the determination of photomultiplier temperature coefficients for gain and spectral sensitivity using the photon counting technique

Transcription

1 technical reprint R/P081 the determinatin f phtmultiplier temperature cefficients fr gain and spectral sensitivity using the phtn cunting technique

2 the determinatin f phtmultiplier temperature cefficients fr gain and spectral sensitivity using the phtn cunting technique A S Singh and A G Wright, ET Enterprises Limited, Riverside Way, Uxbridge, UB8 YF, United Kingdm technical reprint R/P081 abstract The literature,3,8 describes mainly the measurement f the phtmultiplier temperature cefficient, althugh authrs such as Murray and Manning 1 and Kinard 4 fr example, have attempted t quantify these cmpnents separately In the first instance 1 the phtmultiplier was als perated as a dide t prvide the phtcathde sensitivity cefficient In principle it is pssible t deduce the multiplier cefficient, but in practice this is unreliable, as these authrs admit, because the cefficients are small and nt easily measured anyway Kinard 4 adpted the dubtful prcedure f attempting t cl the dyndes alne apparatus and methd The putput signal f a phtmultiplier is sensitive t changes in temperature Tw cefficients are required t specify this dependence: the multiplier gain cefficient α m and the spectral sensitivity cefficient α(λ) α m and α(λ) can be separated by using the phtn cunting technique Results, cnsidered t be superir t thse previusly reprted, are presented fr a range f phtmultiplier types intrductin The lng term stability f any detectr incrprating a phtmultiplier is cnditinal upn the temperature dependence f the phtmultiplier respnse, which itself has tw cmpnents The temperature dependence f the phtcathde sensitivity and f the multiplier gain need t be separately specified by tw independent cefficients In certain applicatins it is acceptable t utilize a cmbined cefficient, which will be referred t as the phtmultiplier cefficient Semi-transparent phtcathdes becme highly resistive at lw temperature 1 At sufficiently high light levels the phtcurrent can distrt the equiptential f the cathde and thereby suppress phtelectrn cllectin at the first dynde This was nicely demnstrated 1 in tubes fabricated with a cnducting underlayer t the phtcathde Cmpared with a cnventinal phtcathde these special tubes exhibited a higher respnse at lw temperature It is clear that t quantify the temperature dependence f a phtmultiplier requires a cefficient α m t accunt fr the multiplier temperature sensitivity and a cmplex cefficient fr the phtcathde α(i k, λ) where I k and λ refer t the cathde current level and the wavelength f the light, respectively At lw and mderate light levels α (I k,λ) α (λ) which is the cefficient actually measured in this wrk The methd adpted is based n the phtn cunting technique Under single phtn cnditins the variatin in the cunt rate is used t deduce α(λ) while the shift in the peak f the single electrn respnse relates t α m In this way the tw cefficients are simultaneusly, yet independently, measured The apparatus, shwn in figure 1, cnsisted f a mnchrmatr (Hilger and Watts Type D9), with a wavelength selectin ver the range nm husing and light splitter test phtmultiplier thermcler MCA TN 1705 mnchrmat narrw band filter reference phtmultiplier figure 1 schematic layut f the apparatus used t measure temperature cefficients Fine wavelength tuning and harmnic eliminatin was accmplished with narrw band dielectric filters, Balzers B40 15 nm bandwidth, lcated at the utput frm the mnchmatr Tw clear glass slides acted as beam splitters directing apprximately equal, very lw intensity, light beams t the tw phtmultipliers The rientatin f the glass slides was adjusted t image the mnchrmatr slit n t the centre f each phtcathde This was accmplished by temprarily substituting the clear plates with mirrrs f the same dimensins

3 Tw phtmultiplier husings were rigidly blted t the main husing The variable temperature husing, PFR Type TE104 was used in tandem with a thermcirculatr The liquid frm the thermcirculatr culd be varied in temperature frm - O C t 40 O C and served as the heat exchange medium fr the thermelectric cler In this manner an verall range frm -0 O C t 40 O C, was attained The temperature was recrded using the thermcuples attached t the edge f each phtcathde windw By using a dummy phtmultiplier cntaining a thermcuple, it was fund that the time fr the dyndes t reach equilibrium was f the rder f half an hur, subsequent t changing the perating temperature; in the experimental wrk, an hur was allwed t elapse The phtmultipliers were perated frm independent psitive high vltage supplies; the chice f psitive plarity was deliberate because best stability with phtmultipliers is always btained with the earthed cathde cnfirguatin The utput pulse height distributins frm the phtmultipliers were encded and recrded using a Tracr 1705 multichannel analyser The chice f the phtmultiplier fr the reference channel is critical; the requirement being that it must remain stable in all respects ver the entire duratin f a cycle f tests - in practice abut twelve hurs A specially selected and aged phtmultiplier was used fr this purpse A cmplete system check was carried ut t verify the stability and cnsistency f the entire apparatus and methd Over a preid f three days f cntinuus measurement the same pair f tubes was repeatedly measured ver the entire temperature and wavelength band The reprductibility was fund t be entirely satisfactry prvided that the high vltage was applied fr at least twelve hurs prir t measurement Because f the design f the experiment, the results are indepentent f fluctuatins in the light surce intensity prvided that utput pulse height distributins are simultaneusly measured frm the pair f phtmultipliers This requires tw multichannel analysers, which prved t be incnvenient In practice it was fund that with the light surce in cntinuus peratin, ne analyser was sufficient, prvided that the acquisitin time fr results was f the rder f a minute Dark cunts were measured fr bth phtmultipliers by interpsing a shutter fllwing the acquisitin f the pair f signal results The dark cunts were subtracted frm the initial measurements t give the true signal rate Measurements were repeated at each temperature until successive sets f signal - backgrund results agreed Usually n mre than tw sets were required at each temperature results Ten phtmultipliers selected frm the types listed in table 1 were measured The spectral sensitivity 5 was measured fr each phtmultiplier t btain λ 0, the lng wavelength cut-ff (the wavelength at which the quantum efficiency is <001% table 1 these particular types were selected because f the high gain capability and well-reslved single electrn reslutin (SER) The frmer is a requirement fr phtn cunting whilst a gd SER is a desirable althugh nt essential attribute fr the methd The effect n the gain upn cling by 40 O C is illustrated in figure Cunts phtmultiplier type phtcathde KSbCs S0 KSbCs RbCsSb dynde material BeO BeO SbCs BeO number studied figure the single electrn respnse f a phtmultuiplier measured at tw temperatures The shift in the peak psitin is used t calculate αm and the change in area under each curve relates t the change in quantum efficiency Nte the peak shifts t the right with cling α Channels 0 C -0 C 3 Using a cmbinatin f neutral density filters and adjustment f the light intensity, the single electrn cunt rate was set in the regin f 1 - x 10 4 per secnd This is a satisfactry cmprmise between attaining acceptable cunting statistics withut incurring excessive analyser dead-time

4 The shift in the peak psitin f the SER can easily be estimated t half a channel, crrespnding t 05% The change in quantum efficiency and hence α(λ) is calculated frm the rati f the integral cunts at the tw temperatures Fr this purpse the integral was taken frm 05 t 4 phtelectrns equivalent, where the peak by definitin is taken as ne phtelectrn equivalent These limits are nt critical in the calculatin f α(λ) as can be readily seen frm inspectin f the SER The quality f the SERs fr the phtmultipliers selected varied with regard t their individual peak-tvalley ratis Hwever, withut exceptin, it was nted that the shape f the SER was invariant t temperature The results fr six phtmultipliers are shwn in figure 3, which illustrates that all samples tested have a negative gain cefficient, althugh f different magnitude α(λ) % C -1 KSbCs λ nm α(λ) % C # 443, λ = 70 nm 9813 # 4406, λ = 680 nm RbSbCs 9954 # 5814, λ = 70 nm # 5813, λ = 715 nm -6 g % x cathde dyndes x bialk bialk S 0 BeCu SbCs BeCu Temperature C x x figure 3 multiplier gain changes derived frm pairs f curves similar t thse seen in figure g is the change in gain raltive t 10 O C The straight line represents α = -0% / O m C Nte that the SbCs dyndes appear t have the same temperature dependence as d the BeCu dyndes The temperature dependence f the phtcathde respnse is illustrated in figure 4 The shapes f the α(λ) vs λ curves appear t have a cmmn prfile (Serial Number 4406 is the exceptin) α(λ) is negative at 400 nm, apprximately zer fr nm, and then negative until λ 0 is apprached There is a crrelatin between the cut-ff f the spectral respnse, λ 0, and the wavelength at which α(λ) begins t change very rapidly with temperature (fr the S0s in figure 4 this wavelength is apprximately 850 nm) Measurements were taken at ten degree intervals, and it was fund that ver the range -0 t 40 O C bth cefficients were temperature independent λ nm α(λ) % C # 6494, λ = 850 nm 9813 # 6497, λ = 860 nm NaKSbCs λ nm figure 4 phtcathde sensitivity cefficients, α (λ) fr the three phtcathde types The cut-ff wavelengths are given fr each phtcathde discussin Miyazawa 6 has suggested that the lng wavelength sensitivity is related t a change in phtelectric wrk functin φ with temperature Fr an S11 phtcathde he qutes the fllwing results: d φ/dt ~ - 4 x 10-4 ev/ O C It is interesting t nte that this relatinship is apprximately true fr the range f phtcathdes studied in the present wrk With regard t temperature dependence f the secndary emissin cefficient, Dekker 7 prvides a frmula that ascribes the effect t a decrease in energy lss due t lattice scattering Fr MgO, a dynde material similar t BeO, Dekker calculates α m as -00% per decree C The phtmultipliers used in this investigatin were either 1 r 14 stage devices which crrespnds t -0016% / O C per stage, which is f the same rder as fr MgO

5 It is difficult t put a figure n the accuracy f the measurements because f the unknwn systematic errrs Hwever, using repeated measurements as a guide, the present results are reliable t 5% There are practical implicatins f this wrk, sme als nted by Yung, which are wrth expanding upn Cpyright 1987 IEEE Reprinted with permissin, frm IEEE TRANSACTIONS ON NUCLEAR SCIENCE, Vl NS-34, N 1, pp , February Cling the phtmultiplier enhances the quantum afficiency t wavelengths appraching λ 0 In the immediate regin f λ0 cling reduces the quantum efficiency If a phtmultiplier is used at wavelengths clse t the cut-ff then clearly the respnse will be sensitive t small changes in temperature t the extent f 1% per degree C In cling a phtcathde ne is usually trading dark cunts fr a quantum efficiency, but belw a certain temperature the signal t backgrund rati may well decrease The extent t which a detectr is sensitive t temperature depends n the applicatin Yung has already pinted ut the advantages f phtn cunting in astrnmical applicatins - the results are insensitive t changes in multiplier gain In applicatins such as scintillatin cunting and spectrmetry, the utput is sensitive t bth α(λ) and α m references [1] R B Murray and J J Manning, Respnse f End-Windw Phtmultiplier Tubes as a Functin f Temperature IRE Trans NS-7, pp 80-86, 1960 [] A T Yung, Temperature Effects in Phtmultipliers and Astrnmical Phtmetry Applied Optics, Vl, N 1, pp 51-60, January 1963 [3] M R Cle and D V Ryer, Cling f PMT Tubes fr Best Spectral Respnse, Electr-Optical System Design, Vl 4, N 6, pp 16-19, June 197 [4] F E Kinard, Temperature Dependence f Phtmultiplier Gain, Nuclenics, pp April 1957 [5] Electrn Tubes Phtmultiplier Brchure, p5, 1986 [6] H Miyazawa, Phtelectric Emissin and Energy Structure f Cs3Sb, J Phys Sc Japan, Vl 8, N, March 1953 [7] A J Dekker, Slid State Physics, Lndn, Macmillan, 1958 [8] H Martin, Cling Phtmultiplier with III-V Phtcathdes, Electr-Optical Systems Design, pp 16-0, Nvember 1976

6 talk t us abut yur applicatin r chse a prduct frm ur literature: phtmultipliers, vltage dividers, signal prcessing mdules, husings and pwer supplies ET Enterprises Limited 45 Riverside Way Uxbridge UB8 YF United Kingdm tel: 44 (0) fax: 44 (0) sales@et-enterprisescm web site: wwwet-enterprisescm ADIT Electrn Tubes 300 Crane Street Sweetwater TX USA tel: (35) tll free: (800) fax: (35) sales@electrntubescm web site: wwwelectrntubescm chse accessries fr this pmt n ur website an ISO 9001 registered cmpany The cmpany reserves the right t mdify these designs and specificatins withut ntice Develpmental devices are intended fr evaluatin and n bligatin is assumed fr future manufacture While every effrt is made t ensure accuracy f published infrmatin the cmpany cannt be held respnsible fr errrs r cnsequences arising therefrm ET Enterprises Ltd, 011 DS_ R/P081 Issue 3 (18/01/11)