Summary of High Energy Particle Detector Elements

Transcription

1 1 Summary f High Energy Particle Detectr Elements Cntents Abstract... 1 Phtmultipliers vs. Phtdides... 2 Phtmultiplier tube... 2 Phtdides... 3 Preamplifier... 4 Amplifier... 5 Multi-Channel Analyser (MCA)... 8 Summary f Detectr Specificatins Detectr respnse Preamp sensitivity Pulse risetime Peaking time Shaping time FWHM(Γ) Energy Reslutin: Detectr efficiency Dead time: Pulse pile-up: Cnversin gain: Series and parallel nise Fan factr (F) Differentiatr circuit Integratr circuit Single Channel Analyser: Abstract The fllwing dcument aims t familiarize the reader with detectr elements that fllw a detectr s active vlume. It als tries t incrprate different detectr specificatins and definitin f related terminlgy.

2 2 Phtmultipliers vs. Phtdides Phtmultiplier tube It is a device that detects scintillatins given ut by the detectr material and generates crrespnding utput pulses. It is essentially like a fast amplifier, which in a few nansecnds amplifies an incident pulse f visible light by a factr f 10 6 r mre. A phtmultiplier cnsists f tw parts: Phtcathde A phtsensitive layer cupled with the detectr which absrbs the scintillatins prduced and generates a flw f electrns (phtelectric effect) Incident phtns transfer their energy t electrns, which migrate t the surface and have t vercme the ptential barrier (wrk functin) t escape Quantum Efficiency (QE) = n f phtelectrns emitted / n f incident phtns. QE is a functin f the wavelength f incident phtns. Hence the phtcathde chsen has the highest QE ver the wavelength range f the scintillatins prduced in the detectr. The average QE is arund 20-30% Electrn Multiplier Structure A lng tube-like structure that is an efficient cllectr gemetry fr phtelectrns. Serves as a near ideal linear amplifier as the utput pulse at the ande is prprtinal t the number f phtelectrns ver a large amplitude Phtelectrns generated at the phtcathde are made t accelerate and strike the surface f an electrde (called a dynde) kept at a high psitive ptential and cause a re-emissin f mre electrns frm its surface. A series f such dyndes are arranged at increasing ptentials, generating mre electrns at each stage The signal is thus amplified t arund electrns at the ande (utput stage)

3 3 Phtdides Phtdides are slid state devices that generate a current flw when light f suitable wavelength falls n them. When perated in reverse bias, the width f the depletin regin (als called the intrinsic r i-regin since it has n impurity atms) at the p-n junctin increases and this is where energy frm the incming scintillatin is absrbed t generate electrn hle pairs. The band gap in semicnductrs is arund 1-2 ev. Phtns frm the scintillatins carrying 3-4 ev pass thrugh the thin p-layer and create electrn-hle pairs in the i-regin These charge carriers are immediately swept away t ppsite ends f the dide by the applied external field befre they can recmbine. This is detected as the utput current pulse whse magnitude is prprtinal t the incident energy f the scintillatin The quantum efficiency is arund 60-80% (much higher than phtcathdes) since charge carriers are directly generated by the light pulse. Als this high QE spans a larger range f wavelengths Other advantages f using phtdides are gd energy reslutin, lwer pwer cnsumptin, cmpact size and insensitivity t magnetic fields One disadvantage is that n amplificatin f the induced charge in the i-regin ccurs (as in a PMT) befre reaching the preamplifier s the utput signal is smaller by rders f magnitude and is affected t a cnsiderable extent by electrnic nise Anther surce f nise is dark current which increases with temperature. Thus phtdides need t be maintained within a certain range f temperature using cling mechanisms t prevent distrtin f the pulse

4 4 Preamplifier Cuples the detectr with the rest f the circuit electrnics Prvides impedance matching with the rest f the circuit t prevent attenuatin f the signal Cnverts the charge cllected at the detectr t a crrespnding utput vltage pulse Designed t minimise any nise that may be generated. One way is by placing it as clse as pssible t the detectr t reduce cable length The preamp utput typically has (1) a small amplitude, in the range f a few mv, (2) a fast rise (tens f nsec t μsec), and (3) fllwing the signal, a slw decay (f the rder f 100s f micrsecnds) In sme preamps, there is n tail after the signal. A radiatin interactin generates a vltage step, and then the utput is cnstant until the next step. When the utput level reaches a certain threshld after many such steps, it is reset t its initial value Fr a charge sensitive preamplifier (CSPA): The charge cllected in the detectr is cnverted t a vltage value. The utput vltage V 0 = Q/C f, under the cnditin that A C f >> C i, where C i = input capacitance and C f = utput capacitance Nise generated at this stage is due t the input FET and input capacitance and resistance. It increases with the input capacitance, which is present due t the detectr itself and the cnnecting cable Output pulse f the preamplifier has a fast risetime f a few nansecnds and a slw expnential decay f ~100 micrsecnds

5 5 Typical CSPA Circuit Preamplifier utputs Preamplifier utputs V ut Threshld level t (r)

6 6 Amplifier Amplifies and shapes the preamplifier pulse utput Uses a cmbinatin f differentiatr and integratr circuits (CR-RC) t shape the pulse in a manner it can be used fr analysis (t study pulse height, risetime etc) The different types f shaping include: CR-RC (Uniplar shaping): This is a differentiatr circuit fllwed by an integratr A unity buffer pamp prvides impedance islatin between the tw circuits The preamp utput is shaped int a uniplar pulse, whse shape depends n the time cnstants f bth circuits Fr a step vltage input f step value E, The utput is a uniplar pulse: with V = (Eτ 1 /τ 1 -τ 2 )*(e -t/τ1 -e -t/τ2 ) CR-RC circuit CR-RC-CR (Biplar shaping): This is a CR-RC cmbinatin fllwed by a differentiatr circuit Unity buffer pamps prvide impedance islatin between the three circuits The preamp utput is shaped int a biplar pulse, with tw lbes f equal area abve and belw the zer baseline

7 7 The time taken by the pulse t drp back t the zer level after reaching the peak depends n the risetime f the unshaped pulse, s this methd may be used fr risetime discriminatin This shaping is generally used fr high cunting rate applicatins, but the S/N rati is lwer than that f a uniplar pulse Biplar pulse CR-(RC) n (Gaussian shaping): This is a differentiatr circuit fllwed by many integratrs The pulse shape appraches a Gaussian with increasing number f integratrs. In practice, n=4 is sufficient fr the difference between the pulse and a true Gaussian t becme negligible A Gaussian shaped pulse has better S/N characteristics than a nrmal uniplar pulse, but greater width f the pulse causes pile-up at higher cunting rates Shaping f the preamplifier pulse is required t: Optimise S/N rati. Fr a specific shape f the pulse, cntributin due t series and parallel nise is minimised Prevent pulse pile-up Differentiate between the types f incming radiatin (Pulse Shape discriminatin). Fr certain detectr materials the rise/decay time f the pulse varies depending n the incming radiatin Ideally shuld have cnstant amplificatin fr all pulses. Hwever, this is nt s due t Electrnic nise generated within the circuit Temperature variatins causing small changes in R and C values and transistr functining Pulse pile-up: depends n cunting rate

8 8 Multi-Channel Analyser (MCA) Is a device that digitises the amplifier utput by using an Analg t Digital Cnverter(ADC) The digital memry is divided int a certain number f channels, each f which crrespnds t a fixed energy (r vltage) band depending n the maximum recrdable vltage (Fr example, if the maximum pulse height crrespnds t 10 V, and the number f channels is 1000, the vltage band f each channel is 10/1000 =.01 V. S the first channel will stre the number f pulses with vltage V; the secnd, the number f pulses with vltage V and s n. Larger the channel number, greater the energy it crrespnds t) When a pulse enters the ADC, 1) An input gate prevents acceptance f anther pulse until the current pulse is fully prcessed and registered 2) A capacitr starts charging and cntinues until the peak f the pulse is reached 3) Vltage n the charged capacitr is nw discharged by a cnstant dc current 4) An scillatr clck (f a fixed high frequency- a few hundred MHz) starts when the discharging prcess begins. The clck stps when the capacitr is fully discharged. The number f scillatins prduced is prprtinal t the pulse height, and thus t the incming vltage (r energy depsited). The time fr which the clck runs is called cnversin time 5) Depending n the number f scillatins, the pulse is stred in the channel crrespnding t that vltage (r energy) band (Memry cycle time) An ADC that uses the abve methd f digitizatin is called a Wilkinsn type ADC. There are ther types f ADCs als available The final utput is a graph f the number f cunts recrded in each channel. This graph is called a pulse height spectrum

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10 10 Summary f Detectr Specificatins Detectr respnse: The utput pulse generated when energy is depsited in the detectr. (It is the respnse f a detectr t incident radiatin.) It is the redistributin f energy int the vltage (r channel) dmain. When mnenergetic radiatins strike the detectr, nly a delta functin is expected t be seen in the pulse height spectrum. Hwever the pulse spreads ut due t: Statistical nature f interactins f the particles: Fr example, tw particles with the same energy need nt depsit equal amunt f energy in the detectr due t even a slight difference in their path r the way they interact with the material. Spreading due t this factr cannt be prevented Electrnic nise, which can be minimised by apprpriately adjusting circuit parameters Preamp sensitivity = V/E, r amplitude f vltage generated by a charge sensitive preamplifier per unit energy depsited in the detectr Pulse risetime: Time taken fr a pulse t rise frm 10-90% f its full amplitude Peaking time: Time taken fr a pulse t g frm the zer level t its maximum amplitude Shaping time: The value f the time cnstant f each RC circuit in the amplifier

11 11 FWHM(Γ): Full Width at Half Maximum. The width f the respnse curve at half f its peak value Energy Reslutin: Fr the respnse curve generated at a particular energy value E 0, reslutin = (FWHM at E 0 )/E 0 and is expressed as a percentage. Lwer this value, better the reslutin. Assuming Γ remains apprximately cnstant ver a small energy range, tw energy peaks at E1 and E2 can be reslved nly if Γ <= E1-E2 Detectr efficiency = number f particles detected per unit time / number f particles impinging upn the detectr per unit time. Its value depends n Density and size f detectr material Type and energy f radiatin Dead time: The minimum amunt f time that must separate tw events in rder that they be recrded as tw separate pulses. It is determined by bth the detectr itself and assciated electrnics. Dead time may be decreased by reducing decay time f the vltage pulse, increasing clck speed f the scillatr in the ADC and reducing the cnversin gain Pulse pile-up: The verlap f ne signal ver anther that ccurs when a pulse is detected during the prcessing f the previus pulse, ie, within the dead time. It causes the secnd pulse t have a higher amplitude and thus gets incrrectly recrded. Decreasing the decay time f a pulse reduces pile-up. Cnversin gain: Number f discrete channels int which the input pulse range is divided in the MCA

12 12 Series and parallel nise in PIN detectrs Series nise riginates frm within the preamplifier input stage and increases with detectr capacitance. It is als inversely prprtinal t the shaping time f the pulse Parallel nise ccurs due t fluctuatins in the leakage current in the phtdide. Leakage current decreases with detectr capacitance and is directly prprtinal t shaping time f the pulse Thus there is an ptimum shape f the pulse t minimise the effect f series and parallel nise Fan factr (F) The number f electrn hle pairs generated in a detectr = E/w, where E is the average energy depsited and w, the energy required t create an electrn hle pair If the prcess is purely statistical, Pissn distributin applies and the standard deviatin (σ) in the number f pairs created is sqrt(e/w). F is defined as σ 2 /n f charge carriers prduced, r F = σ 2 /(E/w), r σ = sqrt(fe/w). F is intrduced t accunt fr the fact that the standard deviatin is lesser than the calculated theretical value, ie, the prcess is nt purely Pissnian F = 0 means that there are n statistical fluctuatins and all the energy depsited prduces charge carriers F =1 means the number f pairs created is gverned by Pissn statistics 0<F<1 means the fluctuatins are less than the theretical value σ, shwing that the events recrded are interdependent. Pissn statistics applies t independent events Differentiatr circuit Under the cnditin that RC<<T(time perid f the pulse), V =RC*dV i /dt Acts as a high pass filter, ie, attenuates the lw frequency cmpnents but high frequency cmpnents retain their amplitude When used n a pulse, it causes it t decay faster

13 13 Integratr circuit Under the cnditin that RC>>T(time perid f the pulse), V =(1/RC)* V i dt Acts as a lw pass filter, ie, attenuates the high frequency cmpnents but lw frequency cmpnents retain their amplitude Can be used t decrease the rise time f a pulse, and als smthen ut effects f nise n a pulse Single Channel Analyser: Sets tw threshlds, a lwer ne (called lwer level discriminatr) and an upper ne (called upper level discriminatr) Cunts all pulses whse vltage peaks lie between these threshld values A multichannel analyser is like a series f cntiguus single channel analysers and cunts the peaks in each band