Cours de méca du point L1 UJF

Johann Collot

Description

Subatomic particle detectors Johann Collot [e-mail] http://lpsc.in2p3.fr/collot UdG P. 1G eneral properties of detectorsA detector always has an active medium in which particlesinteract and generate signal carriers (ion-electron pairs, Signal photons, electron-hole pairs.) which are subsequently collectionparticlecollected to form an electronic signal. deviceexample 1 : ionization chambercharged A ctive medium particle scintillatorexample 2 : scintillating counterphotomultiplier converts light into electronic signalelectronic preamplifiersignalanode collectingionized electrons electronic charged reflector for signal particle photon collectionJohann Collot [e-mail] http://lpsc.in2p3.fr/collot UdG P. 2modes of detector operationt1C urrent mode : Measures current averaged over time interval T : It= it'dt'∫T t−Ti(t)This mode is used when only the average currentinduced by particle interactions is of interest orI (t) when the interaction rate is much too high.Examples : neutron flux detectors in nuclear reactors.A ctive dosimeters. B eam diagnostics. tPulse mode : The characteristics (time, amplitude..) of each pulse are individually i(t) measured. This mode is used when detecting individual particles is ofinterest. I t requires fast electronics.Examples : all kinds of particle detectors used in high energy physics.tt t t1 2 3Johann Collot [e-mail] http ...

Informations

• Publié par: Vyof
• Nombre de lectures: 17
• Langue: English

Extrait

Johann Collot clool@tnip2.3rf              ht  /:ptspl/ni.c.3p2ollofr/ctUdG

P. 1

Subatomic particle detectors

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Johann Collot  

anode collecting ionized electrons

Gene properties of detectors ral A detector always has an active medium in which particles inte gene e signal carriers (ion-electron pairs, ract and rat photons, electron-hole pairs..) which are subsequently collected to form an electronic signal.  particle

electronic signal [e-mail] http://lpsc.in2p3.fr/collot

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electronic signal preamplifier

example 1 : ionization chamber               

charged  particle Active medium scintillator example 2 : scintillating counter photomultiplier converts light into electronic signal

Signal collection device

Measures current averaged over time interval T : I  t = T 1 t − ∫ t T i  t '  dt ' This mode is used when only the average current induced by particle interactions is of interest or when the interaction rate is much too high. Examples : neutron flux detectors in nuclear reactors. t Active dosimeters. Beam diagnostics. The characteristics (time, amplitude...) of each pulse are individually measured. This mode is used when detecting individual particles is of interest. It requires fast electronics. Examples : all kinds of particle detectors used in high energy physics. t t 3 co

Current mode : i(t)

t t 1 2 Johann Collot  

P. 3

Pulse mode : i(t)

I(t)

modes of detector operation

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Mono-channel detectors : channel means electronic channel

active medium

Detector types

Multi-channel detectors :

4π detectors : Detectors which measure particles over full solid angle.

signal collector Counters : Usually a detector which selectively measures one type of particles over a certain parameter range. Multi-detectors : A i-detector is made of several sub-detectors, mult each sub-detector being a multi-channel detector. Nowadays, particle colliders are equipped with  4π multi-detectors. example : detector on e + e - collider Johann Collot   [e-mail] http://lpsc.in2p3.fr/collot UdG P. 4

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Each measurement comes with a measurement error which - in subatomic physics - is always stochastic. Suppose a detector measures the physics observable z. Then z is a random variable t with D(z) its measuremen probability density. D(z) is called the detector response function of z. ∫ D  z  dz = 1 By definition of a probability density. z If the number of error sources on z is big, the central value theorem states that z is normally (Gaussionnally) distributed. This explains why very often, repeated measurements of the same physics observable are normally distributed (but not always) . The absolute resolution of z is the standard deviation of D(z).   if z - z 1 < σ z and z 1 are confused 2 2 σ z an r if z 3 - z 1 >> 3 d z 1 a e separated z 3 z collot

z 1 z 2 Johann Collot  

Detector response f ction un

D(z)

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This is why semiconductor cryogenic detectors are becoming more and more popular. and

If N follows a Poisson probability density law, 

The relative energy resolution improves when number of signal carriers gets bigger !

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and therefore the absolute energy resolution is given by :   E = k   N  h lution is then :   E  = 1 T e relative energy reso E  N

The standard deviation of the energy response function is the absolute energy resolution of the detector.

Energy resolution

To measure the energy of a particle, one collects the signal carriers created in the active medium. a linear medium (immense majority of detectors), the energy deposited by particle is In a proportionnal to the number of charge carriers : E = k N

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Fano factor  

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But for a given deposited energy, N cannot be arbitrarily big ! So N does not strictly follow a   Poisson distribution and in general its fluctuations from the average value are less than predicted by a Poisson distribution. F , the Fano factor , is then defined by :

F = observed variance of N or   E  = F N E observed N

F is always less or equal to 1 . Example : low energy protons in Si , F = 0.16

F depends on the energy and the type of the particle as well as on the medium.

  E  a co ed = N F = F k =  S where : S =  F k is nstant. E observ E E

S is sometimes called the stochastic term of the energy resolution.

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constant term

Noise and inhomogeneity contributions to energy resolution  

To obtain the complete formula of the energy resolution, one needs to add the contributions of the electronic noise and the detector inhomogeneity.

The variance B of the electronic noise contribution to the energy measurement is by definition independant on the signal, then its energy.

If in the formula E = k N, k is not constant (homogeneous) over the detector, then :   E  inho. =  k  N ⇒  E E  inho. =  E  inho.   k  = D = k N k

Stochastic term

noise term   

And finally when all terms are quadratically summed : 2   E E   2 =  S E  2  B   D 2 E

mber of detected ε abs = nu particles / number of particles emitted by the source

Absolute efficiency :

ε int = number of detected particles / number of particles passing through the detector

Intrinsic efficiency :

P. 9

This is the probability to detect a particle.

  abs ≃ int 4 

Detection efficiency

Ω detector

Ω

Source

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fixed dead time Τ generated when a particle is detected n : particle interaction rate in the detector (Hz) t Τ m     : :     d d e e a t d e   c t t i e m d e     p ( a s r ) ticle rate in the detector (Hz)

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This is the minimal time interval that separates two consecutive event (particle) measurements. If two particles pass through a detector with a time separation less than the dead time, the second particle is not recorded. The dead time may be due to the detector, its electronics or the acquisition system (PC and software). Non-paralysable detector : Τ Τ

Τ

nd n   = a m  1  n T 

This detector type will never get paralysed .

1 lim m = n -> ∞ T

m n =  1 − m T 

Dead time of a detector

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Dead time of a detector

lim m = n  1 − n T  Same as non-paralysable case . n T -> 0

m = n exp − n T 

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n l - i > m ∞ m = 0 This detector type may get paralysed.

Τ Τ Τ

Paralysable detector :

If paralysable, this particle is lost !

Dead time Τ generated every time a particle Τ Τ Τ pass through the detector.  n : particle interaction rate in the detector (Hz) t m : detected particle rate in the detector (Hz) Τ : dead time (s)