Post on 24-Sep-2020
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)