cac dau thu quang buc xa

7/30/2019 Cac Dau Thu Quang Buc Xa

1/16

THIT B QUANG T

--------------------------

K54V_CNQT

= Nhm 2 =

Ch:

CC U THU QUANG BC X

Ging vin: Ngc Chung

Sinh vin thc hin:

ng B Tun

L Th Thu H

Ng Vn Hi


2/16

Cc u thu quang bc x

4.4 Cc u thu quang bc x (photoemissive detectors)

Cc u thu quang bc x nh t bo quang in, nhn quang in u da trnhiu ng quang in ngoi. Quang m cc ca cc u thu ny c ph mt hay nhiulp vt liu c cng thot nh (ngha l cc thnh phn kim loi kim hay bn dn).

Di nh sng n sc khi ri sng cso bc sng = c/v quang in t b bc x khiquang m cc vi ng nng cho bi h thc Einstein

Edn =

Sau chng c gia tc bi hiu in th Vo gia dng cc v m cc v cgp li trn dng cc.

Hiu ng quang in

Hiu ng quang in l mt hin tng in lng t, trong cc in t cthot ra khi vt cht sau khi hp th nng lng t cc bc x in t

Khi b mt ca mt tm kim loi c chiu bi bc x in t c tn s ln hn mttn s ngng (tn s ngng ny l gi tr c trng cho cht lm nn tm kim loi ny),ccin t s hp thn ng lng t ccphoton v sinh ra dng in (gi l dng quangin). Khi cc in t b bt ra khi b mt ca tm kim loi, ta c hiu ng quang inngoi (external photoelectric effect). Cc in t khng th pht ra nu tn s ca bc xnh hn tn s ngng bi in t khng c cung cp nng lng cn thit vtra khi ro th (gi l cng thot). in t pht x ra di tc dng ca bc x in tc gi l quang in t. mt s cht khc, khi c chiu sng vi tn s vt trntn s ngng, cc in tkhng bt ra khi b mt, m thot ra khi lin kt vi nguynt, tr thnh in t t do (in t dn) chuyn ng trong lng ca khi vt dn, v ta chiu ng quang in trong (internal photoelectric effect). Hiu ng ny dn n s thayi v tnh cht dn in ca vt dn, do , ngi ta cn gi hiu ng ny l hiu ngquang dn.

mi tn s bc x v mi kim loi, cng dng quang in (cng dngin t pht x do bc x in t) t l thun vi cng chm sng ti.

Vi mi kim loi, tn ti mt tn s ti thiu ca bc x in t m di tn s, hin tng quang in khng xy ra. Tn s ny c gi l tn s ngng,hay gii hn quang in ca kim loi .

trn tn s ngng, ng nng cc i ca quang in t khng ph thuc vocng chm sng ti m ch ph thuc vo tn s ca bc x.

Thi gian trong qu trnh t lc bc x chiu ti v cc in t pht ra l rt ngn,di 10-9 giy.

Nhm 2 [Ng V n H i, L Th Thu H, ng B Tu n] Page 2
http://vi.wikipedia.org/wiki/Kim_lo%E1%BA%A1ihttp://vi.wikipedia.org/wiki/%C4%90i%E1%BB%87n_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/%C4%90i%E1%BB%87n_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/N%C4%83ng_l%C6%B0%E1%BB%A3nghttp://vi.wikipedia.org/wiki/Photonhttp://vi.wikipedia.org/wiki/D%C3%B2ng_%C4%91i%E1%BB%87nhttp://vi.wikipedia.org/w/index.php?title=R%C3%A0o_th%E1%BA%BF&action=edit&redlink=1http://vi.wikipedia.org/w/index.php?title=C%C3%B4ng_tho%C3%A1t&action=edit&redlink=1http://vi.wikipedia.org/wiki/%C4%90i%E1%BB%87n_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/Nguy%C3%AAn_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/Nguy%C3%AAn_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/%C4%90i%E1%BB%87n_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/N%C4%83ng_l%C6%B0%E1%BB%A3nghttp://vi.wikipedia.org/wiki/Photonhttp://vi.wikipedia.org/wiki/D%C3%B2ng_%C4%91i%E1%BB%87nhttp://vi.wikipedia.org/w/index.php?title=R%C3%A0o_th%E1%BA%BF&action=edit&redlink=1http://vi.wikipedia.org/w/index.php?title=C%C3%B4ng_tho%C3%A1t&action=edit&redlink=1http://vi.wikipedia.org/wiki/%C4%90i%E1%BB%87n_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/Nguy%C3%AAn_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/Nguy%C3%AAn_t%E1%BB%ADhttp://vi.wikipedia.org/wiki/Kim_lo%E1%BA%A1i


3/16

Cc u thu quang bc x

4.4.1 Quang m cc (Photocathode):

Photocathode l cc m ca mt thit b cm bin quang, v d nh: Nhn quang in

(Photomultipliers), Pin quang in (Phototube), n c bao ph vi mt hp cht nhyquang. Khi chiu photon n, nng lng c hp th, nng lng ny lm cho electronb bc x ri khi photocathode do hiu ng quang in.

Trong nhiu nm photocathode l phng php thc t duy nht chuyn i nhsng thnh dng in. Do n l yu t quan trng trong cc thit b quang in t.

Photocathode hot ng trong chn khng. Trong hot ng ca quang m cc i himt in trng vi mt anode tch cc gn m bo pht x electron.

Quang m cc (Photocathode)

S pht x quang in t

Mt photocathode thng l mt bn dn, n c th c m t bi cu trc vng nhtrnh by hnh di



4/16

Cc u thu quang bc x

Trong cu trc vng ca bn dn, tn ti vng cm hay khe nng lng (EG) mkhng b chim bi in t. Khi chiu nh sng vo photocathode, cc electron trongvng ha tr hp th nng lng photon (hv) v b kch thch, khuch tn v pha b mtphotocathode. Nu cc electron ny c nng lng vt qua mc chn khng(vacuum level barrier), chng s c gii phng vo chn khng nh quang in t(photonelectrons). iu ny c th c th hin qua hiu sut lng t n(v), tc l t sgia electron thot ra v photon ti c cho bi cng thc:



5/16

Cc u thu quang bc x

Trong :

Pv: Xc xut nng lng electron hp th ln hn mc chn khng

R: H s phn x

k: H s hp th y ca photon

L: chiu di thot ra ca electron khi b kch thch

Ps: Xc xut in t b mt photocathode thot khi photocathode

Photocathode c th c phn loi da trn qu trnh pht x ra quang in t:ch phn x v ch truyn dn. Photocathode ch phn x thng c hnhthnh trn mt tm kim loi, v quang in t c pht ra theo hng ngc li vihng ca photon ti. Photoncathode ch truyn dn thng c ch to l mtmng mng trn thy tinh trong sut, quang in t c pht ra theo cng mt hngvi photon ti.



6/16

Cc u thu quang bc x

Nhn quang in s dng photocathode ch phn x.



7/16

Cc u thu quang bc x

4.4.2 T bo quang in

T bo quang in l dng c c bn nghin cu hin tng quang in. N lmt bnh bng thch anh, thnh mng, bn trong l chn khng, thnh trong ph mt lpkim loi dng lm cathode. gia bng l c mt vng dy kim loi dng lm anode.

Lp kim loi c ph sao cho khng tip xc in vi anode v c cha mt ca s cho nh sng chiu vo cathode



8/16

Cc u thu quang bc x

4.4.3 Nhn quang in (Photomultipliers-PM):

ghi mc nh sng yu, nhn quang in t ra thch hp hn cc t bo quangin v chng hn ch nhiu tt hn bi s khuch i ni ti ca dng quang in khi s

dng bc x in t th cp ni ti nhn s quang in t. cc quang in t bc x tm cc c gia tc nh mt hiu in th vi trm vn v c hi t trn b mt kimloi ca int th nht, mi in t v chm lm bn ra trung bnh q in t th cp.cc in t th cp ny li c gia tc n int th 2, ti y mi in t th cp lito ra q in t th cp v c nh th ta c s in t th cp tng theo 1 cp s nhnc cng bi q. q ph thuc vo th gia tc U, vo gc ti v vo vt liu lm inot.



9/16

Cc u thu quang bc x

Tiu biu khi U=200V th q=3-5. Mt nhn quang in vi 10 inot do c

khuch i dng ton phn l G= 10 5 710 10q .

Mi quang in t trong nhn quang in vi N inot to ra mt thc l in tchti dng cc

Nhng hiu ng c th ng gp cho thi gian tng trng ca xung dng cc,gy ra bi khong thi gian i qua i vi nhng in t khc nhau:

Gi s rng 1 quang in t c bc x t quang m cc v c gia tc nint th nht. Vn tc ban u ca cc in t th 2 thay i do cc in t ny cgii phng t nhng su khc nhau ca vt liu lm int v nng lng ban u, khichng ri khi mt int, nm gia 0 v 5eV.

Thi gian truyn qua gia 2 in cc song song vi khong cch d v hiu th V :

2s

t d m eV v

= =

i vi in t c khi lng m xut pht vi nng lng ban u bng khng.Nhng in t vi nng lng ban u l Ekm n in cc tip theo sm hn 1 khongthi gian:



10/16

Cc u thu quang bc x

1 2 kmd

t mEeV

=

Cc in t i qua qung ng hi khc nhau cht t trong ng nn gy ra thmmt khong thi gian:

2 2t d m eV =

c cng ln. Thi gian tng trng ca xung dng cc, c bt u t 1 quang int, do gim khi tng th t l vi V -1/2.N ph thuc vo dng hnh hc ca cc cutrc int.

Khi mt xung sng mnh v ngn sn sinh ra nhiu quang in t ng thi th thi giantng trng do tng do 2 hin tng:

Vn tc ban u ca cc quang in t b bn ra khc nhau (VD: m cc X-iAntimoin S5 c v t 0 n 2eV). Khong thi gian ph thuc vo bc song canh sng ti.

Thi gian di chuyn ca in t gia m cc v int th nht ph thuc rt nhiuvo v tr ca chm sng trn m cc ti quang in t c bn ra. Khongthi gian thu c c th ln hn do nhng hiu ng khc, tuy nhin c th gimbt do in cc hi t gia m cc v int th nht khi ti u ha th ca n.Thi gian tng trng ca dng cc in hnh i vi nhn quang in trong

khong t 0,5-20ns. i vi nhng ng thit k c bit vi dng hnh hc cti u ha trong vic ri sng m cc t mp ca ng th thi gian tng trng0,4ns t c. Thi gian tng trng ngn hn c th thu c vi nhng bnmt c rnh.

Vn c ch nhiu khi thu nh sng yu trong nhn quang in c tm quan trngc bn. C 3 ngun nhiu chnh:

Nhiu dng ti ca nhn quang in.

Nhiu ca bc x ti. Nhiu t bin.

Chng ta s tho lun ring r cc nhiu ny.



11/16

Cc u thu quang bc x

Nhiu dng ti: sinh ra khi khng c nh sng ti. Khi nhn quang in hot nghon ton ti, nhng cc in t vn b bn ra khi m cc. Dng ti ch yu l do bcx nhit v ch 1 phn do s phn r nhng ng v phng x ca vt liu nhn quang.

Theo nh lut Richarson v dng bc x nhit:

22

1

CTi C T e

=

Ta thy rng dng bc x nhit ph thuc rt nhiu vo nhit ca m cc T v vocng thot .

Nu nhy ph ko di n vng hng ngoi th cng thot cn phi nh. gim dng ti, nhit T ca m cc cn phi gim Nhit hot ng ti u ph thucvo loi m cc (do ph thuc ). Khng nn lm lnh qu mc v c th gy ra hiu

ng khng mong mun nh gim dng quang in hoc gim th qua m cc do in trca mn m cc tng khi nhit gim.

i vi nhiu ng dng quang ph ch cn mt phn nh din tch ca m ccc ri sng, chng hn i vi nhn quang in t pha sau ca khe ra my n sc.Trong trng hp ny ngi ta c th lm gim dng ti hoc dng nhn quang in cdin tch hiu dng nh ca m cc hoc t cc nam chm nh xung quanh m cc rng.T trng s khuch tn cc in t t phn ngoi ca din tch m cc. Cc in t nykhng th n int th nht nn khng ng gp g vo dng ti.

Nhiu ca bc x ti (Nhiu photon):L do cc thng ging ngu nhin t thn ca ccphoton cha trong thng lng ti t

Nhiu t bin (hay nhiu khuch i):

Trong nhn quang in, dng in to cp c khuch i bi hiu ng ion hacc nguyn t bng va chm. Dng in ny cng to ra ting n v hiu ng khuch itrong diode quang lm nhn ln ting n ny. Th nhng qu trnh nhn in bng c chion ha bi va chm cc nguyn t, bn thn n cng l 1 qu trnh ngu nhin v qutrnh ny lm cng thm rt nhiu ting n cho dng quang in cung cp bi diode

quang nhn in.

Vi tha s khuch i G, th hiu nhiu trung bnh qua in tr ti dng cc R do bng:



12/16

Cc u thu quang bc x

( ) 2s cV GR ei f = ci l dng m cc

2 aR eGi f= ai l dng dng cc

Bn thn tha s khuch i G khng c nh m th hin cc nhiu lon donhng bin thin ngu nhin ca h s bc x th cp q l mt s nguyn nh. iu nyng gp cho nhiu ton phn v nhn gi tr trung bnh th hiu nhiu t bin cho mttha s a>1 ph thuc vo gi tr trung bnh ca q.

Nhiu nhit (Nhiu Johnson) : l do thng ging ngu nhin trong chuyn ng nhitca cc ht ti in trong cc in tr ca mch in nhit T. Chuyn ng nhit cacc ht ti in to ra 1 dng in ngu nhin trong vt liu, ngay c trong trng hpkhng c in trng ngoi. Nh vy, dng in chuyn ng nhit trong mt in tr R

l mt dng in ngu nhin m gi tr trung bnh bng khng ( ) 0i t = ngha l trong tt

c cc hng u bng nhau.

Theo cng thc Nyquist cho dng nhiu trung bnh l:

4i kT f R=

Ta c tng s nhiu t bin v nhiu Johnson qua in tr ti dng cc R nhit phng 4kT/e=0,1V

2( ) (2 0,1)J N a

V eR f RGa i+

= + [Volt]

Khi 2 aRGa i >>0,05V ngi ta c th b qua nhiu Johnson. Vi h s khuch i

G=106 v in tr ti R=105 th dng dng cc ai phi ln hn 5.1013A. T khi dng

ti dng cc ln hn rt nhiu gii hn trn th chng ta thy rng nhiu Johnsonkhng cn ng gp g vo nhiu ton th ca nhn quang in.

Mt s ci thin ng k cho t s nhiu trn tn hiu khi ghi nhn bc x yu cth t c vi k thut m tng photon, cho php nhng nghin cu quang ph c ththc hin i vi cc thng lng bc x nh hn 10-17W

4.4.3 Khuch i nh quang in.

My khuch i nh quang in (Photoelectric image intensifiers) gm mt quang mcc, mt dng c to nh quang in v mt mn hunh quang nh khuch i ca



13/16

Cc u thu quang bc x

m hnh bc x trn quang m cc c ti hin bi cc quang in t c gia tc. Cct trng hoc trng in c th c dng to nh m hnh m cc trn mn hunhquang. Thay v nh khuch i c quan st trn mn hunh quang, nh in t c thc dng trong ng camera nhm to ra tn hiu nh c th ti to trn mn nh v

tuyn v c th c lu tr trn my nh hoc my ghi. Trong p dng quang ph, ccc trng quan trng ca khuch i nh l :

o Tha s khuch i cng M: t s cng ra trn cng vo.

o Dng ti ca h: gii hn cng sut nh nht cn ghi c.

o phn gii khng gian ca dng c ni chung c xc nh bi s ng song

song cc i trn mm ca nh trn m cc cn c th phn ly trong nh ra sau khikhuch i.

o phn gii thi gian ca h, khi ghi cc tn hiu vo ngn, xy ra nhanh.

B khuch i nh quang in mt tng vi s hi t t trng

Hnh minh ha mt my khuch i n gin, mt tng khuch i vi t trngsong song vi in trng gia tc.Tt c cc quang in t xut pht t im P trnm cc di chuyn theo ng xon c xung quanh ng sc ca t trng v hi ttrn im P trn mn hunh quang sau mt s t vng quay.

V tr im P, gn ng bc mt, khng ph thuc vo phng ca vn tc banu chm quang in. c th tha s khuch i, gi thit rng hiu sut lng



14/16

Cc u thu quang bc x

t ca quang m cc l 20% v th gia tc l 10kV. Vi hiu sut 20% chuyn it nng lng ca cc in t sang nng lng ca nh sng trn mn hunh quang,mi in t s to ra khong 1000 photon vi hv = 2eV.

Khi tha s khuch i ca s photon ra trn s photon vo s l M=200. Tuy

nhin, nh sng t mn hunh quang bc x theo mi phng v h quang hc ch thuc mt phn nh. iu ny lm gim tha s khuch i cui cng.

Hiu sut gom gp c th c gia tng khi mt ca s bng mica mng csng cho mn hunh quang ta vo do nh tip xc c in ln y. Ngoi ra tac th dng ca s si quang.

B khuch i nh nhiu tng:

a) s h thng ca-tt Ci, mn hunh quang P1, v cc in cc trn cung cp thgia tc

b) chi tit cu trc kp ca ca-tt ln quang

Tha s khuch i ln hn c th t c vi cc ng khuch i nhiu tng,trong ta ni tip cc b khuch i n vi nhau. B phn quan trng ca dng c

ny l lp trung gian gia quang m cc v mn photpho, phn ny nh hng n nhy v phn gii khng gian. Khi nh sng bt u c bc x t mt vt sngxung quanh im P trn cht hunh quang gii phng cc quang in t t vt sngi din xung quanh im P ca quang m cc, khong cch P v P phi l nh nhtc th khng lm nh hng n phn gii khng gian. Do mt lp mngcht hunh quang (c vi m) vi ht rt mn c phun ln mt tm mica dy vi m



15/16

Cc u thu quang bc x

bng phng php in phn. Mt l nhm phn x ngc nh sng t cht hunhquang tr li quang m cc nhm ngn nga s phn hi quang ln m cc trc .

phn gii khng gian ph thuc vo cht lng to nh, cht lng ny b nhhng bi dy ca lp trung gian gia quang m cc v mn hunh quang, bi

tnh ng nht ca t trng v bi khong vn tc ngang ca quang in t. Ccquang m cc nhy vi mu ni chung c phn gii khng gian thp hn do ccvn tc ban u ca cc quang in t ln hn. phn gii tm l ln nht vgim dn ra ngoi.

Bng s liu ca b khuch i nh gm ba tng:

Kiu ng knh

hu ch

(mm)

phn

gii

[linepairs/mm]

tng

ch

RCA 4550 18 32 3.104

RCAC33085DP

38 40 6.105

EMI 9794 48 50 2.105

Multichannelplate

40 35 1.107

B khuch i nh c th dng mt cch thun li khi t n sau my quang ph ghi kh nhy mt vng ph rng. Gi s tn sc di ca mt my quang ph c

kch thc trung bnh l 10 0A /mm. B khuch i nh c kch thc m cc l 30mm

v phn gii khng gian l 30 vch/mm cho php ghi nhn ng thi mt khong

ph 3000

A vi phn gii l 0,30

A vi nhy ln hn nhiu bc so vi knh nh.

Khi quang m cc c lm lnh th nhiu do nhit c th gim n mc bng vinhn quang in v cng sut bc x ti vi photon c th ghi nhn c. Ngi ta trin khai vic phi hp b khuch i nh vi vidicon hoc vi dy diode rt hu ch o mt vng ph rng, nhanh v nhy.



16/16

Cc u thu quang bc x

H khuch i OMA nh vy hu dng khi ghi nh sng c cng nh trongmt vng ph rng. Chng c nhng u im sau:

o Cc bia vidicon lu gi cc tn hiu quang hc v cho php tch li trong mt

khong thi gian di, trong khi nhn quang in ch phn hng khi bc x p lnm cc.

o Tt c cc knh ca vidicon thu nhn ng thi cc tn hiu quang hc. Khi lppha sau cc my quang ph, h OMA c th o mt vng ph rng ng thitrong khi nhn quang in ch thu nhn bc x i qua khe, khe ny xc nh

phn gii. vi phn gii khng gian 30 vch/mm v tn sc di 5 0A /mm ca

my quang ph, tn sc ph l 0,7 0A .Mt bia vidicon c di 16mm c th

ghi ng thi mt vng ph rng 80 0A .

o Tn hiu c c chuyn sang dng s bng in t. iu ny cho php d dng

my tnh x l v phn tch s liu.

o Nhn quang in c quang m cc ln dng ti t tt c yu t ca m cc

c cng vo tn hiu. Trong b khuch i nh pha trc vidicon, ch mt chmnh ca quang m cc to nh trn mt diode duy nht. Do tt c dng ti tm cc c phn b cho ton vng ph m OMA bao trm.

o B khuch i nh c th che chn nn cho php ghi nhn nhng tn hiu c thi

gian phn gii cao. Nu cn o s ph thuc thi gian ca mt phn b ph thxung m c th p t vi thi gian tr c th thay i nn ton b h thng gingnh my tch hp boxcar c thm phn th hin ph. Dy diode hai chiu cngcho s ph thuc thi gian ca cc xung n v s phn b ca chng cng cth hin nu nh sng i qua khe vo ca ph k c qut (chng hn dnggng quay) song song vi khe .

Nh vy cc h OMA hoc OSA phi hp v nhy cao, thu ng thi mt vngph rng v kh nng phn gii thi gian. Cc phm cht ny lm tng vic s dng

chng mt cch rng ri trong quang ph.

cac dau thu quang buc xa

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