Detection of Neutrons: Part II Ralf Nolte
Table of Contents • Introduction – Neutrons in Science and Technology – Interaction of Neutrons with Matter • Neutron Detection – General Properties of Detectors – Detectors for Thermal and Slow Neutrons – Detectors for Fast Neutrons • Recoil Detectors: Prop. Counters, Scintillation Detectors, Recoil Telescopes • (Fission) Ionization Chambers • Techniques for Neutron Measurements – Time-of-flight – Spectrometry – Spatial Neutron Distribution • Absolute Methods, Quality Assurance – Associated particle methods – Key comparison Seite 2 von 58
Recoil Detectors: Proton Telescopes Seite 3 von 58
Recoil Telescopes as Reference Instruments • Scintillation detector used as primary reference instrument? – Properties of the scintillators show variations: Light output, H/C ratio – Full angular distribution for n-p scattering required – Interference from 12 C(n,x) interactions – Detection efficiency difficult to calculate ‘accurately’ (1 -2% uncertainty) Calibration required! • Way-out: Recoil Proton Telescopes (RPTs) – Only n-p scattering contributes – Restricted range of scattering angels E cos 2 E p n p – ‘Localized’ response function – Efficiency determined by geometry, radiator mass and diff. cross section Detection efficiency small: e = 10 -4 - 10 -5 – – Energy range depends of radiator thickness Seite 4 von 58
The Classical Low-Energy Telescope: T1 of PTB Prop. Counters P1 and P2 Los Alamos in-beam design: Two CO 2 prop. counters: D E • • Surface barrier detector: E • Radiator – source distance: 20-35 cm • 1 mm Ta aperture: (20.98 0.01) mm • Energy range : n 1.2 MeV – 15 MeV using three radiators up to 20 MeV with degrader foils • Single rates: < 10 4 s -1 • Coincidence rate: 0.5 – 2 s -1 P1 P2 SB • Coincidence resolution: 2 µs • Multi-parameter DAQ Si SB Diode Aperture Radiator Seite 5 von 58
T1: Recoil Proton Spectra P2 - P1 SB P1 + P2 - SB recoil protons • D(d,n) 3 He, D 2 gas target, E d,0 = 7.11 MeV, < E n > = 10.02 MeV Seite 6 von 58
T1: Analysis • Calculation of the efficiency: – (Semi)analytical integration – Monte Carlo simulation – Relativistic kinematics for CM → LAB! – Anisotropic source: D(d,n) d E np np n A , E p n d p A cos cos e 1 2 d A d A geo 1 2 2 2 E n = 8.4 MeV d d 1 2 A A 1 2 e N n Y p geo H np • Main contributions to uncertainty – Counting statistics: u N / N = 1% - 2% u e / e = 1% – Efficiency: – Diff. n-p cross section: u A / A = 0.2% - 1% Seite 7 von 58
RPT Design Exercise: 75 MeV Test of a proton recoil telescopes for TLABS neutron beam facility: • Neutron Source: nat Li (8 mm) + p (75 MeV): quasi-monoenergetic spectrum, < E n,0+1 > = 71.6 MeV (FWHM 3.2 MeV) Collimated beam (50 50 mm) 2 • Cu coll. + D E - E … which one made the race? D E 1 - D E 2 - E Seite 8 von 58
RPT Design Exercise: Results Double stage RPT: Cu-coll. + D E - E Triple stage RPT: D E 1 - D E 2 - E D E 2 D E E E E E TOF TOF • Good particle discrimination with 500 µm Si-PIPS as D E detectors • Less neutron induced coupling with D E 1 - D E 2 - E scheme Seite 9 von 58
Fast Neutrons: Ionization Chambers Seite 10 von 58
Fission Ionization Chambers +HV U 0 electrons x Drift velocities: v = µ · E / p, v el >> v ion fission frag. Ion-induced signal suppressed by r d time constant of the pre-amp. ions Electron-induced signal depends on the location of the ionizing event fissile layer • Electrical field: E U d 0 e d E 0 ff • Charge per unit track segment: q W d r • Voltage change induced by drift along d x : CU d U q E d x 0 R e 1 d E r • Integration along frag. track: 0 U ( 1 cos ) d r C W d r d 0 Seite 11 von 58
Simulated Pulse-Height Spectra FF energy loss in the fissile deposit Monte Carlo calculations: • ( A , Z ) of the fissioning system: multiple-chance fission! • Range data for U 3 O 8 and Ar/CH 4 • Model for the surface roughness: < r a > • FF distributions: Y ( E n , A ff , Z ff ) FF anisotropy: W ( CM ) = (1+ B· cos cm )/2 • • Incomplete momentum transfer Seite 12 von 58
Analytical Calculation of the Detection Efficiency Absorption of fragments in the fissile layer: W ( ) = (1+ B cos( ))/2 t e 1 ... 0 . 94 0 . 99 f 2 R ff Higher order contributions: • Anisotropic fragment emission • Momentum transfer R t Uncertainty: u e / e f ≈ 1% - 2% • depends very much on sample quality Ref.: G.W. Carlson, NIM 119 (1974) 97-100 Seite 13 von 58
Fission Fragment Detection Efficiency • Background at small pulse heights 100 90 nat Pb-PPFC a decay of fissile nuclei – 80 E n = 145 MeV 70 – recoil nuclei from backing materials 60 fission fragments counts 50 • Extrapolation of fission events 40 light charged particles (normalized) into this region 30 20 – thickness and ‘roughness’ of deposits 10 0 – biasing scheme 0 20 40 60 80 100 120 140 160 180 200 pulse height / arb. units 300 238 U-PPFC 250 a particles 238 U-PPFC 200 counts per bin fission fragments 150 100 50 0 0 100 200 300 400 500 pulse height / arb. units Painted 238 U 3 O 8 layers Electro-sprayed 238 U 3 O 8 layers Seite 14 von 58
242 Pu Fission Chambers for Cross Section Measurements • 242 Pu layers produced by molecular plating (U. Mainz) – m Pu = 42 mg, 242 Pu: 99.9668 % eight layers: 116 m g/cm 2 – – A a = 6.17 MBq – R sf = 34 s -1 • Number of fissile atoms N Pu : – Spontaneous fission rate t 1/2 = (6.77 ± 0.07) 10 10 a – Narrow-geometry alpha counting • Fast pre- amp.’s : a pile-up! • Continuous P10 flow (nanofilters) Seite 15 von 58
The Measurement of Neutron Energy Distributions: TOF Methods Seite 16 von 58
TOF Spectrometry: Principles D t g n d • Neutron energy determined from a velocity measurement: d 1 g g 2 v E ( 1 ) mc , 2 t 1 v c • Energy resolution: 2 2 d d d d d E v v t d g g ( 1 ) , E v v t d Time and distance resolution contribute in same way: express flight time d t by an equivalent distance d d eq Seite 17 von 58
Measurement of TOF Distributions Quasi-monoenergetic source ‘White’ source • Start signal: neutron detector • Stop signal: beam pick-up • Inverted time scale: TOF = t stop – t start • Measured neutron flight time: t m = TOF g + d / c – TOF n NB: Measured flight time t m includes time spent in target and detector! Seite 18 von 58
Width of TOF Peaks • Contributions to the width of TOF peaks : Beam: time spread of the beam pulse d t beam – d t src = d src / v – Source: beam transit time d E src = f kin ( E beam , E n )·(d E /d x )· d src energy-loss broadening f kin ( E n , )· d kinematical broadening d t slow ≈ A / S s v slowing-down time d E spl = f kin ( E n , )· d – Sample: kinematical spread d t det = d det / v – Detector: transit time multiple scattering spread d t ms 2 t ( E , l ) • Total TOF spread: d d j n, j j d 2 2 2 t t E i n, j 2 E n, j i j • Relative importance of time and energy broadening depends on the details of the setup: – Masses of projectiles and target nuclei: source and sample – Flight paths: source and sample Seite 19 von 58
Time Response of Organic Scintillation Detectors • Multiple scattering affects time response: – Width of the main peak: flight time through det. 1 H(n,n) 1 H – Exponential tails for pancake-like detectors ( d >> l ) – Non-Gaussian time response: R ( E , t ) – Modeled with Monte Carlo codes 12 C(n,n) 12 C Calc. (NRESP7) Exp. time / 0.1 ns Seite 20 von 58
Example: PTB TOF Spectrometer D(d,n) E n,0 = 10 MeV – d t beam = 1.6 ns – d E n,src = 106 keV – d src = 17 cm, d det = 12 m d E n / E n = 1.4 % for E n,det = 2 MeV 1.8 % for E n,det = 10 MeV Seite 21 von 58
Example: PTB TOF Spectrometer 51 V 12 C(n,n) 12 C CH 2 1 H(n,n) 1 H 12 C(n,n') 12 C* Separation of TOF peaks Kinematical broadening Vanadium sample – Polyethylene (PE) sample E n,0 = 10.21 MeV – Incident energy: E n,0 = 10.21 MeV – Scattering angle: = 29.3 ° = 36.8 ° Seite 22 von 58
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