Neutron detection efficiency from the 7 Li(p,n) reaction (R405n)

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1 Neutron detection efficiency from the 7 Li(p,n) reaction (R405n) Y. Satou September 3, 2011 Abstract The detection efficiency for the neutron walls used in R405n was evaluated using the 7 Li(p,n) reaction data taken just prior to the secondary beam runs. 1 Introduction For the purpose of making efficiency calibration of the neutron detector, a separate measurement of the 7 Li(p,n) reaction usingthe H 2 + beam was performedduringr405. Thedetector was irradiated by a mono-energetic neutron beam produced by the charged-exchange reaction. By counting the number of neutrons detected to obtain the differential cross section, and comparing it with reported one in Ref.[1], the neutron detection efficiency was deduced. 2 Missing mass spectra for the 7 Li(p,n) reaction In Figs. 1 and 2, missing mass spectra for the 7 Li(p,n) reaction measured in each wall of the neutron detector are shown (the wall-1 and 2 were treated as a single unit, and the same is true for wall-3 and 4). The proton beam energy at the middle of the 7 Li target is estimated to be 67.8 MeV. The software threshold on the pulse height was set at 3 MeVee. Each spectrum was obtained by dividing the center-of-mass scattering angle in an interval of 1.5. In each of them, a peak due to formation of the ground and the first excited 0.43 MeV states in 7 Be can be clearly seen. The typical FWHM width of this peak was 2.92 MeV, and these states could not be separately observed in the spectra. The yield contained in the peak region, extracted by fitting using a Gaussian function, is summarized in Table 1. In the fitting following background function was used: bf = a 1 x e a 2x e 0.5(x 1.59) a 3. (1) Here a 1 a 3 are parameters. The functional shape was chosen so that it starts to take finite values at around 1.59 MeV, considering that the threshold is located at 1.59 MeV for the 7 Be 3 He+α break-up reaction. A constant background component was assumed to exist. The uncertainty in the yield was estimated by repeating the fitting with different yield values and by finding the yields which give a χ 2 value larger than the optimum value by one unit. ysatou/r405n/neutron eff.pdf 1

2 Table 1: Summary of the number of events recorded in each neutron counter hodoscope at each center-of-mass scattering angle. The results are for a neutron threshold value of 3 MeVee. θ c.m. Wall-1 & 2 Wall-3 & 4 Solid angle: Ω q c.m. Yield Error Yield Error (degree) (count) (count) (count) (count) (str) (fm 1 ) Figure 1: Missing mass spectra for the 7 Li(p,n) reaction at 67.8 MeV measured in neutron Wall-1 and 2. 2

3 Figure 2: Missing mass spectra for the 7 Li(p,n) reaction at 67.8 MeV measured in neutron Wall-3 and 4. 3 Lithium target Specifications of the lithium target used for the 7 Li(p,n) reaction measurement are summarized in Table 2. The target was not an isotopically enriched one; it contained both 6 Li and 7 Li with a natural abundance. Table 2: Parameter of the lithium target used for the 7 Li(p,n) reaction measurement. Thickness ± 4.23 (mg/cm 2 ) Isotope abundance ratio 6 Li : 7 Li = 7.5% : 92.5% Atomic weight of 6 Li Atomic weight of 7 Li Areal density ( ± ) 10 4 (/mb) 4 Differential cross section and detection efficiency The angular distribution of the differential cross section dσ dωc.m. for the 7 Li(p,n) reaction was derived for each neutron wall (group of walls) based on the following formula: Yield = N beam N Ω ǫ neff dσ. (2) dω c.m. Here N beam refers to the number of incident protons [2], N the areal density of the target 3

4 Table 3: The neutron detection efficiency for each group of the walls, obtained by normalizing the measured differential cross sections for the 7 Li(p,n) reaction to the reported cross section [1]. The neutron threshold was set at 3 MeVee. Wall Detection efficiency: ǫ neff Wall-1 & ± 0.107% Wall-3 & ± 0.084% particles, ǫ neff the neutron detection efficiency. The scattering angle is given in the centerof-mass system, and so is the differential cross section. In this analysis data acquisition efficiency (live time) was separately treated for beam counting and for neutron counting. The neutron detection efficiency ǫ neff was obtained by normalizing the experimental differential cross section to the reported one in Ref. [1]: [ ] dσ = σ 0 exp q 2 r2. (3) dω c.m. 3 The values of σ 0 =27±0.8 mb/sr and r2 3 =5.8 fm 2 (the value at E p =80 MeV) were adopted in the present analysis. The first three points below θ c.m. =4.5 were used for the normalization procedure (100% geometrical efficiency was ensured for these points). The results of the extracted efficiency values are summarized in Table 3. The uncertainties in the efficiency values are those coming from the fitting procedure (only the statistical errors were taken into account). The experimental differential cross sections obtained by taking into account the efficiency values in Table 3 are shown in Fig. 3. The red line in the figure is the literature distribution [1] given by Eq. (3). 4

5 Figure 3: The angular distributions of the differential cross section for the 7 Li(p,n) reaction measured in each group of the neutron walls. The distributions were normalized to the literature value [1] (shown by a red line) to deduce the detection efficiencies. The forward three points were used for the normalization procedure. The results are for a neutron threshold value of 3 MeVee. References [1] T.N.Taddeucci et al., Phys. Rev. C 41, 2548 (1990). [2] Y.Satou, ysatou/r405n/beam 23oa h2p.pdf 5