Gamma-Ray Detection

J.L. Tain

Jose.Luis.Tain@ific.uv.eshttp://ific.uv.es/gamma/

Instituto de Física Corpuscular

C.S.I.C - Univ. Valencia

Interaction of X-rays and γγγγ-rays with matter

Mainly through:• Coherent scattering• Photoelectric effect• Compton scattering• Pair production

Photons are massless neutral particles subject to electromagnetic interactions

Coherent atomic (Rayleigh) scattering:

( ) ( )( ) factorformatomic:

+=

Elastic scattering (no energy transfer) with the whole atom:

FS: the photon with another direction

Photoelectric effect:

[ ]

×≈ −

−=Ejection of atomic electron (most probably from K-shell)

FS: electron + excited atom

Compton scattering:

( )

′−=

−+=′

Klein-Nishina cross-section:

−

′+

′

′=

Inelastic collision with a (quasi-)free atomic electron

FS: lower energy photon + electron

Pair production:Photon conversion into a pair e+-e- in the field of the nucleus (or an atomic electron)

In the high energy limit:

( ) ( )[ ] [ ] −×≈ −

FS: electron + positron

Energy and charge dependence of photon interactions

Low energy gamma-ray response:

F Monte Carlo simulations are extremely useful

Parameters:• total efficiency• peak efficiency• (intrinsic efficiency)• peak/total• …

Scintillation detector:

SCINTILLATIONMATERIAL

LIGHT-GUIDE /WAVELENGTH-CONVERTER

LIGHT TO

ELECTRIC-PULSE TRANSDUCER

• Simple

• Versatile

• Rugged

• Cheap

Luminescence in inorganic materials

Several mechanisms have been identified: luminescence of doping centers, self-activated luminescence and cross-luminescence

The non-radiativetransfer mechanism between excited centers induces an energy-loss dependent light production

Simple parameterization:

+=

As a consequence there is a particle type and energy dependence of scintillation pulse shape and light output

Both effects can be used to identify particles

100002.41.584201.03Plastic BC-400

49000271.93503.79LaBr3(Ce)

25000471.824207.4Lu2SiO5(Ce)

18000271.953705.37YAlO3(Ce)

44005,271.68310,3406.16CeF3

1500,95000.6,6301.56220,3104.89BaF2

82003002.154807.13Bi4Ge3O12

40000,25000680,33401.805404.51CsI(Tl)

380002301.854153.67NaI(Tl)

Light yield (ph/MeV)

Decay time (ns)

Refractive index

Wavelength at max.(nm)

Density (g/cm3)

Properties of some inorganic scintillation crystals

Semiconductor detectors are widely used in gamma spectroscopy because of their excellent energy resolution

60Co source

They can also have a reasonable position resolution

Their properties as sensors are based on the crystal structure

dist. gapC: 3.56 Å 5.5 eVSi: 5.43 Å 1.1 eVGe: 5.65 Å 0.7 eV

Group IVA: …ns2p2

Not conducting but gap small enough to generate many ionisations (electron-hole pairs)

Other semiconductor materials: GaAs, CdTe, HgI2, CdZnTe

Usual semiconductor detector configurations:

Planar

Coaxial

Strip

Pixel

Ge Ge

HOLESSi

ELECTRONSSi

Pulse formation in semiconductor detectors

Ge detector fabrication:

purification growing cutting

Mounting in a cryostat

diode fabrication

Other semiconductors: CZT (Cd0.9Zn0.1Te)

Advantage:• Large Z, room temperature detectorDisadvantage:• Large hole trapping (many crystal defects)

Z = 47.8 (32)Ee-h (eV) = 4.64 (2.95)

µµµµe (cm2/Vs) = 1000 (3900)µµµµh (cm2/Vs) = 70 (1900)

Coplanar grid technique

PLANAR

COPLANAR GRID

Composite Ge detectors

Euroball CLOVER

Euroball CLUSTERMiniball CLUSTER

crystal

capsule cluster

Escape-suppressed spectrometer (Anti-Compton)

• Scintillation detector surrounds the Ge detector and veto events with escaping radiation

collimator

Ge

scintillator

Suppression factors: 4-60*Peak/Total gain: 3-6

NORDBALL detector + BGO-CSS

CLUSTER detector + BGO-CSS

Ge detector + NaI-CSS(environmental use)

The construction of a level scheme is a highly involved (and human biased) process based on:

1) the coincidence relationships between several γγγγ-rays

2) the matching of γ-ray and level energies

3) the balance of intensities

4) nuclear structure arguments

5) additional information

High Resolution Gamma-Ray Spectroscopy

Requirements:• reduce beam-time F εP ↑• reduce summing F εT ↓• increase sensitivity F P/T ↑• reduce Doppler broadening F ∆Ω ↓

A solution:• many small (or segmented) detectors with CSS: Ge-array

One limitation:• ∆ΩGe << 4π

( ) Pj

Pim

n

n-detector array m-fold coincidence efficiency :

2-fold:6 →→→→ 15

12 →→→→ 6642 →→→→ 861

105 →→→→ 5460

Gammasphere110 Ge+BGO-CSS

EUROBALL

Charged particleSi-ball

CLOVER

n detector

CLUSTER

εP 6-10% P/T 40-60%

∆E/E 0.3%

15 (××××7) CLUSTER + 26 (x4) CLOVER + 30 large Ge= 239 crystals

25x25 Ge strip detector6cmx6cmx1cm

36 fold segmented hexagonal Ge detector

Segmented Ge detectors

Tracking arrays

• Interaction position from pulse shape analysis (Ge detector with segmented electrodes)

• Track reconstruction from (ei,xi,yi,zi) using probabilistic analysis

∆∆∆∆(x,y,z) 5 mm

( )

′−=

−+=′

Compton relationship, interaction probability, mean free path

ΕΕΕΕγγγγ

γγγγ

Prototype triple cluster

Simulations

AGATA

εP 25-50% P/T 50-60%

∆E/E 0.3%

180 hexagonal crystals 36-fold segmented∆Ω/4π = 82%

Scintillation detector arrays

Darmstadt-Heidelberg Crystal Ball

162 NaI(Tl) crystalsRint = 25 cm, Rext = 45 cm∆Ω∆Ω∆Ω∆Ω/4ππππ = 97%εεεεP = 71 % , ∆∆∆∆E/E= 7% @1.33 MeV

Total Absorption SpectroscopyThe determination of level population probabilities (n-capture cross-sections, β-decay intensities,…) requires detection of γ-ray cascades with certainty (known efficiency)

Use a ∆Ω∆Ω∆Ω∆Ω 4ππππ detector, with enough thickness:

E1

E2

E3

ECIf :

εεεεi : total detection efficiency for γ-ray of energy Ei

εεεεpi : peak detection efficiency for γ-ray of energy Ei

then :

( )∏ −−=i

iC 11

∏=i

pi

pC

: total detection efficiency for cascade

: peak detection efficiency for cascade

If εεεεi 1 ∀∀∀∀i, then εεεεC 1 : we count cascades not individual γ-rays

Because experimental reasons is better to have εεεεpi 1 ∀∀∀∀i

156Tm ββββ-decay

St. Petersburg TAS vs. LBL TAS @GSI

30

204.6

7.7

10

35

3515

15

5

St. Petersburg TAS @ JYFL

LBL TAS @ GSI

5 MeV1 MeV

0.890.520.970.65LBL

0.710.250.870.47St. Pt.

εεεεTεεεεPεεεεTεεεεPTAS

40 BaF2 Crystals

Rint = 10cm, Rext = 25cm

TAC @ n_TOF

0.910.805 MeV

0.980.901 MeV

εεεεTεεεεP

237Np(n,γγγγ)