Detectors in Nuclear Physics (40 hours)Silvia Leoni; silvia.leoni@mi.infn.it Università degli Studi di Milano andOliver Wieland; oliver.wieland@mi.infn.it, INFN sezione di Milano.

Material for the course can be found at the web page: http://www.mi.infn.it/~sleoni/

After an introduction on the interaction mechanisms of charge particles, γ-rays and neutrons with matter, the general properties of radiation detectors are presented. Modern and future detection systems based on different kind of materials are discussed in connection with specific experiments at the frontiers of the nuclear physics research. In the last part of the course techniques for production, acceleration and studies of unstable (exotic) beams are presented.

The course is organized as follows:

1 – Radiation interactions:a) charged particles;b) γ-rays;c) neutrons.

2 – General properties of radiation detectors:a) gas detectors;b) scintillator detectors;c) semiconductor detectors;d) neutron detectors;[e) momentum measurement]

3- Modern detector systems:a) large volume composite Ge detectors;b) Ge arrays: the European project EUROBALL;c) Si arrays;d) scintillators arrays;e) electron conversion spectrometers ;f) neutron arrays ;g) heavy ions detectors ;h) magnetic spectrometers.

4- Future developments in γ-detection:a) segmented Ge detectors: pulse shape analysis and tacking techniques;b) the ultimate array for γ-spectroscopy: the European project AGATA.

5- Production and acceleration of radioactive beams:a) fragmentation;b) isotopic separation on line;c) overview of present and future facilities in Europe.

Detectors in nuclear Physics

• Radiationinteractions in mattera) charged particlesb) γ-raysc) neutrons

• General properties of radiation detectorsa) gas detectorsb) scintillatorc) semiconductord) neutron[e) momentummeasurement]

We use mainly:Glenn F. Knoll: “Radiation Detection and Measurement”William R.Leo: “Techniques for nuclear and particle Physics experiments”

measurement

detector signal processing

data handling

analysis&

control

01011101

computersimulation

PhysicsPhysics origin interaction

Particle Detection through energy loss in matter

INTRODUZIONE

Particle energy loss in matter

X rays

gamma rays

PE

CS

PP

electron(s)

dE/dx losscharged particle

thermal neutronsnuclear reaction

energetic neutrons

proton

Detector

dE/dx loss

dE/dx loss

(1896) BecquerelInvestigava la fosforescenza dei sali di uranio.Durante le sue ricerche mise in contatto con ilmateriale una lastra fotografica, accorgendosi che era stata impressionata anche se non era stata esposta alla luce del sole: Becquerel concluse che il materiale emetteva dei raggi senza bisogno di un eccitazione da parte della luce

Rutherford (1911)

"Scattering is the devil"

Early nuclear physics detectors

ZnSGeiger (1909)

INTRODUCTION

DOMANDE:

•Type of Radiation ?

•HOW TO DETECT RADIATION ?

•WHAT happens when radiation passes through matter ?

•WHICH kind of particles makes what kind of interaction ?

Development of detectors!

Detection in the context of this lessons means:measure and quantivy different properties and types of ParticleInteractions which are of interest in nuclear physicsexperiments

• Ionisation• Scintillation• Excitation of lattice vibrations• Breakup of cooper pairs in superconductors• Formation of superheated droplets in superfluids• Excitation of optical states• Bremsstrahlung• Cherenkov radiation• Photoelectric effect• compton scattering• Pair production• Thompson, rayleigh, raman scattering• Rutherford scatering• Spallation• Transmutation• Radiative capture• Fission• Electron ion pair production• Electron hole production• …• …• …

?

Detectors can be suitable for one or moreParticles to identifyDetectors can be suitable to measure one or moreParticle properties.

Particle identification through:-time of flight (velocity) measurement,-position, trace measurement-momentum-energy-charge-…

Concetti di Base

• Radiazione• grandezze fondamentali e loro unità di

misura, sezione d’urto, cammino libero medio, sorgenti di radiazione

Concetti di BaseInterazione nel materiale attraverso:

Interazione forte g~0.1Interazione elettromagnetica g~1/137Interazione debole g~10-5

Interazione gravitazionale g~10-39

Outlook :

•Nonostante le differenze dei mechanissmi e proprieta gli:elettroni si comportano in modo assai simile ad un gamma

•Il neutrone si comporta in mode diverso da tutte le altre

TIPI di Sorgenti di Radiazione(non tutti sono di interesse in fisica

“nucleare”)• Sorgenti alpha (helium nuclei)• Sorgenti beta (electrons, positrons)• Sorgenti gamma (photons w. high energy)• Particle Accelerators (for LCP, HI, elem.part, …)• Solar Sytem and Deep Cosmic Rays• Laser• …

Wave lenght ofγ-rays= electromagnetic radiation in the energy range of nuclear

physics

Radiazioni ionizzanti• Interazione di particelle cariche:• range• perdita di energia per ionizzazione• perdita di energia per radiazione

Interazione di particelle neutre:• neutroni• fotoni:• effetto fotoelettrico• effetto Compton• produzione di coppie• attenuazione•

Particelle cariche: α, β±, π , γionizzazione diretta degli atomi

del mezzo

Particelle neutre: n, γionizzazione indiretta tramite

produzione di particelle carichesecondarie

Tipi di Radiazione

OUTLOOK+RIASUNTO: Interazione radiazione materia

Il principio di funzionamento di un rivelatore di radiazione dipende dalle modalità con cui la radiazione da rivelare interagisce nel materiale che costituisce il rivelatore

I meccanismi di interazione sono diversi e per questo bisogna dividere la radiazione ionizzante in categorie:

1. Particelle cariche leggere ⇒ la caratteristica distanza di penetrazione è ~ 10-3 m2. Neutroni ⇒ la caratteristica distanza di penetrazione è ~ 10-1m3. Raggi X e γ ⇒ la caratteristica distanza di penetrazione è ~ 10-1 m4. Particelle cariche pesanti ⇒ la caratteristica distanza di penetrazione è ~ 10-5 m

• Le particelle cariche sono caratterizzate dal fatto di interagire in modo continuo con gli elettroni degli atomi del materiale mediante la forza Coulombiana

• La radiazione neutra interagisce in specifici punti del materiale attraverso meccanismi che cambiano in modo radicale (“catastrofico”) le proprieta’ (grandezze fisiche) della radiazione incidente

Range of Particles in Matterthrough

Energia E [eV] – energia acquisita da una particella carica sottoposto alla differenza di potenziale di 1 Volt1 eV =1.602 x 10-19 J

Massa a riposo m [ev/c^2] 1 eV/c^2 = 1.8 x 10^-36 kge- mass=9x10^-31kg … 511 keV

Impulso p [ev/c] 1eV/c =0.54 X 10^-27 kg m/s

…spin, polarisazione, ….

Proprieta’

Sorgenti radioattive vengono caraterizzati:

• Attivita numero di decadimenti al secondo : 1 Bq= 1dec/sec• Costante di decadimento – velocita di decadimento:dN / dt = -λ N• Vita media τ (intervallo dopo quale il

numero di nuclei si e ridotto di un fattore eτ=λ-1

• Tempo di dimezzamento t_1/2 (tempo dopo quale meta dei nuclei iniziali e decaduta)

T_1/2=τ ln2=0.693 τ

Fasci di particelle FlussoΦ – numero di particelle per unita di tempo e di area

Mean free Path:Particles pass through matterundergo collision, interactionpossible change of their direction of motion

Average distance between these collisons is a measureof probab. of a particle interaction and depends on the velocity distribution of the particle.

Discrete velocity distribution:Λmean=1/(density of medium * cross section).

Maxwellian velocity distribution (thermal neutrons ): Λmean=1/[√(2)(density of medium * cross section)].

Radiation Lenght (for Electrons and Photons):Is defined as the thickness of a material such that a traveling particle looses 1-1/e (about 63%) of its energy.(Predominant prozesses forelectrons: BremsstrahlungPhotons: Pair production)

Electron radiation lenght:Air = 30050 cmH2O = 36 cmAl = 8.9 cmNaI = 2.6 cmBGO = 1.1 cmPb = 0.56 cm

Lrad becomes usefullIf we measure materials in this unit !Compounds and mixtures follow1/X0=fraction1(1/X1)+fraction2(1/X2)..

The average energy loss due to bremsstrahlung for an electron of energy E is related to the radiation length:

and the probability for an electron pair to be created by a high-energy photon is 7/9 X0.

Ricorda:

Radiation Lenght (for Electrons and Photons):

E LET

LinearEnergyTransfer

Alto LETalta densità

di ionizzazione alta probabilità di

colpire e danneggiare un oggetto biologico Grande variabilità

Rapporto tra l’energia totale T trasferita alla materia

lungo un camminoe la lunghezza R

del cammino percorso

LET = T/R(misurato in keV/μm, MeV/mm)

Termine usato in fisica medica:

0.01 0.1 1 10 100 1000 (MeV)0.1

1

10

100(cm)

R(E)H2O

E

e

elettroni

p

protoni

alfa

Water (~ tessuto biologico)

“Radio”-Protezione:

Fundamental Detector properties:(important to choose the right detector for a specific purpose)

•Energy (or other measured quantity) Resolution (response function)(for specific and different particles, time, energy dependance) and linearity:•Detector Efficiency(for specific and different particles, time, energy dependance):

•Timing(dead,repeat time,..)•…

REMEMBER (seems obious but it isnt !):o All detectors (even of the same type) are different

and have to be checked before used !o All (properties) have to be re-measured and calibrated !o All detector properties change with temperature, count rate etc…o All detector properties change with the time

due to irradiation or simple electronical/mechanical/chemical ageing!

•Energy (or other measured quantity) Resolution (response function)(for specific and different particles, time, energy dependance):

One characteristic property of a detector is its resolution fo measuring a certain quantity Z. Let z be the response of the detector; Then the resolution is defined as the standard deviation σz or theFull width at half maximum (FWHM) Δz of the distribution D(z) inThe measured quantity z for a monochromatic input distributionδ( Z - <Z> ).The mean value of the measured quantity is <z>=∫zD(z)dz, the variance is σz

22 .

If the only source of fluctuations is the statistical fluctuation of thenumber of N primary charge carriers,

The formation follows the Poisson statistics:This gives (for N>20 this wil give approx. a gaussian distribution)σz / <z> = 1/ √NThis should give lower limit to resolutionBUT:It has been found that in several detectors the resolution can be much better (up to 4 times)!

Assumption of Poisson statistics is WRONG.

Fano effect: Fano Factor=F=( observed_resolution/resolution_expected_from_Poissonstatistics)^2

The reason is that the Energy deposition E in a detector bycreation of electron ion pairs takes place in many <N>steps k (and E=ΣkEk )where mean energy W is needed tocreate ion pairs number of pairs is Ek/W .

Linearity:

Relation between measured <z> value and the value Z of theOriginal quantity of the incident particleHAS to be established by a calibration of the detector with(best possible) monochromatic beam of particles (sources) !If <z>=cZ than the detector is called ‘ linear’(in a specific range!)

If c varies with Z the relative variation

it is called non-linearity.

dZ/dc Z/c

•Detector Efficiency(for specific and different particles, time, energy dependance):

“intrinsic” Efficiency means the probabilityof recording correctly a pulse from a particleemitted in an elementary reaction impinging on the detector surface.(taking into account impinging angle, electronic threshold, ….)

The “absolute” efficiency is determined by 2 factors:-solid angle Ω-Intrinsic efficiency

Solid angle for point source:

If detector surface is cirlce with r at distance R: then

with

∫ −=ΩD

dd ϕθ )cos(

2

2

22)1(2)cos1(2

Rr

RrR ⋅

=+

−=−=Ωππαπ

RrandRr

r<<

+=

22)sin(α

The efficiency(in function on geometry of detector, positioning, size, interaction circumstances …) can be very well calculated with“Monte Carlo” methods(GEANT) when the basic underlying interaction mechanisms are known.

•Timing(dead,repeat time,..)

If no precise time of passage os needed:Simple measure the dc-current delivered by the detector

If timing is needed: recording of each individualparticle (pulse mode) is done:Output current is transformed to a voltage signal by a PREAMPLIFIER.The time structure is determined by the respectiveINPUT IMPEDANCE of the circuit.Time constant τ=RiCi has to be small compared withCHARGE-collecting time tc.

For some detectors the capacitance (e.g. semiconductor detectors)is not constant ! In this case a charge-sensitive amplifier is used

Inverting amplifier with feedback loop through a capacitance Cf.Amplification is very large compared to (Cf+Ci)/Cf.This means the output voltage is approx. Proportional to the inputCharge Q:

Vout= = ≈ –Q/Cf C1)(AC QA -

if ++ Be aware of Dead Time

Fine introduzione