germanium layer in a high purity germanium detector after a long time operation applying MCNP code.
Nuclear InsUuments and Methods in Physics Research A 573, pp. 384-388,2007.
[7]. Ngo Quang Huy. The influence of dead layer thickness increase on efficiency decrease for a coaxial HPGe p-type detector. Nuclear Instruments and Methods in Physics Research A 621, pp. 390-394,2010.
[8], Ngo Quang Huy. Dead layer thickness effect for gamma spectra measured in an HPGe p- type detector. Nuclear Instruments and Methods in Physics Research A, 2011, m press.
