Production & Measurement of Thermal Neutron at RCNP Chhom Sakborey Nguyen Thi Duyen An Tran Hoai Nam Li Chunjuan Wang Mian

2 Outline Introduction Methodology Experiments arrangement γmeasurement and results β-γcoincidence measurement and results Conclusion

3 Outline Introduction Methodology Experiments γmeasurement and results β-γcoincidence measurement and results Conclusion

4 Introduction (1) About thermal neutrons: Discovered by Enrico Fermi (1938 Nobel prize was awarded for his work on thermal neutrons). Produced when fast neutron enter and are slowed down in material with large concentration of hydrogen such paraffin or water. More readily absorbed by atomic nucleus (large reaction cross section)

5 Introduction (2) –Application of neutrons: Therapy Neutron Activation Analysis Material structure Nuclear reaction …

6 Purpose of our experiments To produce neutrons by Be(p,n) reaction with 53MeV protron beams from the cyclotron accellerator, and then thermalize them in the water-drum. To measure the space distribution of the thermal neutron in the water-drum.

7 Methodology Activation method to detect neutron

8 1) 27 Al + 1 n[ 28 Al*] γ + 28 Al (n, γ) reaction 1 H + 27 Mg (n, p) reaction 4 He + 24 Na (n, α) reaction 2 1 n + 26 Al (n, 2n) reaction 1 n + 27 Al elastic scattering 2) 197 Au + 1 n[ 198 Au* ] γ Au (n, γ) reaction Methodology –In our experiments,we choose 197 Au foils and 27 Al foils

9 C : Counting rate (s -1 ) N 0 : Number of nuclei Φ : Neutron flux (cm -2 s -1 ) σ : Cross section (1barn = cm 2 ) λ : Decay constant ( = Ln2 / T 1/2 ) t i : Irradiation time ( h) t w : Waiting time (h) t m : Measurement time (h) I γ : Relative intensity (%) ε : Detector efficiency (%) g : Geometry efficiency (%) Activation equation

10 Experiment(1) Target preparation for 9 Be(p,n) 9 B reaction Set the position and make sure that the beam is in the center of the target. Proton beam: –E = 53 MeV –I = 80 nA Beryllium target Collimator

11 Experiment(2) Moderate fast neutron with water Water tank

12 Experiment(3) Set some kind of foils into the water- drum –Gold foils –Gold foils with cadmium outside –Aluminum foils

13 D C B A Z Assignment of the foils 5 0 cm

14 Experiment(4) Activity measurement γ measurement HP-Ge β-γ coincidence Pla. scin. NaI(Tl)

15 Outline Introduction Methodology Experiments γmeasurement and results β-γcoincidence measurement and results Conclusion

16 Apparatus –HV = V –Gain : 0.72 x 20 –Shaping time: 6μ s (1/3)

17 (2/3) Setup Detector Source 5cm Lead shielding

18 Measurements 2 measurements with golden foils: –2 hours after activation, measured time: 300s –3 days after activation, measured time: 600s 1 measurement with aluminum foils: 20 hours after activation, measured time: 5400s ReactionHalf-timeMain gamma- rays (keV) Intensity (%) Isotope abundance (%) 197 Au(n,γ) 198 Au2.695 d Al(n,a) 24 Na h (3/3)

19 Energy Calibration Energy (keV) Channel of centroid Error (channel) Fitting function: Y = A + B * X A = ± B = ± 1E-4 (1/5)

20 Energy (keV) EfficiencyError (%) Efficiency Calibration (2/5)

21 (3/5) Result: Thermal Neutron Distribution A B C Z D 1.12E E E E E E E E E E E E E+07 Be Target

22 Fast neutron flux density: Epithermal neutron flux density: PositionDensity flux (cm -2 s -2 ) Error (%) A 5cm1.96E B 2.5cm9.97E C E Result: Epithermal & Fast Neutron Flux (4/5) PositionDensity flux (cm -2 s -2 ) Error (%) A 10cm2.97E B 0cm1.58E

23 PositionΦ fast Φ ther Φ ther / Φ fast A 51.96E E B E E C E E (5/5) PositionΦ epi Φ ther Φ ther / Φ epi A 10cm 2.97E E B 0cm 1.58E E Comparison

24 Outline Introduction Methodology Experiments γmeasurement and results β-γcoincidence measurement and results Conclusion

25 β -γcoincidence measurement Principle –Principle of coincidence –Principle of absolute activity measurement with β-γcoincidence system Experiments and Results

26 Principle of coincidence –β–β γ β-β- β γ Det.1 Det.2 Coincidence

27 Principle of coincidence Pulse 1 Pulse 2 Coincidence Pulse t t <t >t

28 Coincidence technique True coincidence & accidental coincidence –True coincidence events: correlation –Accidental coincidence events: no correlation. eg.βfrom one source and γfrom another source. Resolving time for coincidence system –The shortest time which the system can distinguish between two signals –t 1 :the width of signal 1 –t 2 :the width of signal 2

29 Resolving time measurement 0 -t d tdtd Counting rate 2τ2τ τ-electronic resolving time Delay Coin. scaler Dis. Pulse generator Delay

30 Resolving time measurement counting rate -t d 0 tdtd 2τ’2τ’ τ’-physical resolving time Delay Det.1 Det.2 Dis. Coin. HV Scaler β γ Dis.

31 Absolute activity measurement with β-γcoincidence system HV β γ Pla. Dis. NaI(Tl) Dis. Delay n βγ (βγ) Scaler Delay Coin. Scaler nγ(β)nγ(β) nβ(γ)nβ(γ) –no delay for Pla. in our experiment

32 Absolute activity measurement with β -γcoincidence system Corrections for the counting rate

33 Absolute activity measurement with β-γcoincidence system Solid angle Correction factors of scattering and absorption Discrimination coefficient of the discriminator Efficiency of the detector Probability of detectingγrays while one βsignal being detected Source activity

34 Absolute activity measurement with β-γcoincidence system Advantages –The results have no relationship with the efficiency of the detector, data analysis is simple. Limits –To make sure that There should be

35 β -γcoincidence measurement Principle –Principle of coincidence & some concepts –Principle of absolute activity measurement with β-γcoincidence system Experiments and Results

36 Experimental setup –Gold foil’s position in water-drum: (41.32, 0, 5)cm, the center of the front surface as (0,0,0) –Distance from source to Plas.:3cm –Distance from source to NaI(Tl):2cm

37 Experimental process-1 Check the detectors with oscilloscope and MCA –HV for NaI(Tl): -1850V; HV for Pla.:-2000V

38 Experimental process-2 Set the threshold of the discriminator –Very important! Gate Generator NIM-TTL Amp Det. Dis Shaping MCA input gate

39 Spectra after setting the threshold Threshold(NaI)=-85.8mV Threshold(Pla.)=-406.0mV

40 Experimental process-3 Resolving time measurement

41 Experimental process-4 –With 198 Au source, Al foils(0.31mm) before NaI detector. –With 198 Au source, Al foils(0.31mm) before Pla. Detector –Without source

42 Results n β (β)[s -1 ]n β (b.g.)+n β (γ )[s -1 ] n β [s -1 ] n γ (γ)[s -1 ]n γ (b.g) [s -1 ] n γ [s -1 ] n βγ (βγ) [s -1 ]n βγ (b.g.) [s -1 ]n βγ (accidental) [s -1 ] n βγ [s -1 ]35.98 –Counting rates of βsignals,γsignals andβγcoincidence signals

43 Results A[Bq]N Au197 (A5cm)σ[b]Φ[cm -2 s -1 ] 1.60E E E+07 source of uncertaintyuncertaintytotal uncertainty A statistical1.87% 2.74% system2.00% N Au % σ0.14% Uncertainty estimation Neutron fluence rate at (41.32, 0, 5)cm Comparision with HPGE’s result : 1.12E8 ±5.99E6

44 Outline Introduction Methodology Experiments γmeasurement and results β-γcoincidence measurement and results Conclusion

45 Conclusion Neutrons were produced by Be(p,n) reaction with 53MeV proton beams from the cyclotron accelerator, and then were thermalized in the water-drum. The space distribution of the neutron fluence rate in the water- drum was measured with activation methods,and the results showed that the distribution is isotropic. The activities of the gold foils were measured both with HPGE detector and β-γcoincidence system, and the results were compared with each other. Energy spectrum of neutron may need more measurements or calculation.

46 Acknowledgement JICA Osaka University Professors, assistant teachers RCNP …….. Thanks a lot!!

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