Chap. 9 Photomultiplier Devices

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Chap. 9 Photomultpler Devces The scntllaton process produces photons n proporton to the prmary onzaton (or n some cases, the range) we need to count the number of photons to obtan the energy deposted

1 Chap. 9 Photomultpler Devces The scntllaton process produces photons n proporton to the prmary onzaton (or n some cases, the range) we need to count the number of photons to obtan the energy deposted by the prmary radaton n the detector. Fg. 9. Knoll, 3 rd Ed. Photocathode / photoelectrc effect Varous coatngs, low w & hgh quantum effcency Electrons avalanche down a strng of dynodes (8-4) Dynodes are also coated to enhance cascades HV can be postve or negatve (schematcs later) Vacuum tube nternal getter to mantan vacuum Low potassum glass ( 40 K) KE of electrons start out very low some electron optcs and external magnetc shelds DJMorrssey, oo9

Insulators/semconductors are better than metals, all electrons are bound.

2 PMTs Photocathode Photocathode materal should be matched to the output spectrum of the scntllator and s characterzed by a quantum effcency η N e / N hν or radant senstvty n ma/w or by lumnous senstvty n μa/lm. Insulators/semconductors are better than metals, all electrons are bound. Free electrons n a conducton band tend to rescatter the Photoelectron and thermalze t before t can leave the metal. Balkal K CsSb η ~ 5% The layer s very thn (~30 nm) to allow the P.E to escape the layer lowers absorpton Fg. 9. Knoll, 3 rd Ed. DJMorrssey, oo9

3 PMTs Secondary Emsson The electrons are accelerated between dynodes ΔV ~ 00 V, penetrate nto the materal (recall de/dx ) and release secondary electrons. Secondary emsson from the dynodes s characterzed by a coeffcent: N s / N p. Typcal dynode materal s MgO, BeO, Cs 3 Sb, recent materals are negatve electron affnty GaP:Cs ( see Fg. 9.4 ) Secondary energy spectrum (ncdent energy) From Burle s Photomultpler Handbook DJMorrssey, oo9 Typcal value 5, range 3 0 Total gan n tube: Μ N Example: 5, N0 gves Μ x0 6 % change, 5.05, (5.05 / 5) 0 ~ 0 %

4 PMTs Tme Dstrbuton Electron optcs between cathode and frst dynode focus adjustment Nonrelatvstc veloctes: qv ½ mv Transt tme: t ~ N Δx/ v (N / V) Tme Varance: total Cath Dyn /( ) Fgs. 9.0, Knoll, 3 rd Ed. DJMorrssey, oo9 Beware of cartoon, t ~ 50 ns, ~ - ns

5 PMTs Sngle Photoelectron Spectrum PMTs can be run n hgh-gan modes that are senstve to sngle photoelectrons. In such cases the anode current must follow a Posson dstrbuton characterzed by a mean and wdth of one. Multple photoelectrons can be easly dstngushed. Only sngle photoelectron events Curve from many events made up from ndvdual events wth dscrete numbers of photoelectrons. From C.Andreopoulos, et al. NuMI-ANA-994 US/PHYS/HEP/ DJMorrssey, oo9 (Related Fg. 9.6 n text shows separate peaks for n 4 )

6 DJMorrssey, oo9 PMTs Resoluton The cascade across one dynode: ( ) N collecton IP collecton Photons PhotoElectrons N S N N N where M α ηε ηε α α α α x & > large then f L L L -th sequental, dentcal dynode: α If number of photoelectrons s Posson : Potental problems: Are photons monochromatc? η Are photoelectrons monoenergetc? ε Is S a constant? η < so the number of photoelectrons essentally determnes the resoluton

7 PMTs other resoluton ssues Stray B-felds use so-called mu-metal or ron shelds Dfferental senstvty of photocathode surface dffuse lght over surface Dark current thermal photoelectrons, electronc nose, cosmc rays Hgh voltage stablty ~ V n where n ~ (number stages mnus a few) Gassy tubes electrons onze resdual gas and gve afterglow. Photocathode glass transparent to uv or not? DJMorrssey, oo9

8 PMT Bases Voltage Dstrbuton Vacuum barrer ORTEC 66 (mnmal) base HV can be postve or negatve If () then anode s HV, AC-coupled DJMorrssey, oo9 If ( ) then anode s Gnd, DC-coupled cathode s HV glass becomes charged Example: Estmate the anode current n a typcal PMT for a NaI(Tl) scntllator. NaI(Tl) ~40k photons/mev, τ 30ns, PMT: η 0.5, M0 6 rough Photoelectron Rate ~ Ν(e - )/τ ~ 43 /ns I d/dt ~ (43 e - /x0-9 s) 0 6 (.60x0-9 coul/e - ) I ~ 7x0-3 A 7 ma!! for one pulse Crude approach: set current n Resstor chan 0x I but ths leads to hgh power dsspaton P V I ~ 00W Fg. 9.3 Knoll, 3 rd Ed. Bypass capactors: C ~00 /V so that the voltage drop s no more than % of V ORTEC 96 actve base

9 PMTs DC Pulse shape Model of the connecton to the anode of a DC-coupled PMT C (t) R (t) 0 e λ t wth λ from scntllator, wth boundary condton: (t) λ e λ t ( t) λe ( t) λe dv dt λ t λ t λ e C λ C R dv C dt t V RC V R ( t) dq dt V ( t) R 0 λ t 0e dt 0 / λ Fg. 9.9 Knoll, 3 rd Ed. V ( t) λ Θ λ C Θ t λ t [ e e ] Θ RC Pulse maxmum, set dv(t)/dt 0 Θ Θ Θ λ t max ln ln V ( tmax ) Θ λ λ λ If slow electroncs: RC >> τ then Θ << λ and V(t max ) /C If fast electroncs: RC << τ then Θ >> λ and V(t max ) λ/θc << /C DJMorrssey, oo9 λ C Θ Θ Θ λ Θ λ Θ λ [ ] λ Θ λ λ

10 Electron multplers other devces Channeltron: essentally a contnuous dynode n a curved tube. Typcal Phllps devce.5 μm φ 5 μm apart ε geo ~ 0.55 Chevron confguaton s opaque GEM gaseous electron multpler: dervatve of gas-flled proportonal counter Copper Kapton Copper DJMorrssey, oo9 Typcal devce 70 μm φ 40 μm apart, M ~ 0 3

Light Collection. Plastic light guides

Light Collection. Plastic light guides Light Collection Once light is produced in a scintillator it must collected, transported, and coupled to some device that can convert it into an electrical signal (PMT, photodiode, ) There are several