Cross sections of the 144 Sm ( n , α ) 141 Nd and 66 Zn ( n , α ) 63 Ni reactions at 4 . 0 , 5 . 0 and 6 . 0 MeV

Cross sections of the 144Sm(n,α)141Nd and 66Zn(n,α)63Ni reactions were measured at En = 4.0, 5.0 and 6.0 MeV performed at the 4.5-MV Van de Graaff Accelerator of Peking University, China. A doublesection gridded ionization chamber was used to detect the alpha particles. The foil samples of Sm2O3 and enriched 66Zn were placed at the common cathode plate of the chamber. Monoenergetic neutrons were produced by a deuterium gas target through the 2H(d,n)3He reaction. The neutron flux was monitored by a BF3 long counter. Cross sections of the 238U(n,f) reaction were used as the standard to perform the (n,α) reaction measurement. Present results are compared with existing measurements and evaluations. They are generally in agreement with TALYS-1.6 code calculations. For the 144Sm(n,α)141Nd reaction our measurements support the data of JEF-2.2. For the 66Zn(n,α)63Ni reaction present results support the data of EAF-2010 and


Introduction
Cross section data for the charged particle emitted reactions induced by the fast neutron are important both in basic nuclear physics and nuclear engineering applications.Samarium isotopes are relatively high-yield fission products in nuclear reactors and zinc is one of the structural materials for nuclear reactors and other high energy installations, so accurate knowledge of their neutron cross sections can be important for nuclear technology applications.The (n,α) reactions, in particular, are gas-producing and exothermic ones.The helium gas accumulated in the material will cause [1] serious embrittlement problems.Experimental measurements, however, are rather scanty for 144 Sm(n,α) 141 Nd and 66 Zn(n,α) 63 Ni reactions.For the 144 Sm(n,α) 141 Nd reaction, only two measurements exist for neutron energies around 14 MeV, and there are large differences between them [2].For the 66 Zn(n,α) 63 Ni reaction, no measurements were made for neutron energies from 0 to 20 MeV.Although most of evaluated nuclear data libraries contain these two reactions, large discrepancies exist among them both in the trend and magnitude [3].So, accurate measurements are demanded to address the application needs and clarify exciting discrepancies among different libraries.

Details of experiments
The experiments were performed at the 4.5-MV Van de Graaff accelerator of Peking University, China.The experimental apparatus consists of three main parts: the neutron source, the charged particle detector (with samples inside) and the neutron flux monitor.
A deuterium gas target was used to produce the monoenergetic neutron through the 2 H(d,n) 3 He reaction.
The α-particle detector is a double-section gridded ionization chamber (GIC) with a common cathode, and its structure can be found in Ref. [7].Working gas of the GIC was Kr +2.83% CO 2 .
In the present work, enriched 144 Sm 2 O 3 and 66 Zn foil samples were prepared.The foil samples were prepared using the press method with which nearly all material can be utilized without loss.Data of the samples are listed in Table 1.
A sample changer was set at the common cathode of the ionization chamber with five sample positions, and back-to-back double samples can be placed at each of them [7].The 144 Sm 2 O 3 and enriched 66 Zn foil samples were attached to the tantalum backings 0.1 mm in thickness.With back-to-back samples, forward (0 • ∼ 90 • ) and backward (90 • ∼ 180 • ) emitted α-particles can be detected simultaneously.A 238 U sample was placed in the GIC at the forward direction of the other positions of the sample changer to determine the absolute neutron flux by measuring the fission fragments.
The neutron flux monitor is a BF 3 long counter.The axis of the counter was along the normal line of the electrodes of the ionization chamber as well as the 0 • direction of the deuteron beam line.
Two-dimensional spectra of the cathode-anode coincidence signals for forward and backward directions were recorded separately, from which the number of α-events from the measured (n,α) reactions can be obtained.The data-acquisition system (DAqS) can be found in Ref. [8].Generally, for each neutron energy point, the experimental process in turn is as follows: 1) compound α source measurement for energy calibration of the DAqS, 2) foreground measurement for expected α events, 3) background measurement with tantalum sheets, 4) 238 U fission fragment measurement for neutron flux calibration with the BF 3 long counter, and 5) α source measurement again for checking the stability of the DAqS.However, because the Q value of the 144 Sm(n,α) 141 Nd reaction is relatively large (7.874MeV) comparing with that of the background reaction, the background measurement had not been done for the 144 Sm(n,α) 141 Nd reaction.At neutron energy point E n = 4.0, 5.0 and 6.0 MeV, the beam duration was 46, 31 and 26 h, respectively for 144 Sm(n,α) 141 Nd reaction and 29, 16, 12 h for 66 Zn(n,α) 63 Ni reaction.
Since the 238 U(n,f) reaction were used as the standard to perform the measurement, the cross section σ of the (n,α) reaction can be calculated by the following equation: The σ f is the standard 238 U(n,f) cross sections taken from ENDF/B-VII.1 library [3].The N α and N f are the detected counts above the energy threshold from the (n,α) reaction (after background subtraction) and from the 238 U(n,f) reaction, respectively.The ε α and ε f are the detection efficiency for α-particles and fission fragments.The N 238U and N samp are the numbers of 238 U and 144 Sm (or 66 Zn) nuclei in the samples, respectively, N B F3− f and N B F3−α are the counts of the neutron flux monitor (BF 3 counter) for 238 U fission and for (n,α) event measurements, respectively.The detection efficiency for α-particles ε α and fission fragments ε f can be written as Where N det is the detected counts, i.e., N α orN f , N th is the number of events with amplitudes below threshold (the threshold correction), and N ab is the number of events absorbed in the samples (the self-absorption correction) [9].The calculation method of ε will be discussed in the next section.
The cross sections of the 144 Sm(n,α) 141 Nd and 66 Zn(n,α) 63 Ni reactions in forward and backward directions can be calculated separately using Eqs.( 1) and ( 2).The complete cross section can be obtained by adding them up together.

Data processing, results, and discussions
The data processing methods are almost the same for both (n,α) reactions to be measured at all neutron energies.As an example, the following descriptions are given for the data processing of the 144 Sm(n,α) 141 Nd at E n = 4.0 MeV and 66 Zn(n,α) 63 Ni reactions at E n = 5.0 MeV for the forward direction.
Firstly, a two-dimensional spectrum of compound α-sources was obtained and used for energy calibration and valid-event-area determination.The four groups of α particles, i.e., α 1 , α 2 , α 3 , α 4 are emitted from 234 U, 239 Pu, 238 Pu and 244 Cm alpha sources with energies of 4.775, 5.155, 5.499, 5.805 MeV, respectively.The determined valid-event-area is between curves of the 0 • and 90 • emitted angle of α-particles as shown in Fig. 1.
Secondly, cathode-anode two-dimensional spectra for the foreground and background are obtained from the experimental data.As mentioned before, due to the large Q value background measurement for the 144 Sm(n,α) 141 Nd reaction were not made as shown in Fig. 1.
Figure 2 shows the results of 66 Zn(n,α) 63 Ni reaction after background subtraction.The valid-event-area was used to pick out the expected α-events from the 144 Sm(n,α) 141 Nd or 66 Zn(n,α) 63 Ni reactions.
Figures 3 and 4 show the anode spectrum after the projection of the selected α-events.As can be seen from the blue curves (pure event), α-events corresponding to different energy levels of the residual nuclei cannot be separated and they become one broad peak due to the close energy levels of the residual nuclei and the energy loss of α-particles in the sample.The α counts above the threshold can be obtained from the anode spectrum.The threshold are shown as the arrows in Figs. 3 and 4.   As shown in Figs. 3 and 4, in addition to the α counts above threshold, there are also α events below the threshold which cannot be detected.Therefore, threshold and self-absorption corrections are needed.To determine the detection efficiency of α-particle, the anode spectra were simulated as shown in Figs. 3 and 4 with the red curves.The SRIM code was used to get the stopping power of α-particles in the samples, and the TALYS-1.6 code to predict the angular and energy distributions of the emitted α-particles [10,11].To obtain the well fitted anode spectrum, level densities from Hilaire's combinatorial tables (Idmodel 5 in TALYS) were used within the TALYS-1.6 code to predict the angular and energy distributions of the emitted α-particles for the 144 Sm(n,α) 141 Nd reaction.For the 66 Zn(n,α) 63 Ni reaction, the default level densities, constant temperature model (Idmodel 1 in TALYS), were used.
ii Predictions of TALYS-1.6 with adjusted parameters.
The corrected α counts are obtained through the division of α counts above threshold and the correction factor, which is calculated through the simulated anode spectra.The calculated correction factor value for the 144 Sm(n,α) 141 Nd reaction is from 77% to 82% and that for the 66 Zn(n,α) 63 Ni reaction is from 82% to 87%.
The measured results of cross sections for the 144 Sm(n,α) 141 Nd and 66 Zn(n,α) 63 Ni reactions are listed in Table 2.The calculated results using the TALYS-1.6 code with default and adjusted parameters are also listed in Table 2. Parameters of the optical model potential were adjusted to obtain a good agreement with present results.For the 144 Sm(n,α) 141 Nd reaction, the factor of geometry radius parameter, r v , were adjusted from 1 (the default) to 0.8 and the constant c of the adjustment function for tabulated level densities were adjusted from 1 (the default) to 0.3.The adjustable range of the parameter r v and c is from 0.1 to 10 and from −10 to 10, respectively.Although the adjustments of the parameter r v and constant cfor tabulated level densities are in the adjustable range, they are relatively large.This may due to the fact that the 144 Sm is heavy nucleus and special structure may exist in the level densities.For the 66 Zn(n,α) 63 Ni reaction, the r v were adjusted from 1 to 1.02 [11].
The uncertainty was calculated using the error propagation formula.Major source of uncertainty is the counting error for α particles (including statistics and background subtraction) produced by the valid-event-area cut and the energy threshold cut.The uncertainty of detection efficiency for α particles are due to the threshold and self-absorption correction based on Talys-1.6 code.Numbers of 144 Sm, 66 Zn nuclei were determined by the mass weighing.Numbers of 238 U nuclei were determined by α decay and the uncertainty of the numbers of nuclei is less than one percent.
Results of the present work are compared with those of existing measurements, evaluations, and TALYS-1.6 calculations as shown in Figs. 5 and 6.

Conclusions
In the present work, cross sections for the 144 Sm(n,α) 141 Nd and 66 Zn(n,α) 63 Ni reactions are measured E n = 4.0, 5.0 and 6.0 MeV.Our results are generally in agreement with TALYS-1.6 calculations.For the 144 Sm(n,α) 141 Nd reaction there are big disagreements among different evaluation libraries.Our measurements support the data of JEF-2.2 libraries.For the 66 Zn(n,α) 63 Ni reaction, no measurements exist for neutron energies from 0 to 20 MeV.Present results at 4.0 and 5.0 MeV support the data of EAF-2010, and good agreements are obtained between our results and those of TENDL-2015.Further measurements for both the two reactions are needed at neutron energies from 8 to 12 MeV to clarify the discrepancies among different libraries.

Figure 1 .
Figure 1.Two-dimensional spectrum of the 144 Sm(n,α) 141 Nd reaction at E n = 4.0 MeV for the forward direction.The axes represent the amplitudes of cathode and anode signals of event.

igure 2 .
Two-dimensional spectrum of the 66 Zn(n,α)63 Ni reaction at E n = 5.0 MeV for the forward direction (after background subtraction).

Figure 3 .
Figure 3. Anode spectrum of the 144 Sm(n,α) 141 Nd reaction at E n = 4.0 MeV for the forward direction.

Figure 4 .
Figure 4. Anode spectrum of the 66 Zn(n,α) 63 Ni reaction at E n = 5.0 MeV for the forward direction.

Figure 5 .
Figure 5. Present cross sections of the 144 Sm(n,α) 141 Nd reaction compared with existing measurements and evaluations for neutron energy region from 1.0 to 8.0 MeV.

Figure 6 .
Figure 6.Present cross sections of the 66 Zn(n,α) 63 Ni reaction compared with existing measurements and evaluations for neutron energy region from 2.0 to 8.0 MeV.

Table 1 .
Description of the samples.