Lecture 2: Introduction to detector sensors

Transcription

1 Lecture 2: Introduction to detector sensors Radiation detectors are needed for any work involving nuclear radiation Specifically: For assessing the need for shielding in a given situation and for determining the effectiveness of an existing shield. We consider detectors for uncharged radiation: Gamma rays and Neutrons Introduction to detector sensors 1

2 Gamma detectors: Radiation detectors * Scintillation detectors Semiconductor (germanium) detectors Neutron detectors: * Gas detectors * Scintillation detector * Activation Introduction to detector sensors 2

3 SCINTILLATION DETECTOR General purpose device Medicine: diagnostics; computer tomography (PET) Industry: level gauging; radioactive waste assay Environment: survey; geological applications Physics: nuclear, high energy, particle physics Introduction to detector sensors 3

4 Typical arrangement Radiation(here, a γ-ray) deposits energy in a scintillator, causing a region of excitation. De-excitation releases photons, some of which release photo-electrons from the photo-cathode of a photo-multiplier tube (PMT). Electrons are multiplied and produce a pulse of current at the anode. Introduction to detector sensors 4

5 Properties of some common scintillators Material Type Density a Relative to NaI(Tl) b Nuclear Enterprises Ltd. Emission max. (nm) Time constant (ns) Light output a NaI(Tl) Inorganic crystal CsI(Tl) Inorganic crystal BGO Inorganic crystal Stilbene Organic solid NE102A b Plastic (org. solid) NE213 b Organic liquid Introduction to detector sensors 5

6 Advantages of scintillation detectors (+PMT): High gain large signal Inexpensive Efficient for X-rays and gamma rays (inorganic) Fast response (especially organics) Introduction to detector sensors 6

7 Disadvantages of scintillation detectors (+PMT): Bulky and fragile (especially crystals) Magnetic field sensitive Gain drifts very sensitive to applied voltage Introduction to detector sensors 7

8 Semiconductor (Germanium) detector Formed of intrinsic (hyper-pure) material with (thin) p- and n-type regions deposited at either end p Hyper-pure germanium n An applied voltage creates an electric field, whereby the charge released in the detector by the radiation is collected. Mainly used to make large volume (LN 2 cooled)ge detectors e.g. cylindrical crystal 50 mm 100 mm Introduction to detector sensors 8

9 Germanium γ-ray detector The Ge detector is widely used It has excellent energy resolution: Typically 1.8 to 2.0 kev for 1.33 MeV γ radiation. A cut-away view of a coaxial Ge detector. Introduction to detector sensors 9

10 Response to Gamma Rays Important Factors: Energy resolution Peak-to-Compton ratio Efficiency Introduction to detector sensors 10

11 Scintillation detector: 662 kev γ ray P FWHM Resolution 6 8% (FWHM/P) ~50 kev for 662 kev γ ray Introduction to detector sensors 11

12 Germanium detector : 662 kev γ ray Resolution ~1.8 kev (FWHM) for 662 kev γ ray Introduction to detector sensors 12

13 Comparison of NaI and Ge detectors (mixed Ag sources) Introduction to detector sensors 13

14 Peak to Compton Ratio Measured using 60 Co Ratio = (count in max channel of 1332 kev peak) (mean count/channel in the Compton region) The part of the Compton region used is kev Introduction to detector sensors 14

15 Peak to Compton Ratio Best indicator of detector quality Its value increases with both Resolution: higher value of A and efficiency: larger full-energy peak area Introduction to detector sensors 15

16 Comparison Ge detector has best performance - but - Ge system is much more complex: Detector has to be cooled with liquid nitrogen Needs high-quality electronics Ge crystal suffers from radiation damage Introduction to detector sensors 16

17 NEUTRON DETECTORS Slow neutrons: Gas detectors, scintillators, activation. Exploit the high absorption cross sections at low energy Fast neutrons: Scintillators: Usually measure the recoil energy from elastic scattering Introduction to detector sensors 17

18 NEUTRON DETECTORS Requires a nuclear reaction Choice depends on neutron energy He(n,p)t H(n,n) 1 H 10 6 Li(n,α)t 1 10 B(n,α) 7 Li ev 1 kev 1 MeV Introduction to detector sensors 18

19 Slow neutrons Common gas detectors: BF-3 and He-3 E.g. BF-3: Boron tri-flouride gas in a proportional counter Reaction: 10 B(n,α) 7 Li(gs) Q = 2.79 MeV (6%) 10 B(n,α) 7 Li(exc. state) Q = 2.31 MeV (94%) Q shared between α and 7 Li emitted back-to-back E.g. Q = 2.31 MeV: 7 Li 0.84 MeV α 1.47 MeV Introduction to detector sensors 19

20 BF-3: Ideal spectrum (n,α) 94% MeV (n,α) 6% γ ray 0 Reaction product Full-energy peak dn/de Excited state α interactions, noise, etc Introduction to detector sensors 20

21 BF3: Actual spectrum Structure below main peak due to wall effects Introduction to detector sensors 21

22 Wall effects Deposited energy < maximum if either charged particle hits detector wall Minimum if alpha hits wall and only 7 Li is deposits energy second step if 7 Li hits wall and only alpha deposits energy Wall-effect continuum Introduction to detector sensors 22

23 3 He DETECTOR 3 He has almost 40% higher neutron cross section than 10 B 3 He is the proportional counter gas (expensive) Reaction: 3 He(n,p)t Q = MeV Q is shared between p and t emitted back-to-back t MeV p MeV Introduction to detector sensors 23

24 Wall effects dn/de Wall-effect continuum Reaction product Full-energy peak Introduction to detector sensors 24

25 Typical spectrum for a 3 He detector (2 atmos) Introduction to detector sensors 25

26 Fast neutrons - scintillators * Lithium iodide * Plastic and organic Introduction to detector sensors 26

27 6 Li iodide detector 6 Li cannot be used in a gas detector (no suitable gas containing lithium). However, it can be incorporated into a scintillator for both slow and higherenergy neutrons. Advantages compared with a gas detector:: Higher efficiency (solid-v-gas) - smaller sensitive volume. Higher counting rates (smaller dead time). Geometric flexibility. Large Q value: Reaction: 6 Li + n 3 H + 4 He Q = 4.78 MeV better separation of n and γ events than in a BF 3 detector. Natural lithium contains 7.5% 6 Li and 92.5% 7 Li. Can obtain material enriched to 96% in 6 Li. Introduction to detector sensors 27

28 Use LiI with ~ 0.1% europium (Eu) activator. Light output ~ 35 % of a NaI(Tl) crystal. 1 cm crystal absorbs ~ 69% of incident thermal neutrons 2 cm crystal absorbs ~ 90% With enriched (96%) 6 Li, 90% absorption occurs in about 0.2 cm Short range of reaction products is good No significant wall effect Slow neutrons should give a full-energy peak Introduction to detector sensors 28

29 Potential problems Lithium iodide has a high efficiency for detecting γ rays. (recall the NaI detector) Minimize γ efficiency (for thermal n) by using thin crystals (< 1 cm) much less if using enriched 6 Li. Neutron detector efficiency is reduced by n capture in iodine and europium: σ capt (I) = 6.2 b, σ capt (Eu) = 4600 b. LiI is hygroscopic (like NaI) - so requires careful encapsulation. Introduction to detector sensors 29

30 Organic and plastic scintillators Fast neutrons: Use a plastic or liquid organic scintillator (high hydrogen content). Detect the signal from the recoil energy of an elastically-scattered proton Efficiency > gas detector: Solid material. High hydrogen content. σ scatt > σ reaction for E n > few kev. Introduction to detector sensors 30

31 For single E n : get continuum of E recoil - from ~ 0 (grazing collision) to E n (head-on collision) (a) assumes σ scatt is isotropic in the c-m system. (b) response of an actual (stilbene) detector. - well reproduced by the solid curve (from a detailed calculation) - includes detector resolution and scintillator non-linearity. Detector signal contains information about E but difficult to unfold the continuum spectrum if there are several different neutron energies. Introduction to detector sensors 31

32 Activity counting Measurement of n flux by counting the neutron-induced activity in a sample Activation Detector Need σ capture large (τ ½ not too large) to produce a measurable activity in a reasonable time. σ capture largest for low E n activation detectors usually measure fluxes of slow neutrons. Use a small sample to avoid disturbing flux. Introduction to detector sensors 32

33 Activation and decay Assume an irradiation time t 0, Initial activity (at time t 0) : A 0 = A [1 exp(-λt 0 )] where A = saturation activity [= nσφ]. A(t) A A 0 0 t 0 t 1 t 2 t Introduction to detector sensors 33

34 After irradiation, count sample (over interval t 1 to t 2 ): t t 2 1 A( t)dt = C B ε ε = counting efficiency (including self-absorption effects). B = background expected in time interval (t 2 t 1 ). A = R = nσφ n = nuclei in sample σ = cross section Φ = neutron flux Introduction to detector sensors 34

35 Note: Activity detectors are integrating devices they give no information about time variations of the flux during exposure. Advantages: Small Insensitive to γ rays. Low cost. Tolerate extreme environments. No electrical connection to the outside world. Widely used to map steady-state neutron fluxes in reactor cores. Introduction to detector sensors 35

36 Gamma detectors: Radiation detectors * Scintillation detectors Semiconductor (germanium) detectors Neutron detectors: * Gas detectors * Scintillation detector * Activation Introduction to detector sensors 36

37 Lecture 2: Introduction to detector sensors Radiation detectors are needed for any work involving nuclear radiation Specifically: For assessing the need for shielding in a given situation and for determining the effectiveness of an existing shield. We consider detectors for uncharged radiation: Gamma rays and Neutrons Introduction to detector sensors 37