L

zG')c-mz-=1.,)

::::J:>-1z:I)>>z

NGUYEN DUC HOA

E T

Li ni u

Gio trnh in t ht nhn nhm cung cp cc nguyn l c bn ca thit b ghi o bc x c s dng cho nghin cu v ng dng trong lnh vc Vt l ht nhn, l cn thit cho i ng nghin cu, cho cc sinh vin i hc, cao hc v nghin cu sinh trong cc trng i hc cng nh cc ngnh k thut c lin quan ti ghi o bc x.

Vi s pht trin ca ngnh k thut ht nhn cho thy kh nng ng dng rng ri v hiu qu vo cc lnh vc khoa hc cng nh i sng. Hin nay, Vit Nam ang s dng l phn ng ht nhn, my gia tc nng lng thp v cc thit b ng dng cht phng x, c bit chng ta ang chun b xy dng nh my in ht nhn d kin pht in vo nm 2020. Do , vic o to i ng lm vic trong lnh vc ht nhn tr thnh mt nhim v i vi cc trng i hc trong giai on mi. Gio trnh in t ht nhn c bin son trn c s cc bi ging ca tc gi cho bc i hc v sau i hc trong nhiu nm qua, nhm phc v cng tc o to ngun nhn lc nguyn t.

Tc gi by t li cm n n PGS. TS. L B Dng, TS. L Hng Phong, ThS-NCS. Nguyn An Sn, Trng i hc Lt; TS. Nguyn Xun Hi, ThS-NCS. ng Lnh, Vin Nghin cu ht nhn Lt; TS. GVC Phm nh Khang, Trung tm o to ht nhn, Vin Nng lng nguyn t Vit Nam ng gp v b sung nhiu kin c gi tr cho gio trnh ny. Tc gi mong mun nhn c nhng kin ng gp t cc ng nghip v bn c gio trnh c hon chnh hn trong ln xut bn sau.

Lt, ngy 10 thng 3 nm 2012

Tc gi

2

MC LC

Li ni u2MC LC3M U10BNG K HIU VIT TT TING ANH12Chng I. TNG TC CA BC X VI VT CHT151.1. NGUYN T15Cu to nguyn t15S kch thch v ion ho nguyn t161.2. TNG TC CA TIA BETA VI VT CHT171. Ion ho (Ionization)172. ion ho ring (Specific ionization)19H s truyn nng lng tuyn tnh (LET)20Bc x hm (Bremsstrahlung)20Qung chy ca ht beta trong vt cht211.3. TNG TC CA HT ALPHA VI VT CHT22Truyn nng lng ca ht alpha22Qung chy ca ht alpha trong vt cht231.4. TNG TC CA BC X GAMMA VI VT CHT24Hiu ng quang in24Hiu ng Compton25S to cp electron-posistron26Tng hp cc hiu ng khi gamma tng tc vi vt cht28Cu trc ph gamma29Chng II. DETECTOR GHI O BC X V S LIN KT322.1. BUNG ION HO32Nguyn tc hot ng32Qu trnh vt l32Hnh thnh xung332. S ni vi tin khuch i342.2. NG M T L36Qu trnh vt l v to xung36Minh ha thng k ca qu trnh nhn kh373. S tin khuch i382.3. DETECTOR NHP NHY39Nguyn l hot ng ca detector nhp nhy39Hnh thnh xung423. S tin khuch i ghp ni vi detector nhp nhy432.4. DETECTOR BN DN441. Nguyn l hot ng ca detector bn dn442. S tin khuch i47Chng III. CC KHI IN T TNG T513.1. CC C TRNG CHUNG CA B KHUCH I513.2. CC LOI TIN KHUCH I52Chc nng ca tin khuch i52Phn loi tin khuch i52Cc cch ghp ni P.Amp vi detector553.1 Ni AC gia P.Amp v detector553.2. Ni DC gia detector v P.Amp563.3. CC PHNG PHP HNH THNH XUNG60Mch b tr im khng60Mch hnh thnh xung CR-RC v CR-RC-CR63Hnh thnh xung chun Gauss64Mch hnh thnh xung chun tam gic67Hnh thnh xung bng tch phn cng683.4. MCH PHC HI NG KHNG72Chc nng ca mch phc hi ng khng72Cc s hi phc ng khng74BLR loi i xng (Robinson)74BLR loi khng i xng75BLR khng ph thuc thi gian763.5. CNG TUYN TNH78Loi hai diode (ni tip - song song)80Loi cu diode (cng tuyn tnh lng cc)803.6. CC MCH M RNG XUNG823.7. H THNG KHUCH I PH84Chng IV. CC S BIN I TNG T S864.1. NGUYN L C BN CA ADC86Khi nim chung86Mt s phng php bin i A/D87Phng php iu khin m87Phng php so snh lin tc88Phng php dng tn hiu dc ln892.4. Phng php dng tn hiu hai dc90Cc c trng chnh ca ADC91 chnh xc91 phn gii92 tuyn tnh924.2. ADC LOI SO SNH SONG SONG (ADC Flash)921. Nguyn l chung932. Phng php hiu chnh934.3. ADC LOI GN NG LIN TIP951. Nguyn l95S khi ADC gn ng lin tip96Phng php thang trt984.4. ADC WILKINSON1001. Nguyn l1002. S khi ADC Wilkinson1024.5. PHN TCH A KNH102Gii thiu chung102T chc b nh v b nh thi gian106B nh lu tr d liu (RAM)1062.2. S khi ca RAM tnh (SRAM)106Cc thit b SRAM chun107Khi hin th110Vng din tch quan tm (ROI)111Chc nng pht k t112Chng V. NG DNG PSD V FPGA TRONGTHIT K GHI O BC X1135.1. VAI TR CHC NNG CA DSP V FPGA113Vai tr chc nng ca DSP v FPGA113X l tn hiu s113Mng cc phn t logic lp trnh c114ng dng ca DSP v FPGA trong thit b in t1175.2. PHNG PHP IN T S119Phng php kh tch chp bng k thut ly mu quaca s ng (MWD) trong pht trin thut ton DSP119Gii thiu119Ti cu trc in tch ca s kin120Phng php thit k b ghi-o v x l tn hiubng thut ton DSP126Gii thiu h ph k trn c s DSP126Cc khi chc nng chnh127B tin lc tng t129H s khuch i ca h thng131Cc Tin khuch i phn hi lin tc v c xo131Hnh thnh xung132Hi phc ng c bn134Chn la xung135Qu trnh xo v phn bit thi gian tng137Phng php p dng vi mch FPGA thc hin thut ton DSP140Phng php tit kim140Phng php chuyn nghip141Phng php lp trnh cho FPGA s dngmi trng Max+Plus II141Phng php lp trnh cho FPGA sdng mi trng ISE1435.3. B VI X L XUNG S143Gii thiu143Mi tng quan gia cc cu hnh MCA theo phngphp tng t truyn thng v phng php s145S cu trc ca DSP-MCA148B to dng xung s hnh thang148Nhn xt1514. u v nhc im ca in t truyn thngv in t s1515.4. MCH NG DNG DSP V FPGA153Thit k khi MCA8K dng FPGA153B x l trung tm v hot ng cabn mch FPGA-MCA8K154c trng chnh MCA 8k ch to155Chng VI. BIN I THI GIAN THNH BIN VCC PHNG PHP X L THI GIAN1566.1. B BIN THI GIAN THNH BIN 1566.2. B PHN BIT TCH PHN1636.3. PHN BIT CT KHNG (ZERO-CROSSING)1656.4. PHN BIT CT KHNG THEO PHNG PHPT S KHNG I1676.5. PHN BIT THEO PHNG PHP NGNG SUY BIN171Chng VII. CC H THNG O BC X V CC PHNGPHP O TRONG VT L HT NHN1747.1. H THNG PH K HT NHN1747.2. PHNG PHP TRIT COMPTON1757.3. PHNG PHP TRNG PHNG THI GIAN1777.4. PHNG PHP XC NH S LIN QUANK TIP CA BC X1817.5. PHNG PHP TRNG PHNG TNGGHI S KIN - S KIN1837.6. NG DNG PHNG PHP TRNG PHNG CHM1887.7. PH K THI GIAN BAY1917.8. H THNG O TN X CNG HNG PROTON193TI LIU THAM KHO198

M U

Hnh thanh x ly xungin t c mt ngha to ln trong i sng, k thut v khoa hc. Phng php in t c ng dng rng ri gii quyt cc bi ton khc nhau. i vi cc php o vt l, c th l o cc i lng vt l (m ch yu l i lng khng in), mt h thng o trc ht phi bin i cc i lng khng in thnh cc i lng in. S khi nh sau:

10SBienoi

Hnh thnh x l xung

ADC

May tnh

i vi cc bi ton khc nhau, hay ni cch khc l vi cc i lng vt l cn o khc nhau, vi yu cu thc tin khc nhau th li vo v c cu x l l khc nhau.

Cc i lng o trong Vt l ht nhn gn lin vi php o ht nhn u dng khng in. V th, cc phng php ghi nhn bc x pht ra t ht nhn u da vo cc tng tc bc x i qua vt cht. Dng c lm nhim v bin i cc bc x thnh dng tn hiu in c nhiu tn gi l u d, ng m, detector,

H thng in t o m, xc nh gi tr cc i lng lin quan ti bc x ca ht nhn c gi l h thng in t ht nhn.

Mt h thng in t ht nhn c bn c cu trc nh sau:

DetectorDetectorP. Amp

S

P.AmpAnalog Processor

ADC MCD

Detector lm nhim v bin i cc bc x thnh dng tn hiu in,

Tin khuch i (P.Amp) l b khuch i c h s khuch i nh lm nhim v khuch i cc tn hiu t li ra ca detector. Tin khuch i thng c t st detector, cch xa trung tm o,

Khuch i ph k hay cn gi l b x l tng t (Analog Processor) c nhim v khuch i tn hiu ln vi trm cho n vi ngn ln, ng thi x l dng xung in cho chnh xc cao trong php o,

ADC, MCD l cc b bin i tn hiu tng t thnh tn hiu s (ADC), v tn hiu c a vo my tnh x l kt qu o thng qua b giao din MCD.

Gio trnh ny nhm cung cp cc kin thc c bn nht v cc khi in t chc nng nu trn, ng thi t cc khi chc nng ny, tu vo mc ch o m h thng in t ht nhn i km c th c cu trc khc nhau. V vy, nm bt c cc kin thc v in t ht nhn th cc kin thc v C s k thut in t, K thut xung, K thut s, Vt l ht nhn v Phng php thc nghim Vt l ht nhn cng rt quan trng v cn thit.

BNG K HIU VIT TT TING ANH

(Theo th t A, B, C)

T vit ttTing AnhNgha

ADCAnalog to Digital ConverterB bin i tng t sang s

ADCLADC LatchingCht a ch pha ADC

AMPAmplifierKhuch i

BLRBase-Line RestorerPhc hi ng khng

BUSYBusyBn bin i

CICarry InputNg vo c nh

CLOCK GEN

Clock GeneratorMy pht xung nhp

COCarry OutputNg ra c nh

DACDigital to AnalogConvertor

B bin i s sang tng t

DACCData AcceptedNhn xong d liu

DPPDigital Pulse ProcessingX l xung s DRData ReadyD liu sn sng DSPDigital Signal ProcessingX l tn hiu sDSPsDigital Signal ProcessorB x l tn hiu s

DTDead TimeThi gian cht

ECONEnable ConvertingCho php bin i

FFAFast Filter AmplifierKhuch i nhanh

FPGAField Programmable GateArray

Mng cc phn t lp trnh c

FWHMFull Width Half Maximum rng cc i na chiu cao

GIGate IntegrateTch phn cng

I/VCurrent to Voltage(i) dng sang th

LETLinear Energy TransferTruyn nng lng tuyn tnh

LLLow LevelMc (ngng) thp

MMemoryB nh

MCAMulti Chanel AnalyserMy phn tch a knh

MCDMulti Chanel DataProcessing

X l d liu a knh

MIOMemory Input-Output busTuyn nhp-xut b nh

MWDMoving WindowDeconvolution

Kh tch chp bng k thut ly muqua ca s ng

OEOutput EnablingCho php xut

P.A mpPreamplifierTin khuch i

PMTPhotomultiplier Tubeng nhn quang in

PSELPort SelectionChn cng

ROIRegion of InterestVng din tch quan tm

S/NSignal per NoiseT s tn hiu trn tp m

SCASingle Chanel AnalyserMy phn tch n knh

SRAMStatic Random AccessMemory

B nh tnh thm nhp ngu nhin

TRPTransisitor Reset Pre-AmpTin khuch i xo bng transistor

ULUpper LevelMc (ngng) trn Filter AmplifierKhuch i lcGain StageTng khuch i

Difference AmplifierKhuch i vi sai

Wrap-Around BLRMch iu khin BLR vng sau Slow Pass Active FillterB lc thp quaFast Gate BLRCng phc hi nhanh ng c bn

Control LogicLogic iu khin

Chng I

TNG TC CA BC X VI VT CHT

Nguyn t,

Tng tc ca tia beta vi vt cht,

Tng tc ca ht alpha vi vt cht,

Tng tc ca bc x gamma vi vt cht.

1.1. NGUYN T

1. Cu to nguyn t

xem xt s tng tc ca bc x vi vt cht, phn ny s trnh by tm tt cu to ca nguyn t - thnh phn c bn ca vt cht. Cc nguyn t c cu trc ring ca mnh ph thuc vo loi nguyn t. Nhng c im chung ca n l cu to t ht nhn nguyn t (nucleus) c in tch dng nm gia v cc electron in tch m chuyn ng trn cc qu o xung quanh ht nhn. M hnh nguyn t nh trn tng t m hnh h thng Mt Tri, c gi l m hnh nguyn t Bohr. Bnh thng nguyn t trung ho v in. in tch dng ca ht nhn bng tng s in tch m ca cc electron. S electron qu o cng ln khi nguyn t cng nng. V d nguyn t hydrogen c 1 electron qu o, cn uranium c 92 electron qu o.Nguyn t c ng knh khong 10-10 m cn ht nhn c ng knh khong 10-15 m. Khi lng ht nhn chim phn ln khi lng nguyn t, cn khi lng cc electron khng ng k. V d, khi lng nguyn t hydrogen bng 1,67343.10-27 kg trong khi khi lng electron ch bng 9,1091.1031 kg.

Cc electron chuyn ng trn cc qu o m ti , electron tn ti mt cch c lp v c nng lng xc nh. Bn knh qu o v nng lng electron c xc nh bi s lng t chnh ca nguyn t, s lng t qu o l v s lng t t m. S lng t chnh ca nguyn t l s nguyn dng xc nh lp

qu o: Lp K l lp trong cng ng vi n = 1, lp L tip theo ng vi n = 2, lp M ng vi n = 3, lp N ng vi n = 4, ... i vi nguyn t hydrogen c s nguyn t Z = 1, ti mi lp qu o nguyn t, nng lng Wn ca electron c xc nh theo cng thc sau:

W Rhnn2

(1.1)

trong , R = 3,27.1015 s1 l hng s Rydberg, cn h = 6,625.1034 J.s l hng s Plank. i vi cc nguyn t khc c Z > 1 th hng s Rydberg c nhn vi Z2. Cng thc (1.1) cho thy, Wn c gi tr m v t gi tr thp nht khi n = 1. Nh vy, cc electron lp K c nng lng thp nht, sau cc electron lp L c nng lng cao hn, cc lp tip theo c nng lng cao dn. Mi lp li gm mt s trng thi con, c xc nh bi s lng t qu o l v s lng t t m. Ti lp th n, nguyn t c 2n2 electron, tc l lp K c 2 electron, lp L c8 electron, lp M c 18 electron,

2. S kch thch v ion ho nguyn t

Cc electron ca nguyn t chim y cc trng thi thp nht cc qu o thp nht. V d nguyn t Na c 11 electron, trong 2 electron nm lp K, 8 electron nm lp L v 1 electron cn li nm lp M, l trng thi c bn (ground state) ca nguyn t. Cc electron nm lp cng thp th cng b lc tc dng ht mnh vo ht nhn. chuyn n ln lp cao hn phi c nng lng cung cp t bn ngoi. Khi mt electron no c cung cp nng lng chuyn t lp di ln lp trn th li mt l trng (empty slot) lp m n va b i v nguyn t lc ny trng thi kch thch (excited state). Nu c cung cp nng lng rt ln, electron c th thot ra ngoi nguyn t tr thnh electron t do v li mt l trng ti lp n va b i. Khi ta ni nguyn t b ion ho, tc l nguyn t vi in tch dng c gi tr bng in tch cc electron bay ra ngoi.

Khi nguyn t b kch thch hay b ion ho, v tr c ca electron tr thnh l trng. Nu mt electron no lp cao n2 chuyn xung chim v tr ca l trng lp thp n1 th nguyn t gii phng mt nng lng bng hiu s gia hai mc nng lng tng ng ca hai lp ny:

E W W

1 Rh 1 vi n > n

(1.2)

n2n1

n2

n2 21

12

Nng lng E c gii phng ra khi nguyn t di dng mt bc x in t, chng hn l nh sng i vi nguyn t hydro. i vi cc nguyn t nng, tc l c s nguyn t Z ln, nng lng bc x c gi tr ln. Trong trng hp ny, khi cc electron chuyn xung cc mc thp, bc x pht ra c nng lng kh ln, gi l tia X. Cn i vi cc lp cao hn nng lng bc x b, khi nguyn t pht ra cc tia nh sng t ngoi, nh sng nhn thy hoc nh sng hng ngoi. Tia X v cc bc x nh sng u l sng in t, chng ch khc nhau v tn s sng. Chng cng c tnh cht ht nn cn gi l photon hay lng t nh sng.

Cc bc x c kho st bao gm cc ht tch in nh alpha v beta, cc tia gamma v tia X cng nh ht neutron. Trong qu trnh tng tc ca bc x vi vt cht, nng lng ca cc tia bc x c truyn cho cc electron qu o hoc cho ht nhn nguyn t, tu thuc vo loi v nng lng ca bc x cng nh bn cht ca mi trng hp th. Cc hiu ng chung khi tng tc ca bc x vi vt cht l kch thch v ion ho nguyn t mi trng.

1.2. TNG TC CA TIA BETA VI VT CHT

1. Ion ho (Ionization)

Do ht beta mang in tch nn c ch tng tc ca n vi vt cht l tng tc in vi cc electron qu o. iu dn ti s kch thch v ion ho cc nguyn t mi trng. Trong trng hp mi trng b ion ho, tia beta mt mt phn nng lng Et nh bt mt electron qu o ra ngoi. ng nng Ek ca electron b bn ra lin h vi th nng ion ho ca nguyn t E v mt nng lng Et nh sau:Ek = Et E(1.3)

Trong th nng ion ho E l nng lng cn thit mt electron chuynt mc c bn K (n1 = 1) tr thnh electron t do mc vi n2 :

E = Wn2 Wn1 = 0 Wn1 = Rh(1.4)

Trong nhiu trng hp, electron bn ra c ng nng ln c th ion ho nguyn t tip theo, l electron th cp v c gi l electron delta. Electron delta ban u vi ng nng c 1000 eV c th to nn mt chui cc electron delta th cp v do to nn mt chui cc cp ion.

Bng 1.1. Th ion ho E v mt nng lng trung bnh sinh cp ion i vi mt s cht kh.

KhTh ion ho

E (eV) mt nng lng trung bnh sinh cp ion W (eV)

H213,636,6

He24,541,5

N214,534,6

O213,630,8

Ne21,536,2

Ar15,736,2

Kr14,024,3

Xe12,121,9

Khng kh33,7

CO214,432,9

CH414,527,3

C2H211,625,7

C2H412,226,3

C2H612,824,6

Do ht beta ch mt phn nng lng Et ion ho nguyn t, nn dc theo ng i ca mnh, n c th gy ra mt s ln cp ion. Nng lng trung bnh sinh mt cp ion thng gp 2 n 3 ln th nng ion ho. l do ngoi qu trnh ion ho, ht beta cn mt nng lng do kch thch nguyn t. Chng hn, i vi oxygen v nitrogen, th ion ho tng ng l 13,6 eV v 14,5 eV, trong lc mt nng lng trung bnh sinh mt cp ion l 30,8 eV v 34,6 eV. Bng 1.1 trnh by th ion ho E v mt nng lng trung bnh khi sinh ra cp ion W i vi mt s cht kh.

Do ht beta c khi lng bng khi lng electron qu o nn va chm gia chng lm ht beta chuyn ng khi hng ban u v nh vy, ht beta chuyn ng theo hng ng cong gp khc sau nhiu va chm trong mi trng hp th, cui cng s dng li khi ht nng lng ion ho. Dc theo ng i ny c rt nhiu cp ion to nn do qu trnh ion ho s cp ca ht beta ban u ln qu trnh ion ho th cp do cc ht electron delta. Qu o chuyn ng c th ghi nhn bng phng php nh tng nh hay bung bt.

2. ion ho ring (Specific ionization)

ion ho ring l s cp ion to ra trn mt n v ng i ca ht beta. ion ho ring kh cao i vi cc ht beta c nng lng thp, gim dn khi tng nng lng ca ht beta, t cc tiu nng lng khong 1 MeV, ri sau tng chm.

ion ho ring c xc nh qua tc mt nng lng tuyn tnh ca ht beta do ion ho v kch thch, mt thng s quan trng dng thit k thit b o liu bc x v tnh ton hiu ng sinh hc ca bc x. Tc mt nng lng tuyn tnh ca ht beta tun theo cng thc sau:

2q4 NZ 3.109 4

2

dE

ln

Em Ek

2 MeV

(1.5)

dxE 2 1,6.106 2

I2 1 2

cm

m

trong :

N l s nguyn t ca cht hp th trong 1 cm3, Z l s nguyn t ca cht hp th,

NZ = 3,88.1020 e /cm3 l s electron ca 1 cm3 khng kh nhit 00C v p sut 760 mmHg,

Em = 0,51 MeV l nng lng tnh ca electron,

Ek l ng nng ca ht beta,

= v/c, trong v l vn tc ca ht beta, c = 3.1010cm/s,

I (c gi tr 8,6.10-5 MeV i vi khng kh v 1,35.105Z MeV i vicc cht hp th khc) l th ion ho v kch thch ca nguyn t cht hp th.

Nu bit trc i lng W, l mt nng lng trung bnh sinh cp ion, th ion ho ring s (Specific ionizaion) c tnh theo cng thc sau:

20s dE/dx (eV/cm) W(eV/c.i)

(1.6)

trong c.i l s cp ion.

3. H s truyn nng lng tuyn tnh (LET)

ion ho ring c dng khi xem xt mt nng lng do ion ho. Khi quan tm n mi trng hp th, thng s dng tc hp th nng lng tuyn tnh ca mi trng khi ht beta i qua n. i lng xc nh tc hp th nng lng ni trn l h s truyn nng lng tuyn tnh.

H s truyn nng lng tuyn tnh LET c nh ngha theo cng thc sau:

LET dELdl

(1.7)

trong dEL l nng lng trung bnh m ht beta truyn cho mi trng hp th khi i qua qung ng di dl. n v o thng dng i vi LET l keV/m.

4. Bc x hm (Bremsstrahlung)

Khi ht beta i n gn ht nhn, lc ht Coulomb mnh lm n thay i t ngt hng bay ban u v pht nng lng di dng bc x in t, gi l bc x hm. Nng lng cc bc x hm phn b lin tc t 0 n gi tr cc i bng

ng nng ca ht beta. Rt kh tnh ton dng phn b nng lng ca cc bcx hm nn ngi ta thng s dng cc ng cong o c thc nghim.

nh gi mc nguy him ca bc x hm, ngi ta thng dng cng thc gn ng sau y:f = 3,5.104 ZEmax(1.8)

trong f l phn nng lng tia beta chuyn thnh photon; Z l s nguyn t cacht hp th v Emax (MeV) l nng lng cc i ca ht beta.

Cng thc (1.8) cho thy kh nng sinh bc x hm t l thun vi s nguyn t ca cht hp th. Do , vt liu dng che chn tia beta thng c lm t cc vt liu nh. Nhm vi Z = 13 l vt liu che chn tia beta nng nht v cng t khi c s dng.

5. Qung chy ca ht beta trong vt cht

Do ht beta mt nng lng dc theo ng i ca mnh nn n ch i c mt qung ng hu hn. Nh vy, nu cho mt chm tia beta i qua bn vt cht, chm tia ny b dng li sau mt khong ng i no . Khong ng i ny gi l qung chy (range) ca ht beta, n ph thuc vo nng lng tia beta v mt vt cht ca mi trng hp th. Bit c qung chy ca tia beta vi nng lng cho trc c th tnh c dy ca vt che chn lm t vt liu xc nh. Mt i lng thng dng khi tnh ton thit k che chn l dy hp th mt na (absorber half-thickness), l dy ca cht hp th lm gim s ht beta ban u i mt na sau khi i qua bn hp th. o c thc nghim cho thy dy hp th mt na vo khong 1/8 qung chy.

Ngoi qung chy tuyn tnh du tnh theo cm, ngi ta cn dng qung chy tnh theo mt din tch ddt c n v g/cm2 v c xc nh nh sau:

ddt (g/cm2) = (g/cm3) du (cm)(1.9)

trong l mt khi ca cht hp th, tnh theo g/cm3. Trong tnh ton thit k dy, vt liu che chn, ngoi b dy tuyn tnh (linear thickness) tnh theo cm, ngi ta cn dng b by mt (density thickness) tnh theo n v g/cm2 haymg/cm2. Vic s dng i lng b dy mt lm n gin php tnh v khi

b dy khng ph thuc vo vt liu c th. V d, mt nhm bng 2,7 g/cm3, do mt bn nhm dy 1 cm c b dy vt liu l:

ddt = 2,7 g/cm3 1 cm = 2,7 g/cm2

Mt bn plexiglass vi mt 1,18 g/cm3 c kh nng hp th tia beta tng ng vi bn nhm dy 1 cm khi b dy mt ca n bng 2,7 g/cm2, khi b dy tuyn tnh ca bn plexiglass bng:

d2,7 g/cm2

du

dt 2,39 cm1,18 g/cm3

(1.10)

i vi nng lng beta trong vng 0,01 E 2,5 MeV:

lnE = 6,63 3,2376.(10,2146 lnR)1/2.(1.11)

1.3. TNG TC CA HT ALPHA VI VT CHT

1. Truyn nng lng ca ht alpha

C ch mt nng lng ca ht alpha l kch thch v ion ho nguyn t. Khi i qua khng kh, ht alpha mt mt lng nng lng trung bnh khong 35 eV to ra mt cp ion. Do ht alpha c in tch ln hai ln so vi in tch ca ht beta v khi lng rt ln so vi ht beta nn vn tc ca n tng i thp, ion ho ring ca n rt cao, vo khong hng chc nghn cp ion trn 1 cm trong khng kh.

Tc mt nng lng tuyn tnh ca tt c cc ht tch in nng hn ht electron, trong c ht alpha, tun theo cng thc:

4z2q4 NZ 3.109 4

222

dE

ln 2Mv

v ln 1

v MeV

(1.12)

dxMv2 .1,6.106I

c2

c2 cm

trong : Z l s nguyn t ca ht gy ion ho, Z = 2 i ht alpha,

q = 1,6. 1019 C l in tch ca electron,

zq l in tch ca ht ion ho,

M l khi lng tnh ca ht gy ion ho, M = 6,6.1024g i vi htalpha,

v l vn tc ca ht gy ion ho,

N l s nguyn t ca cht hp th trong 1cm3, Z l s nguyn t ca cht hp th,NZ l s electron ca cht ht th trong 1cm3,

c = 3.1010 cm/s l vn tc nh sng,

I l th ion ho v kch thch trung bnh ca nguyn t cht hp th.

2. Qung chy ca ht alpha trong vt cht

Ht alpha c kh nng m xuyn thp nht trong s cc bc x ion ho. Trong khng kh, ngay c ht alpha c nng lng cao nht do cc ngun phng x pht ra cng ch i c mt vi cm, cn trong cc m sinh hc qung chy ca n c kch thc c micromet.

ng cong hp th ca ht alpha c dng phng v n l ht n nng. cui qung chy, s m ca cc ht alpha gim nhanh khi tng b dy cht hp th. Qung chy trung bnh c xc nh na chiu cao ca ng hp th, cn qung chy ngoi suy c xc nh khi ngoi suy ng hp th n gi tr 0.Qung chy ca ht alpha trong khng kh 0 0C v p sut 760 mmHg cbiu din mt cch gn ng vi sai s tng i 10% nh sau:

Rcm = 0,56 E (MeV)i vi E < 4 MeV(1.13) Rcm = 1,24 E (MeV) 2,62i vi 4 MeV < E < 8 MeV(1.14)Qung chy Rm ca ht alpha trong cc mi trng khc nhau c th tnh quaqung chy i vi mi trng khng kh R nh sau:

Rm (mg/cm2) = 0,56A1/3R(1.15)

trong A l s khi ca mi trng hp th.

1.4. TNG TC CA BC X GAMMA VI VT CHT

Nu khng tnh n phn ng ht nhn, di tc dng ca bc x gamma th tng tc ca bc x gamma bao gm: hiu ng quang in, hiu ng Compton v hiu ng to cp electron - posistron.

1. Hiu ng quang in

Hiu ng quang in l qu trnh tng tc ca lng t gamma v in t lin kt vi ht nhn. Trong qu trnh ny, ton b nng lng ca lng t gamma c truyn cho in t.

Te = E Ii(1.16)

trong : Te l ng nng ca electron pht ra (photo electron), E l nng lng ca lng t gamma,Ii l nng lng lin kt ca in t lp th i trong ht nhn.

Khi E < IK th hiu ng quang in ch c th xy ra trn lp L, M... v khng th xy ra trn lp v K; khi E < IL hiu ng quang in ch c th xy ra trn lp v M, N... v khng th xy ra trn lp K, L,

Hiu ng quang in khng th xy ra vi cc in t t do - cc in t khng lin kt vi ht nhn. Nng lng lin kt ca in t vi nguyn t cng nh so vi nng lng ca lng t gamma th xc sut hiu ng quang in cng nh.

Tng tc xy ra vi xc sut ln nht khi nng lng gamma va vt qu nng lng lin kt, c bit l i vi cc lp v trong cng. Khi nng lng

tng, xc sut tng tc gim dn theo hm

1 . Xc sut tng cng ca hiu ngE3

quang in i vi tt c cc electron qu o khi E EK, EK l nng lng lin

kt ca electron trn lp K, tun theo quy lut

qui lut 1 .E

1E 7/ 2

, cn khi E >> EK th tun theo

Do nng lng lin kt thay i theo s nguyn t Z nn tit din quang inph thuc vo Z theo qui lut Z5. Nh vy tit din quang in:

Z 5 photo ~ E 7/2

Z 5khi E EK v photo ~E

khi E >> EK.

Hiu ng quang in c tit din ln i vi cc nguyn t nng ngay c vng nng lng cao, cn i vi cc nguyn t nh, hiu ng quang in ch yu ch xy ra vng nng lng thp.

Khi hiu ng quang in xy ra, mt electron b bt ra khi mt lp no ca nguyn t s li mt l trng. L trng ny s c mt electron t cc lp ngoi ca nguyn t chuyn xung chim ch. Qu trnh ny dn ti lm pht cc tia X c trng hay cc electron Auger.

2. Hiu ng Compton

h

Hnh 1.1: Tn x Compton.

Trong hiu ng Compton, lng t gamma tn x n hi ln mt electron qu o ngoi ca nguyn t. Lng t gamma thay i phng bay v b mt mt phn nng lng, cn electron c gii phng ra khi nguyn t. Qu trnh tn x Compton c th coi nh qu trnh tn x n hi ca gamma ln electron t do.

Cng thc tnh nng lng ca lng t gamma b tn x vi gc nh sau:

h ' h / 1

h

em c2

(1 cos )

(1.17)

T cng thc (1.17), gc bay ca gamma sau tn x cng ln th h cng b, ngha l gamma cng mt nhiu nng lng. Gamma chuyn nng lng ln nht cho electron khi bay ra gc 1800, tng ng vi tn x ngc. Gc bay ra ca

gamma tn x c th thay i t 00 n 1800, trong lc electron ch yu bay v pha trc, ngha l gc bay ca n thay i t 00 n 900.

Tit din ca qu trnh tn x Compton t l thun vi in tch Z ca nguyn t v t l nghch vi nng lng ca lng t gamma, nh vy:

Z

compton ~E

(1.18)

Trong tn x th electron sau tn x tiu tn ng nng ca n theo c ch kch thch, ion ho mi trng mt cch trc tip nh ht beta.

3. S to cp electron-posistron

Khi tia gamma c nng lng rt cao (E > E0) cng vi hiu ng quang in v hiu ng Compton, trong qu trnh tng tc ca gamma vi vt cht cn xy ra hin tng to cp electron-posistron.

Qu trnh to cp khng th xy ra trong chn khng, m i hi phi ln cn ht nhn hoc in t. Thc vy, nu hin tng to cp xy ra trong chn khng, th theo nh lut bo ton nng lng v ng lng cn tho mn hai biu thc:

m c2

m c2

E

e

1 2e

e

1 2e

(1.19)

eeT cng thc (1.19) ta c:

P P P

(1.20)

E

m cm cm cm c

P ee

e e

e e

P P

c1 2e

1 2

e

1 2

e

1 2

e

ee

ngha l

P P

P

, iu ny tri vi (1.20), tc nh lut bo ton ng

ee

lng khng c tho mn. Nh vy, khi khng c mt ca ht nhn hocelectron, qu trnh to cp ca lng t gamma khng th xy ra.

e+

e

Hnh 1.2: Hin tng to cp.

Khi hin tng to cp xy ra trong trng Coulomb ca ht nhn hoc proton, ng nng git li ca ht nhn l nh. Nh vy, nng lng ngng E0 xy ra hin tng to cp ca lng t gamma cn ln hn hai ln khi lng ngh ca electron.

2E0 2mec

1, 022 MeV,

h T e

T e

2m c2 .

e y cn lu rng e+ v e c sinh ra trong in trng ca ht nhn, v th e+ b y ra xa v c tng tc, cn e b lm chm. Do , ph nng lng o c s khc nhau i vi 2 loi ht ny.

Khi hin tng to cp xy ra trong trng Coulomb ca electron, nng

lng ngng ca lng t gamma l

E 4m c2 2, 04 MeV .

e0Tit din to cp electron - posistron trong trng Coulomb ca in t b hn tit din hnh thnh cp trong trng ca ht nhn c 103 ln. Biu thc cho tit din to cp trong trng ht nhn kh phc tp. Trong min nng lng 5mec2 < E < 50mec2, tit din to cp c dng:

pair

~ Z 2 ln E

(1.21)

Theo cng thc (1.21), tit din to cp electron - posistron gn nh t l viZ2 nn c gi tr ln i vi cht hp th c s nguyn t ln.

4. Tng hp cc hiu ng khi gamma tng tc vi vt cht

Nh trnh by trn, khi gamma tng tc vi vt cht c 3 hiu ng chnh xy ra, l hiu ng quang in, hiu ng Compton v hiu ng to cp electron - posistron. Tit din vi phn tng tc tng cng ca cc qu trnh ny bng:

photo compton pair

(1.22)

trong tit din qu trnh quang in l photo ~

Z

Z 5E7 / 2

, tit din qu trnh tn x

2

Compton l compton ~E

v tit din qu trnh to cp l pair ~ Z

ln E .

tphophoto

tpair

compcompton

E

Hnh 1.3: S ph thuc ca tit din vo nng lng.

T s ph thuc cc tit din vo nng lng E ca tia gamma v in tch Z ca vt cht nh trn, suy ra rng trong min nng lng b hn E1, c ch c bn trong tng tc gamma vi vt cht l qu trnh quang in, trong min nng lng trung gian E1 < E < E2 l qu trnh tn x Compton v trong min nng lng cao E > E2 l qu trnh to cp electron - posistron. Cc gi tr nng lng phn gii E1, E2 ph thuc vo tng mi trng vt cht.

5. Cu trc ph gamma

E0

a)

E0-511keV

b)E0-1022keV

c)

d)

E0-1022keVE0-1022keV

E0-511keVE0

Hnh 1.4: Cu trc ph l tng ca ph tia gamma theo cc hiu ng:a) quang in; b) to cp; c) Compton; d) ph thc.

n gin, ta xt trng hp tia gamma i n detector c gi tr nng lng E0. Theo hiu ng quang in, nng lng ny bin thnh ng nng ca

photo - electron hoc cc electron Auger v b hp th ngay trong detector. Kt qu l tt c nng lng Eo ca photon b hp th trong detector to nn xung in, m xung ny c bin t l vi nng lng E0 v to nn mt nh trong ph gamma. nh ny c gi l nh quang in hay nh hp th ton phn. Vi ph k l tng, ph nng lng tng ng vi nh quang in c biu din bng mt vch thng ng nh hnh 1.4. Khi cc tia gamma n nng E0 gy nn tn x Compton vi vt cht, th cc electron tn x c ng nng Ec phn b lin tc t

30

gi tr 0 n gi tr cc i:

Emax

2 E01 2

, trong

E0

m c2e

. Trn hnh 1.4c

biu din ph l tng lin tc ca hiu ng Compton.

Gii hn cc i ca ph nng lng lin tc gi l mp Compton. Phn nng

lng

E0 EC

ca cc electron tn x c th b hp th do cc qu trnh khc

nhau v ng gp vo s hnh thnh xung in. Ngoi ra trn hnh cn c th xut hin mt nh rt nh nm vng nng lng thp, do tn x ngc ca cc lng t gamma vi lp bo v hay mi trng xung quanh detector. Nng lng Eng ca tn x ngc lin quan vi nng lng E0 ca photon ti v gc tn x theo cng thc:

Khi = th Eng

E0.1 2

Nu nng lng gamma ln hn nng lng E0 = 2mec2 = 1,022 MeV, th khi i qua in trng ca ht nhn n sinh ra mt cp electron - posistron (posistron c khi lng bng khi lng electron nhng mang in tch dng +1e). Posistron khi dng li c th kt hp vi 1 electron no t hu v sinh ra hai photon vi nng lng 0,511 MeV. Cc photon ny c th b hp th trong detector bi cc qu trnh bit. Do , trn ph ng vi qu trnh to cp (hnh 1.4b) cng xut hin mt nh hp th ton phn. Ngoi ra c kh nng mt hoc hai photon th cp bay khi detector. V th trn ph hnh 1.4b hnh thnh thm hai nh tng ng vi thot n v thot i ng vi nng lng E0 0,511 MeV v E0 1,022 MeV.

trn ta xt ring l tng hiu ng ca tia gamma khi i vo detector tng ng vi cc ng ph hnh thnh, nhng trong thc t c ba hiu ng nu trn xy ra mt cch ng thi, v th ph nng lng ca gamma thu c c dng phc tp hn, l s chng cht ca c ba hiu ng. Ngoi ra trong thc t chm gamma l

khng n nng v h thng khng phi l l tng, do ph nng lng gamma thc t c phn b nh hnh 1.4d. rng ca nh hp th ton phn ng vi rng na chiu cao ca nh (FWHM) gi l phn gii ca detector.

Chng II

DETECTOR GHI O BC X V S LIN KT

Bung ion ho,

ng m t l,

Detector nhp nhy,

Detector bn dn.

2.1. BUNG ION HO

1. Nguyn tc hot ng

1.1. Qu trnh vt l

Nh ta bit, s tng tc ca tia bc x vi vt cht lm ion ho cc nguyn t, phn t hoc dn ti kch thch chng. Nguyn l hot ng ca hu ht cc detector o bc x da trn c s ca s ghi in t hoc ion (nh bung ion ho, ng m t l, ng m Geiger-Muller, detector bn dn) hay ghi cc photon nh sng c pht ra bi cc nguyn t hay phn t b kch thch (nh detector nhp nhy).

IkIKNguyn l hot ng ca bung ion ho c ch ra trn hnh 2.1.

Ik

-Out

+

CRUk

.

Hnh 2.1: Cu to ca bung ion ha.

Nguyn tc ghi bc x ca cc detector cha kh u da vo hin tng ion ho xy ra trong mi trng cht kh khi bc x i vo detector. Ht tch in s mt mt phn nng lng W ca mnh trong cht kh gia cc in cc ca bung v to ra N cp in tch vi in tch ton phn Q Ne . Do c in trng gia hai in cc, cc ht tch in s chuyn ng v cc in cc tngng to nn dng Ik trn mch ngoi, y l mch RC. in trng trong bungion ho t l vi hiu in th ngoi Uk v in trng cn phi ln loi b c s ti hp ca cc ht in tch v nh trit c cc hiu ng, dn ti s gia tng s lng ht mang in do s t pht hoc ion ho do va chm.

Nng lng cn thit to nn mt cp ht in tch l (nng lng ny l khng i v khng ph thuc vo loi ht ion ho cng nh nng lng ca n, hoc l iu kin hot ng ca detector) nn ta c:

WN ;

W Q e

(2.1)

T biu thc (2.1), ta thy rng:

Nu W ch l 1 phn ca nng lng ton phn ca cc ht ion ho, ngha l nu kch thc hay th tch hot ng ca bung ion ho b hn qung chy ca cc ht bc x, th phn b thng k N tun theo l thuytLandau, ngha l ng phn b rng hn phn b Poisson N N ,

Nu nng lng ca cc ht ion ho hon ton truyn cho cht kh cabung, th quy lut thng k cng lch khi phn b Poisson, mc d

lch bnh phng trung bnh N

y l b hn.

Ngha l N

fN , trong f < 1 gi l h s Fano (th Poisson f 1).

1.2. Hnh thnh xung

n gin, chng ta gi thit rng s ion ho hnh thnh N ion dng v N

electron vi in tch ton phn Q Ne

xut hin trong bung v ti im x0.

Qu trnh phn b in tch dc theo trc x i vi ba dng bung ion ho cch ra trn hnh 2.1.

Dng bung Ik bao gm dng ion v electron: Ik =Ii +Ie

trong

I Q E ( x)v(i)v

iUk

E

I Q E ( x)v(i)

eUk

E

vi

v(i) iP

vv(e) eP

Do linh ng ca 103 , khi lng ca electron rt b so vi khi

eilng ion, v th thi gian chng chuyn t cc ny n cc kia i vi3

electron e

s nh hn 10

ln so vi ion

i . Gi tr ny thng vo

khong e 1s .

Bin cc i ca xung in Umax = Q C t c ch sau mt khong thi

gian ln

t 5i . Thng thng hng s vi phn tho mn theo iu kin

eimax RC . Suy ra trong bung ion ho phng: U Q e

w

vi nng

CClng trung bnh.

2. S ni vi tin khuch i

BITin khuch i (P.Amp) thng c t cnh detector, cch b tr ny nhm gim in dung k sinh li vo ca tin khuch i. i vi mt tin khuch i, phi m bo khng ch h s khuch i ln m cn m bo ti thiu t s tn hiu trn tp m.

Det

+3

A6Out2 -

2R1

U Q R1

UkCR

0C R

R2

.

Hnh 2.2: S P.Amp nhy in th.

Trong bung ion ho, cc ht ti in c to ra c 3000 eV hoc5.1015 C/1 MeV nng lng mt. Khi , in dung ca mch tch phn

C = 20 nF cho ra mt xung vi bin 0,25 mV/MeV. V th, xung ny cn phi c khuch i ln. Tp m ca cc s khuch i s hn ch phn gii nng lng hoc nhy ca h thng.

Cf

Det

BIUk

+362A-

U 0 C

Out

QC f

Hnh 2.3: S P.Amp nhy in tch.

Thng th s khuch i c thc hin bi s tin khuch i c tp mthp, sau l khuch i ph cho cc xung ra t tin khuch i.

Xung in p trn t C s c khuch i. in dung C l in dung ton phn ca mch ra bung ion ho (bao gm in dung ca bung ion ha, ca cp ni, in dung ghp ni bung ion vi tin khuch i v in dung mch vo ca tin khuch i). S khuch i c n nh nh vo lin kt ngc (hnh 2.2), m s lin kt bao gm b chia p R1, R2. H s khuch i bng R1/R2 khi R2 > R1. Thng thng la chn RC >> e (hoc RC >> i i vi bung ion ho c li). Khi t s tn hiu trn tp m (S/N) l tt nht. Nu trong cc b khuch i c hai mch vi phn, th mch vi phn th nht c th chn hng s thi gian RC e.

Bin xung ra ca tin khuch i nhy in p l U0

Q

Q R1 , cn vi tinC R2

Ckhuch i nhy in tch, bin xung ra l U0 f

khi

A ,

bin ny

khng ph thuc vo C cng nh cc yu t khc c lp trn mch. S khng ph thuc linh kin chnh l u im ca b khuch i nhy in tch. B tin khuch i l thit b c bit quan trng i vi cc detector bn dn.

2.2. NG M T L

Cc loi bung ion c khuch i nh trnh by trn u c cu to v nguyn tc hot ng c bn l ging nhau. ng m t l, in cc dng (anot) lm bng volfram hay st nhn c ng knh 0,05 0,3 mm. Cc m l mt tm ng hoc nhm dy c 0,05 mm c cun thnh ng. Cht cch in l thu tinh, s hay cht do. p sut trong ng m c 50 760 mmHg hoc c khi cao hn.

1. Qu trnh vt l v to xung

Nu in trng trong phn th tch nhy ca bung ion ho t n gi tr ln, cc electron trn qung chy t do c tch t nng lng ln. i vi s va chm ion ho phn t th s ht ti in trong mi ln va chm s tng ln hai ln, v

dng ca bung s tng ln

M 2n , vi n l s va chm trung bnh ca electron s

cp trn qung ng i ti anot. Vi qu trnh thc l, in tch ton phn:

Q M W e

Theo cng trnh ca Hanna G. Kirkwoad trn Phys. Rev, V.75, p.985 (1949) th tnh t l s c m bo, nu Q khng vt qu in tch ti hn Qt = 106 107 electron. Bc x ti c nng lng thp ch to ra mt lng khng ln electron nhng M c th t gi tr 106 n 107.

Thng ng m t l c dng hnh tr, dng ny cho in trng khng u. S khuch i kh s din ra trong min in trng ln gn trc (anot). Bi vy sau khi kt thc qu trnh cc nhanh ca s khuch i, in tch ton phn Q nm gn anot. Cc electron khi chuyn ng ti anot ch chu mt hiu in th rt nh, v th thnh phn in c th l 10% trong tng ca dng tch phn v c th b qua ton b dng tn hiu xut hin anot. Dng xung in p U(t) ca t tch phn C c xc nh bi cng thc:

Qb2 t

U (t) lg 1 22C lg(b / a)a T

khi b >> a

i

Hiu in th U(t) u tin tng rt nhanh v t n na gi tr cc i Q/C khi t1/2= i(a/b), sau tng chm v thng t s a/b c 1/103 th t1/2 = 0,1 1s. Do tn hiu U(t) c th khuch i vi hng s thi gian ca mch vi phn c1s, trong ch na bin xung b mt mt.

2. Minh ho thng k ca qu trnh nhn kh

Nh nu trn, bnh phng trung bnh N ca s N ht ti in th cp

bng N

f N .

Trong ng m t l, cn tnh n tnh thng k ca qu trnh

nhn s ht . Tho mn l thuyt Shider, phn b P1(m) ca s ton phn Melectron thc l, m thc l ny c gy bi mt electron th cp l1M

P1 M

exp , trong M l gi tr trung bnh ca h s nhn. Nh vy

MM

M lch bnh phng ca nhn kh M bng 2

M 2.

Theo l thuyt thng k th lch tng i

2 ca NM electron s l:

M N

22

2 1 M .

N M N

N M

Khi gi tr Fano f = 1 th

2 1 1 .

N MNN

Theo kt qu thc nghim th lch tng i c th biu din 0, 6 .N MN 0,4

3. S tin khuch i

m bo ch t l, in tch ton phn ca xung in t ng m phi nh hn gi tr in tch ti hn Q1 = 1013 n 1012 C. Bi vy bin cc i ca xung in khi in dung c 10 nF vo khong 10 mV 100 mV.

Bin ny rt b, v vy cn phi c khuch i ln. Cn lu rng, bin ny c mc ln hn mc tp m ca cc mch in t v thng th khng th

loi b tp m . V l i vi bung ion ho, in dung vo C thng l n nh nn cho php s dng tin khuch i nhy vi in p. c mc n nh

cao hn, c bit khi

M 100 hoc b hn th phi s dng cc b tin khuch

i vi mc tp m b v nhy vi in tch.

-12V

BID11MDet50

1MD21M

Q1 2N1307

Q22N1307 1047

Out

100K2K100K

+V high

Hnh 2.4: S tin khuch i ghp ni vi ng m t l.

Thng thng xung t ng m t l c bin ln, nn thng s dng tin khuch i vi cch mc emiter chung trn vi transistor (phi hp tr khng). Trong tng khuch i u tin, tn hiu c ly vi phn bi mch vi phn c hng s thi gian l 1 s, v th tt c cc hng s thi gian sau ca mch vi phn trong tin khuch i cn c gi tr ln hn. i vi xung m, tt hn ht l s dng loi transistor p-n-p. Cc diode Ge D1, D2 gii hn bin xung v bo v tin khuch i trong thi im ng ngun cao th cho ng m t l. Cc tnh ton chnh xc ca s c p dng trong cc trng hp khi tin khuch i lm vic trong h thng vi in trng cng cao, m in trng gy nn tp m ca tn hiu. Lc ny h thng cn b sung thm cc mc lc LC.

Hnh 2.5 ch ra h thng tin khuch i, khuch i c bn (AMP) v cc ng cp in cho n. Trng in t cng cao s to nn cc th U1 n U5 trn cc dy dn ghp ni. cc in p khng nh hng ti li vo tin khuch i, phi s dng cc cm khng L1 v L2. Vi cp ra tin khuch i di hn 2 m th phi bt buc xem xt n vn ny. gim nh hng ca U1, b tin khuch i phi c h s khuch i ln, cn tng vo b tin khuch i phi c ni trc tip vi cp.

LVHVAMPLow voltageHigh voltageU5U4

U1U2U3

Det

L1

P.A mpL2

Hnh 2.5: H thng ph k trong in t trng cao.

2.3. DETECTOR NHP NHY

1. Nguyn l hot ng ca detector nhp nhy

Tng tc ca cc bc x vi vt cht ngoi gy ion ho nguyn t v phn t cn dn ti s kch thch chng. Kt qu ca s kch thch lm pht sng. Detector nhp nhy hot ng da trn c s bin i cc photon pht ra t cht nhp nhy do s kch thch ca bc x thnh tn hiu in.

Cht nhp nhy c th l v c hoc hu c. Di y s trnh by vn tt mt s u, nhc im ca mt s cht nhp nhy ph bin c s dng trong ch to detector nhp nhy.

Cht nhp nhy v c:

NaI(Tl): c im ni bt nht l kh nng pht sng rt tt. nh sng pht ra rt tuyn tnh theo nng lng ca cc electron (v cc tia gamma). Hn ch l tinh th d b v do va p hoc sc nhit. Thi gian phn r ca xung nhp nhy vo khong 230 ns nn khng ph hp vi cc ng dng cn thi gian phn gii hoc tc m cao,

CsI(Tl) v CsI(Na): Cesium iodide c h s hp th gamma ln hn so vi Sodium iodide, c s dng trong cc ng dng cn cc detector c kch thc nh, khi lng trung bnh nhng hiu sut ghi ln,

LiI(Eu): c ch to vi giu Li cao v thng s dng xc nhneutron da trn phn ng 6Li(n, ),

Bismuth Germanate (BGO): u im chnh ca loi vt liu ny l c mt rt cao (7,3 g/cm3) v c s khi ln (83) nn tit din ca hiu ng quang in rt ln. Tuy nhin cng nh sng pht ra ch bng 10 20% so vi NaI(Tl) cn phn gii th km hn hai ln so vi NaI (Tl),

Barium Fluoride (BaF2): C Z cao, thi gian phn gii b hn 1 ns, thch hpdng cho cc detector nhp nhy c hiu sut cao, thi gian phn gii nhanh.

Cht nhp nhy hu c:

Dng tinh th tinh khit: Anthrancene cho hiu sut pht sng cao nht, cn Stilbene cho dng xung r rng. Do , lm tinh th nhp nhy hu c, cc vt liu trn c s dng ph bin. C hai vt liu ny u d v v kh ch to vi kch thc ln,- Dng dung dch: Cc cht nhp nhy hu c c s dng di dng dung dch ho tan. Nh hiu sut cao, cc cht ny c s dng ph bin trong o hot beta nng lng thp nh C14 v Tritium. Hoc s dng nhng ni i hi th tch detector ln, trong trng hp ny, t l nh sng pht ra ph thuc vo tng loi ht (d nng lng nh nhau). Cc cht nhp nhy ny cn c s dng o neutron. i khi s dch chuyn bc sng xy ra to thnh phn ui trongph bc x thu c t ng nhn quang,

Cht nhp nhy do: l cht nhp nhy hu c c ho tan trong dung mi, sau polymer ho v to dng cn thit. Cc detector ny khng t, c cung cp sn v c th ch to di nhiu kch thc khc nhau nh hnh tr, tm phng, chng kh thun tin s dng cho nhiu ng dng khc nhau. Cc cht nhp nhy do c thi gian phn r ngn (vi nano giy), do thun li cho cc thc nghim c tc m cao hoc trng phng nhanh,

Cht nhp nhy pha tp: cht nhp nhy hu c thng thun tin cho vic xc nh trc tip cc ht alpha hoc beta. Chng cng c th c s dng xc nh cc neutron nhanh thng qua cc proton git li. V cc cht nhp nhy lng c Z thp nn hu nh khng xy ra hiu ng quang in vi cc tia gamma, do lm tng phn b lin tc trong ph bin . ci thin tit din tng tc

40

quang in, mt s vt liu c s Z cao c pha thm vo trong cht nhp nhy (~ 10% trng lng ch hoc thic). Tuy nhin s b sung ny li lm gim cng sng.

Do nh sng pht t cc cht nhp nhy l rt yu, nn phi dng mt dngc c bit gi l b nhn quang in (PMT).

Ia(t) A-

Out

RC

KCDDD

Hnh 2.6: Cu to ca b nhn quang in.

Nguyn tc hot ng ca detector nhp nhy nh sau: Khi mt bc x i vo bn nhp nhy, chng s lm kch thch cc nguyn t hay phn t. Khi dch chuyn v trng thi c bn th s pht ra mt nh sng nhp nhy, l cc photon nh sng. Qua lp dn sng, cc photon i vo ng nhn quang in, tn hiu c nhn ln nhiu ln to ra mt tn hiu in kh ln li ra ca ng nhn quang in.

Bin xung li ra:

AA

C0CU0 eWPmax ()()d e N

trong , e l in tch electron, l s photon/1 n v nng lng W, () l ph pht x ca tinh th nhp nhy, nh vy () cho ta s photon pht ra trong mt n v chiu di sng.

() c chun ho bi: ()d 10

Pmax () xc nh xc sut gii phng ca mt photon electron t catod. Nh

vy (max ) 1; A l h s nhn ca nhn quang in trn mt photon electron.

C l in dung ca ti anod ca ng nhn quang in, N l s e t c cab nhn. Nh vy nng lng to ra mt photon electron l W , nh vy,Ni vi detector NaI(Tl), nng lng ny vo c 300 eV n 1000 eV i vi lng t gamma.

H s khuch i A ca b nhn quang bng tch ca cc h s pht th cp:

A n

vi n l s dinod. Thng thng

A 106 108 .

2. Hnh thnh xung

Cc phn t hoc nguyn t ca cht nhp nhy c kch thch thi im t = 0, chng c mt thi gian sng nht nh, bi vy cng ca nh sng pht x L gim theo quy lut hm m:

L(t) L

t expH (t)*

0 0

0 l thi gian lo sng ca cht nhp nhy,

H (t)*

l hm Heaviside.

Nh vy gia va chm ca photon v s gii phng cc electron khng ctnh tr, th tc pht electron t photo catod theo quy lut:

d N

N exp t H (t ).

dt 0

0

Sau b nhn quang in s hnh thnh mt xung dng:

2Ae t

ai (t) expt t

, vi t t g l thng ging ca thi gian bay.

tg

tg

Theo iu kin chun ho:

ia (t)dt Ae , suy ra vi N photo electron:

Ia (t)

AeN

1

t t' t exp[ ]exp H (t ')dt '

ttg 0

ttg

0

t

t

U0 (t) U0

exp

exp

H (t)dt ;

0

RC

0

U0

AeN .C

3. S tin khuch i ghp ni vi detector nhp nhy

Trong detector nhp nhy, bin xung ca tn hiu thng c gi tr ln hn mc tp m ca tin khuch i. Bi vy cc tin khuch i thng mc theo kiu lp li emiter. Trn hnh v l s nguyn l mch ra ca mt tin khuch i dng vi tn hiu li ra ca ng nhn quang in.

-12V

NaI(Tl)

2M

50Q1

Q22K

Q3

1M2M

+V high

47K

1075

10K2K

Out

Hnh 2.7: S P.Amp ghp ni vi detector nhp nhy.

Nu in th catod bng th t th tt c in p ra c 1 kV l in p ra trn anod. V vy tin khuch i cn ni qua t chia C1, cn Ca v Cb ni song song trn li ra ca b nhn quang in. Mt cch tng ng trn li vo ca tin khuch i: Cs = Ca + Cb l in dung ton phn mc song song. Ra l in tr anod, v Rb l in tr vo ca tin khuch i, thng Ca Cb 10 pF.

Khi

Ra , Rb , ta c:

U(t)

1I ( p)C1

1 .C1

ab1ababoutP(C C ) a(C C .C ) / (C C )

pCs

(C1 Cs ) / 4

Hng s thi gian ca mch:

(Ra || Rb )(Ca Cb ).

Khi tho mn iu kin a, a l thi gian lo sng ca b nhp nhy th:

U0 (t)

AeN

exp t H (t) U exp t H (t)

0Hm Heaviside:

Cs

H(t) = 1 vi t > 0 v bng 0 vi t < 0.

Thc t lch cn bc hai trung bnh ca in p ra khi ng c bn

(0 V) c xc nh:

U 2 U

1 R

00 2

trong R l gi tr trung bnh ca xung, R = 20H(t),

100 s ;

W 1 MeV ;

1000 eV / electron

A 106 ;

C 16 nF U

10V ;

U 2 10 V .

s00

2.4. DETECTOR BN DN

1. Nguyn l hot ng ca detector bn dn

Tng t nh qu trnh ion ho cht kh trong bung ion ho, qu trnh hnh thnh cc ht ti in trong cht rn cng c th s dng ghi o bc x. Thi gian ti hp trong cht rn ln hn do linh ng ca cc ht ti in l ln. Mt tnh cht quan trng na l do nng lng b nn bin tn hiu ln v phn gii tt.

Gi s ta s dng mt lp tip xc p-n c dy khc nhau, lp bn dn n l rt mng v c nng pha tp cht cho rt cao lm ca s vo i vi bc x, min vi cht bn dn p c nng tp cht va . Kt qu ca s khuch tn cc ht ti in trong vng cm lm xut hin in tch khng gian (x), in trng E(x) trong vng ny bin i theo quy lut tuyn tnh, cn in th theo quy lut parabol. in p ngc bn ngoi cng vi s khuch tn (tip xc trong) (Uk = 0,3 V vi Ge v Uk = 0,6 V vi Si) to nn hiu in th Uk gia lp

p-n. Cc ht ti in c to ra bi cc bc x ion ho trong vng cm s to nn dng tn hiu tng t nh bung ion ho trong in trng.

n -+p-+-++-+-+

x=0x=l

+-

E(x)

U(x)

Hnh 2.8: Phn b in tch, in th U(x), in trng E(x) trongdetector bn dn p-n.

T s thay i dng parabol ca U(x), rng l ca vng cm l:

l 2 eN p

U N Uk

trong l hng s in mi ca cht bn dn v Np l nng tp cht trongmin p, UN l in th ngun nui.

in dung ca vng cm: Cd

eN Sp2

1U N U K

vi S l din tch tip xc.

in trng cc i:

Emax 2

U N U K .l

Cu trc detector bn dn nh trnh by trn gi l detector hng ro mt. in

tr sut

102 104 v chu th ngc 200 V 300 V. Vi chiu dy lp

singho (p) c 0,5 n 1 mm ghi nhn bc x beta vi nng lng 500 keV, ht ca cc ng v phng x t nhin v ht p c gia tc nng lng thp. Mt in dung k sinh c 55 pF v tp m c th lm ti kh nng phn gii n 7 keV.

n -i+p-+-+-+-+

x=0

x=l

+-

E(x)

U(x)

Hnh 2.9: Phn b in tch, in th U(x), in trng E(x) trongdetector bn dn p-i-n.

m rng vng cm vi in trng khng i, c th thc hin bng cch a vo gia lp n-p mt lp khng cha tp cht (min i) to nn detector p-i-n. in trng:

E(x) U N U K .l

Nh vo vic m rng thm vng khng tp cht i, in dung Cdet ca detector gim ng k, c khong 10 pF vi din tch ln c 5 cm2, cn trong detector p-n vi din tch c 2 cm2, Cdet vo khong 50 n 100 pF.

Dng r ca detector bn dn (dng ngc cn bng khi khng c s ion ho bn trong) bao gm dng mt, dng khi, dng khuch tn. Dng ny c gy nn bi cc ht ti in khng c bn (trong cht bn dn, phn t ti in no c mt ln hn gi l phn t ti in c bn v phn t cn li gi l khng c bn. Nh vy trong bn dn n phn t ti in c bn l electron, cn trong bn dn p l l trng). Cc phn t ti in ny pht sinh do dao ng nhit trn mt khong cch nh hn chiu di khuch tn t cui ca lp i. Khi chiu di ca lp i l ln th dng khuch tn c th b qua so vi dng khi I0. Khi dng khi c dng:

I Sl e.ni0

ni l mt ca cht bn dn khng pha tp,

l thi gian sng ca cc ht ti in.

Trong Si vi tp cht nng thp (detector loi p-i-n), nhit T = 300 K,

1s , ni = 1,5.1010 cm3, th

I / Sl 1 A / cm3.

Bi vy trong detector, vi

0S = 2 cm2 v l = 5 mm th

I0 1 A.

2. S tin khuch i

Trong detector bn dn, in dung tip xc Cdet ph thuc vo cao th lm vic UH v khng th coi n l hng s, ngay c khi UH l n nh. Bi vy tch phn xung dng cn phi s dng cc b tin khuch i nhy in tch. Hng s

thi gian mch vo: H s khuch i:

T0 (R || ri )Cn

A( p) Sn (R || ri ) A11 p 0

+VCC

R

A1Out

Cn

SiRi

Cf

IoC

Hnh 2.10: S tin khuch i ghp ni vi detector bn dn.

C

B chia in dung C, Cf cho h s

b fC C f

, v vy h s khuch i vi

lin kt ngc bng

A( p) / 1 bA( p), th nh ca li ra c dng:

U ( p) I

Ii.

A( p)

0i p(C C

) 1 A( p)C

/ (C C )

fff

= Ii ( p)

1p C f (C C f ) / A0

.11 p

Trong :

A0 Sn (R || ri ) A1 , Sn l h s bin i, hay dc bin i ca

transistor trng (FET) hoc transistor vi in tr ni l ri. Khi

A0C f

C C f

th:

dU0 dCC( 1

) dC

1 , trong

Ad A0 (C f

/ C) l

u0C A0C f

1 (C C f ) / A0C f

C Ad

h s khuch i d tr.

Hng s thi gian:

C C f

f0 f0 C C A C

Ai R || rd

+V high

1M

Cd

100K R

100K

0.1

91K

82K2K

10

3K9

+12V

Out

4.72K

1MR

0.01

In

1.1 39K

Q4 Q2

2N779

NPN

10

Test

1.0

Q12N779 10

Q3 2N779

100

100K30K

2K710

Hnh 2.11: S tin khuch i s dng transistor.

Trn hnh 2.11 l s tin khuch i s dng transistor. tn hiu in p t c bin ln th t phn hi Cf khng c qu ln (Cf c gi tr t 0,5 n 5 pF, ngha l Cf