Detection of γ-rays from nuclear decay: 0.1 < E γ < 20 MeV

Transcription

1 Detection of -rays from nuclear decay: 0.1 < < 0 MeV Basic concepts of radiation interaction & detection Compound Nucleus reactions and -ray emission High resolution detectors: the semiconductor Ge s Present Ge Arrays: UROBALL Future Arrays: AGATA

2 -ray interaction ionization occurs in limited regions of the absorber I/I 0 e -µt t Linear attenuation coefficient (probability per unit path) µ σ + σ + σ ph C pp µ σ ph Z n n σ C Z ln σ pp Z ln Ge

3 Detector response We detect recoil electrons and NOT photons! gap gap gap C 1 + / m c e mec 0.56 MeV ( if >> m e c ) Important characteristics: energy resolution: δ / FWHM/ peak-to-total: P/T Area peak /Area total

4 -decay of Compound Nucleus 14 Sn 164 r 40 Ar ~ 5 MeV/A Giant Dipole Resonance ~ 15 MeV FWHM ~ 5-7 MeV P /P part Ce 70 α GDR n p T 0 rotazione smorzata -decay below n-threshold < 8 MeV FWHM ~ -10 kev M ~ 0-30 (within few ps) 163 r T0 rotazione discreta

5 3 6 -ray patterns reveal nuclear structure detector t 10-9 sec sec 4 Collective rotation leads to regular band structures Single-particle generation of spin leads to an irregular level structure

6 Present status there is NO detector providing simultaneous measurement of high-energy and low-energy -rays with optimal conditions Low-energy: Ge detectors good energy resolution Arrays dedicated or coupled High-energy: Scintillators (NaI, BaF, ) high efficiency, good timing, particle discrimination Z Future Arrays of segmented Ge detectors? Pulse shape & tracking (particle- discrimination?)

7 Low-energy -rays Counts FWHM1keV 48 kev Ge NaI(Tl) FWHM40 kev 108m,110m Ag 66 kev 1.33MeV The energy resolution depends directly on the number N of charge carriers in a scintillator ~ 100 ev/photoelectron N 10 1 MeV in a semiconductor ~ 3 ev/photoelectron N3x10 1 MeV R.35 N

8 High-energy -rays photopek efficiency 7.5x7.5 cm ε p 4.x4. cm n energy delay (after 30 cm) for 10 cm source absolute efficiency ε a ε p (Ω/4π) Ge BaF 14cm 18 cm 10 4 from ORTC

9 High Purity Ge Detectors 15% impurity concentration N ~ atoms/cm 3 0.3µm 150% active region d 600 µm Characteristics size shape n-type operating temperature rates Ø~10cm, L~9cm coaxial less sensitive to radiation damage < 85 K ~ 10 khz to prevent pile-up d εv en energy resolution kev at 1.33 MeV (0. %) time resolution 4-5 ns (with CFD) ns total rise time V~ V efficiency* up to 00% * relative to 7.5x7.5 cm NaI(Tl) for 1.33 MeV -rays emitted by 60 Co source at 5 cm from detector (ε a 1. x 10-3 )

10

11 nergy resolution versus kev FWHM [kev] FWHM [kev] band structure effect Temperature [K] Temperature [K]

12 Pulse shaping Preamplifier : FT (at 130 K, to minimize noise) FT energy Amplifier: CR-RC shaping circuit time true pulses if C 1 R 1 C R τ out t e τ t τ mv from preamp τ ~ 50µs pile-up V after shaping τ ~15 µs τ ~ 15 µs is a good compromise between reduced pile-up and good energy resolution (depending on large charge collection)

13 Preamplifier τ RC Amplifier (RC-CR shaping) Rinput resistance Cinput capacitance+ detector capacitance + cables RC-integrator (low-pass filter)... in out ir + out (1 e t /τ ) operation mode for time information, high rates, operation mode for energy information CR-differentiator (high-pass filter)... in out Q + C e t /τ out t c charge collection time ~100 ns τ RC decay time ~ 50 µs

14 Ge Response function (+ Anti-Compton Shield) Anular detector used with heavy metal collimators in front P/T~60% P/T~0% used material: BGO (Bi 4 Ge 3 O 1 ) density ~ 7.3 g/cm 3 Z 83 3 times more efficient than NaI ideal for very compact geometry (small spaces) N.B. in some cases NaI nose is used to improve the light output far away from PM tube

15 Compton scattering angular distribution incident ' 1+ ( / m c e )(1 cosθ ) high-energy -ray: forward scattering low-energy -ray: forward & backward NaI nose: improvement of light output far away from PM tubes (low-energy -rays) BGO back-catcher: improvement of high-energy Compton scattering (high-energy -rays)

16 Study of Detector response 133 Ba in-beam spectrum after unfolding Backscattering peak: θπ (Ε ) min ~m e c / 56 kev important for θ Accurate study of detector response is done with MonteCarlo GANT simulations

17 Towards a 4π detection array: The uropean -ray detection systems UROGAM (45 CSGe) UROBALL III (> 00 CSGe) TSSA SSA30 P ph ~ 5 % P ph ~ 10 % 5 Ge (5%) +ACS (NaI) 30 Ge (5%) +ACS (NaI) P ph ~0.5-1% GASP (40 CSGe) + IB UROBALL IV (> 00 CSGe) + IB

18 GAMMA -DTCTOR Systems Ge detector + BGO shields Multiplicity filter (BGO or BaF) Si detectors for particles (p, α, d) RMS, PPAC (for recoil detection)

19 UROBALL 4π Ge detector Array

20 IRS (Strasbourg)

21 UROBALL (39 Ge Crystals) Composite Ge detectors HPG CLOVR inner radius 5-40 cm 4 Ge BAM LIN ε 0% HPG CLUSTR 30 TAPRD G-DTCTORS 6 CLOVR G-DTCTORS 15 CLUSTR G-DTCTORS 7 Ge ε 60% UROBALL M 30, v/c% ε 7 % P/T 40 % Ω 40% 1.3 MeV, S 70 kev, M 30, v/c% Full Ball: Ge+BGO 4π

22 Composite & encapsulated detectors bare crystal encapsulated(0.7 mm alluminum) under vacuum (10-6 mbar) 78mm 50mm 70mm 70mm CLOVR B-CLUSTR

23 Mounting of a UROBALL Cluster Ge detector (Kholn, Milano, LNL, Manchester, Lund) encapsulated detectors have proven to be reliable, easy to handle and repair: they have also been used in space mission

24 Advantages of composite Ge Detectors: 1. enhanced efficiency (add back). reduced Doppler broadening 3. polarization studies

25 1. fficiency more Ge volume: V cluster 183 cm 3, Ω 0.91 % ε ph 9.%, ε a ε ph x Ω 0.65% add-back procedure: improved efficiency & spectrum quality As a consequence of Compton scattering -rays can scatter in adjacent capsules: Similar to large BaF (but no good timing!) CLUSTRs add-back scheme - Full energy peak: (add-back) - the higest energy is released in the first interaction - the y-ray incident angle θ is given by the first interaction (better Doppler correction) CLOVR P/T0.1 P/T0.18 add-back factor summing effects (# photopeakevents) F (# photopeakevents) SINGL HIT Basic idea of future tracking arrays TOTAL

26 . Doppler broadening Doppler ffect in-beam NORDBALL data: 163 Tm v 1 c 0 (1 + v 1 cos θ c v sin θ ) c 0 Doppler broadening 0 c v sin θ sin θ v c << 1 θ79 0,101 0 θ37 0,143 0 θ90 0 Ge detectors with large opening angle suffer of a considerable energy deterioration maximum broadening is reached at 90 0

27 Composite Detectors: compromise between efficiency & resolution ε a ε p Source at rest: 4π - intrinsic resolution is reached πr Ω - ε a decreases with d increasing distance d detector-source (smaller Ω) Ω Moving Source: - effective resolution depends on d (Doppler broadening) poor FWHM good FWHM composite Ge θ kev small d large Ω large d small Ω single Ge 6.1 kev θ Si+ 14 Sb 149 Gd beam 158MeV v/c.1% Composite detectors allow to obtain large solid angle without suffering of energy resolution deterioration

28 3. Polarisation measurements: composite detectors as Compton polarimiters the or M character of the -ray can be determined by the polarization P B P I I vertical vertical + I I horizontal horizontal > < 0 0 M 16 Ba: UROBALL data

29 Large Number of Detectors: Resolving Power Resolving power F F T P S R F T P S -ray energy spacing -ray energy resolution Peak to-total (Compton) Measured fold M P F ph > < Ω i ph Ge P N ε array capability of identifying weak cascades out of a large backgroud increses with fold F F # measured s coincidence data Peak-to-bg improvement unc unc B N S T P B N B S B N T P N unc unc B N S T P B N B S B S B N T P N T P N 1 1 kev kev S T P Total photopeak efficiency

30 SD band in 151 Tb 5 Ge <F> 45 Ge <F>4 Approximate selection of angular momentum is achieved by selecting the fold number. ARRAY Trigger: high folds events of clean or dirty Ge (with or without Compton suppression)

31 NORDBALL DATA: 143 u SD band 37 Cl+ 110 Pd, beam 160MeV Triple gated Observation Limit Double gated Single gated Observational Limit TOTAL spectrum Year

32 Light ions scintillators detectors Light ions Ge detectors: 1 Heavy Ions Ge detectors: 1 Heavy Ions Ge detectors: Heavy Ions Ge detectors: 3