Week 10: Chap. 14 Slow Neutron Detection

Transcription

1 Week 10: Chap. 14 Slow Neutron Detection (Gamma ray) Backgrounds Slow Neutron Detection -- nuclear reactions --- spectra properties -- Proportional Counters --- fill gas --- lined detectors Fast neutron detection NIM A618 (o10) 75

2 Chap. 14 Slow Neutron Detection All neutron detection relies on observing a neutron-induced nuclear reaction. The nuclear cross sections have a characteristic variation with energy: Charged particle reactions are dominated by the coulomb energy since both reaction partners have a positive charge: s(e) ~ pr (1 V/E CMS ) where V is the coulomb barrier. Neutron-induced reactions also have a characteristic shape.. The interaction is always attractive and the cross section for l=0 capture reactions always grows with 1/v at (very) low energies. The form is derived from the Breit-Wigner lineshape: 113 Cd + n 114 Cd + g s s s s Cap ~ = p" = p" 1 v = 4p" ( E - E 4G G n g G G G n g 0 G G ) " n g + ( G / ) E = = E! mv 0 ; G = G G n ~ v n + G g + G g 113 Cd s Total

3 Popular Slow Neutron Reactions n + 3 He ( 4 He)* p + 3 H, Q=0.765 MeV, target abundance = 1.4x10-4 % n + 6 Li ( 7 Li)* 4 He + 3 H, Q=4.78 MeV, target abundance = 7.5% n + 10 B ( 11 B)* 7 Li* + 4 He, Q=.31 MeV, Branch=94%, target abundance = 19.9% 7 Li + 4 He, Q=.79 MeV, Branch=6% n Cd ( 114 Cd)* 114 Cd + g, Q~8 MeV, target abundance = 1.% (1k barns) n Gd ( 158 Gd)* 158 Gd + g, Q~8 MeV, target abundance = 15.6% (55k barns) n + 35 U ( 36 U)* (fission frags) Q~00MeV, TKE~160MeV, abundance = 0.7% (n,p) (n,a) (n,a) (n,g) (n,g) (n,f) Check : y = ax + b " Logσ = Δy % $ 'LogE + b # Δx & Δy Δx = σ = E ~ 1/ E Fig Knoll, 3 rd, 4 th Eds.

4 Slow n Detection: Gas-filled counters Gas-filled proportional counter.. usual gas-gain amplification on the central anode n + 3 He p + 3 H, Q=0.765 MeV s ~ 5333 l /1.8 barns K.E. <ev ~0 Q3/4 Q/ MeV Range in Si ~6µm ~5µm Range in gas ~ 10 3 x Range in solid few mm s [N.B. (Rr) ~ 0.5 mg/cm for these a s in He gas] What about the pulse-height distribution? Reaction in gas volume: both products range out -- full energy signal Reaction elsewhere??

5 Slow n Detection: Gas-filled Pulse Height n + 3 He p (573 kev) + 3 H (191 kev) 3 He gas W ~ 33 ev Typical: 5mm diameter, 50µm anode P = 5-10 bar (!) V~ 1.5kV.. M ~ 0.. C ~ 0pF Ideal Fig Knoll, 3 rd, th Ed.

6 Slow n Detection: BF 3 Ideal 7 Li 4 He BF 3 gas n + 10 B 7 Li + 4 He, Q=.8 MeV [7%] 7 Li* + 4 He, Q=.3 MeV [93%] [ K.E MeV (strong branch)] ( R-air ~ 7.5mm ) 7 Li + g, E=0.477 MeV More Realistic Fig Knoll, 3 rd,4 th Eds. BF 3 gas W ~ 33 ev, enriched to ~96% Typical: 5mm diameter, 50µm anode P = Torr, (nero 900 Torr) V~.5kV.. M ~ 40 Real Annl Nucl Energy 6 (013) 1

7 Slow n Detection: BF 3 Ideal 7 Li 4 He BF 3 gas n + 10 B 7 Li + 4 He, Q=.8 MeV [7%] 7 Li* + 4 He, Q=.3 MeV [93%] [ K.E MeV (strong branch)] ( R-air ~ 7.5mm ) 7 Li + g, E=0.477 MeV BF 3 gas W ~ 33 ev, enriched to ~96% Typical: 5mm diameter, 50µm anode P = Torr, (nero 900 Torr) V~.5kV.. M ~ 40 More Realistic Fig Knoll, 3 rd,4 th Eds. Poor Annl Nucl Energy 6 (013) 1 NIM A 63 (010) 1035

8 Multiple Gas Proportional Counters High Efficiency Position Sensitive NSCL NIM A618 (o1o) 75 FRM II He/CF 4 9.7/0.3 bar NIM A580(oo7)1414

9 Multiple Gas Proportional Counters High Efficiency Position Sensitive General Electric RS-P NSCL NIM A618 (o1o) 75 FRM II He/CF 4 9.7/0.3 bar NIM A580(oo7)1414

10 Slow n Detection: Fission Chambers n + 35 U ( 36 U)* (fission frags) Q~00MeV, TKE~160MeV, abundance = 0.7% Fig. 14. Knoll, 3 rd, 4 th Eds. Absorption efficiency of all nuclear reaction-based devices (from text): UO deposit on wall Change by ~ 5x Fig Knoll, 3 rd, th Eds. ε(e n ) =1 e Σ(E )Δx where Σ(E) = ρ N σ (E) ε(e n ) ~ ρ N σ (E)Δx for small values

11 Slow n Detection: Fission Chambers 35 U 38 U blank fission Beam LAMPF NIM A336 (1993) 6 blank 68 kpa gauge pressure P-10 Parallel plates, 6mm gap, -300 V

12 Boron-loaded GEM device n + 10B 7Li + 4He, Q=.8 MeV [7%] 7Li* + 4He, Q=.3 MeV [93%] CASCADE Detektor, 00 mm x 00 mm, Stapel aus GEM-Folien mit D-Auslesestruktur in der Mitte, komplette Auslese- und HistogrammierElektronik auf der Rückseite des Detektors

13 Slow n Detection: lithium scintillators Lithium loaded materials.. usual scintillation device with PMT for ToF 1) Li glass.. Scintillation efficiency ~ 0.45% 395nm, with ~ 7k photons/n 4% of NaI(Tl), current manufacturers quote 15% ) LiI (Eu)...8% 470 nm with ~51k photons/n [~ same as NaI(Tl) ] n + 6 Li 4 He + 3 H, Q=4.79 MeV s ~ 940 l /1.8 barns

14 Chap. 14 Slow n Detection Question Estimate the intrinsic efficiency and signal height from a fission chamber for thermal neutrons made up as follows: a natural uranium metal coating of 1 mg/cm on the inside of a 1cm diameter tube with a 50µm central anode. The tube is filled with Ar/Methane (P-10) at 1 atm pressure and is operated at 1000V and has a (stray) capacitance of 50 pf. e ( E n ~ ) ~ ( ) A -4 r Ns ( E) Dx = 400b 10 cm / b 3 10 ( / ) 19( 3 MM ç ø / ) è mg g g cm ( ) ( ) -6 x x x cm x cm x cm = 7x10 e detector = e ( E n æ ç è ) N rf ö æ 1mg / cm ö ø ( ) V ~ Q C = MΔE C W = M TKE / C W ln(m ) = V ln ln(b / a)δv ln " $ # V pa ln(b / a)k % ' &