A system for transmission measurements of total neutron cross sections using two silicon detectors

Nuclear Instruments and Methods in Physics Research A 356 ( 1995) 408-41

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I NUCLEAR INSTRUMENTS B METHODS IN PHYSICS RESEARCH

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A system for transmission measurements of total neutron cross sections using two silicon detectors G. Doukellis *, S. Kossionides, G. Galios, T. Paradellis Institute of Nuclear

Physics. N.C.S.R. Received

“Democritos”,

I

Apia Paraskevi. Atrikis. Greece

September 1994

Abstract A new method is presented for transmission measurements of total neutron cross sections. The detection system has low background with good monitoring of the incident neutron flux and can be used for measurements at & > 5.5 MeV. This method has been applied to total cross section measurements for the system “B + n at & = 7.3-8.5 MeV. The results show the potential of this method for nuclear spectroscopy measurements of high accuracy.

1. Introduction When measuring excitation functions of total neutron cross sections with the transmission method, one needs for accurate measurements: a low background detection system, good monitoring of the incident neutron flux and measurement of the neutron beam energy. Usually for such a system one uses a pulsed neutron beam. Here we present a system that fulfills the above requirements but does not require a pulsed beam. This system consists of two silicon semiconductor detectors. One is placed in front of the scattering sample (front detector) to monitor the incident neutron flux and the other (back detector) is placed in the back of the scattering sample to measure the attenuation of the neutron flux and the neutron energy. This system has been used to measure the *IB + n excitation function at En = 7.3-8.5 MeV [ 11.

2. Experimental

method

In transmission measurements of total neutron cross sections, the usual experimental arrangement consists of: a neutron source, the scattering sample and a neutron detector. Two measurements of the neutron flux are taken with the sample in and the sample out. In our case a monochromatic neutron beam is produced via the D( d, n) “He reaction. The neutron source is a gas cell 3.7 cm long, filled with deuterium gas [ 21. The entrance window is a 2.5 mg cm p-Zthick Ti foil. A beam of deuterons, provided by the “Democritos” Tandem accelerator, enters the gas cell and stops on a 1 mm thick Pt foil. Fig. 1 shows * Corresponding

author. Tel. +30 651311 l/+30

6518911

016%9002/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSD10168-9002(94)1314-4

the experimental setup. The energy distribution of the neutrons hitting the detector was calculated from the deuterium gas pressure and the D(d,n)3He cross section using Monte Carlo techniques [ 21. As a neutron detector we have used a silicon semiconductor detector. The energy spectrum of a silicon semiconductor detector bombarded with a monochromatic beam of neutrons displays several peaks. These peaks are due to the nuclear reactions that are induced by the neutrons on the isotopes of silicon. The position and area of each peak can be used for neutron energy and flux measurements [2]. Fig. 2 shows an energy spectrum taken with a 100 mm’ silicon semiconductor detector 150 pm thick, at & = 8.391 MeV. The first four higher energy peaks, are due to the 28Si(n,~a)25Mg, Q = -2.654 MeV, 28Si(n,~r)25Mg, Q = -3.239 MeV, ‘8Si(n,cr2)25Mg, Q = -3.629 MeV, and. 28Si(n, pa + p~)~‘Al, Q = -3.885 MeV, reactions. In Fig. 2 we see that in the energy region where the four peaks lie the background is very low. The background neutrons in this energy region are due to: a) source neutrons scattered from the floor, walls, scattering chamber etc. Most of these neutrons have energies much lower than the primary neutron beam. b) Neutrons produced when the beam hits the collimators, entrance foil and beam stop of the gas cell. This background can be measured by removing the deuterium

Fig. 1. The experimental setup. G: the gas cell. M: the monitor detector, 150 mm*, 50 ,.an thick silicon semiconductor detector, T: the uansmission detector, 100 mm’, 150 pm thick, silicon semiconductor detector. S: the scattering sample.

G. Doukellis

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semiconductor detector, 50 pm thick, placed directly in front of the sample. Fig. 3 shows an energy spectrum of the monitor detector which is similar to that of Fig. 2. Again we see that the background is very low in the energy region of the first four peaks. These four peaks were used for monitoring the neutrons crossing the monitor detector and the scattering sample and finally hitting the back detector. The number of counts under each peak, in the back detector, were normalised to the number of counts under the corresponding peak in the monitor detector as follows: If we denote quantities pertaining to the front (back) detector with the superscript F(B), the counts under the peak i with the sample out, are given by:

( LeL’)

Fig. 2. Energy spectrum taken with a 150 ductor detector (back detector), bombarded of En = 8391 keV. The peaks labeled pi *‘a( n,~)~~Al and 28Si(n,a)25Mg reactions

in Phys. Res. A 356 (1995)

@I thick silicon semiconwith monoenergetic neutron and ai correspond to the respectively.

gas from the gas cell. It was found to be negligibly small. c) Primary neutrons produced in the gas cell at large angles but scattered forward by parts of the gas cell. d) Light particles escaping from the silicon detector leaving part of their energy. All of the above produce a low and smooth background under the first four peaks of Fig. 2. This background can be easily subtracted when integrating the peaks. As we see in Fig. 2, the background becomes increasingly important at energies lower than the 28Si( n, pa + PI ) 28A1 peak. This low energy background is due, in addition to the factors mentioned above, to: a) deuteron break-up which contributes a continuous low energy background and b) the activated parts of the gas cell which contribute a background below 200 keV. For the reasons stated in the last two paragraphs, we have used only the first four peaks in Fig. 2 for the transmission measurements. The relative normalization between the two runs with the sample in and sample out, can be based on the accumulated deuteron charge on the gas cell. However, effects causing random fluctuations in the primary neutron flux can be taken into account by using a monitor detector. As a monitor detector we have used a transmission type 150 mm* silicon

In these expressions @ is the neutron flux, in neutrons per unit solid angle, which varies with deuteron beam energy. The flux in the back detector is reduced due to the absorption of the first detector:

where Zr is the energy averaged total neutron cross section for silicon and NF is the area1 density of the first detector. For a silicon detector 50 pm thick and a & taken from Ref. [3], we get HTN~ = 5.4 x 10M4 which can be neglected compared to the statistical errors. The effective solid angle 0 takes into account the geometric solid angle plus the variation of the D(d, n) cross section with angle. The effective solid angle varies with deuteron energy. The efficiency E takes care of the fact that some of the 2RSi(n, zi) reaction products escape the detector, this quantity depends also on the incident neutron energy. Finally Ci( E) is the properly averaged reaction cross section taking care of the incident

‘+I + 6 286

0.5’

’

6.0

6.2

(keV)

Fig. 3. Same as in Fig. 2 but for the 50 pm thick detector detector).

(monitor

1

1 6.4

En E

Mev

rl

(MeV)

Fig. 4. t2C + a total cross section measurements. The data points represent the present measurements. The error bars represent the statistical error. TIlhe line is an average of previous cross section measurements, shifted in energy to fit OUTdata.

410

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7200

Fig. 5, The “B + n total neutron cross section measurements. The error bars represent the statistical error. The lines are resonance ftts.

neutron energy dist~bution on the detector and possible resonances of the reaction leading to the formation of the peak i. Since the spectra in the front and in the back detector are measured simultaneously the ratio:

is independent of the flux and contains quantities which vary only with deuteron energy. With the sample in (between the detectors) and all other conditions unchanged the only modification is now that: (an)’ = (aF)‘T, where 7’ is the sample transmission and the primed quantity is referring to the measurement with the sample in. Taking again the ratio between the corresponding peaks, we obtain: N,! = TNi , summing over the four higher peaks yields: kNi=TkNi. i=l

i=l

Thus the neutron t~nsmission can be obtained from the sum of the ratio of the four peaks in the two detectors, with the sample in and out. All other factors as the thickness of the detector, the 28Si(n, z ) cross section, light particle escape, effective solid angle or absolute neutron flux, are divided out and do not enter in the determination of the neutron transmission.

The position of the peaks in the energy spectrum of Fig. 2, can be used for energy measurements of the incident neutrons [2]. From the uncertainties involved in the Monte Carlo simulation of the the peaks we performed in Ref. [ 21, we obtain an absolute error in the determination of the incident neutron energy of about f25 keV, for the neutron energy region covered in this work. The relative error is about &7 keV. It should be noted that the average neutron beam energy can be obtained from the Monte Carlo calculation of the neutron energy distribution [ 21. The uncertainty to the calculated mean neutron energy is about f 1S keV. The measured mean neutron beam energy was determined at a few incident neutron energies and was compared to the calculated mean neutron energy. A very good agreement was found between the measured and calculated values. The energy values shown in the energy axes of Figs. 4 and S are the calculated mean neutron energy values. To test our method we measured the total neutron cross section near the & = 6294 * 5 keV resonance [4] in 13C. The target was a cylinder 1.78 cm in diameter 3.12 cm long of graphite. Corrections were made for neutrons scattered elastically into the back detector by the sample. The incident neutron energy was varied between 6.04 and 6.5 MeV and the neutron energy spread at the back detector was about 40 keV FWHM. Since we wanted only to test our method and not to obtain data of high statistical accuracy, each run lasted 20 to 30 min. This was long enough to accumulate counts for a 4% statistical error to the cross section. It should be noted that at E,, = 6.05-6.5 MeV, the number of counts under the peaks in Figs. 2 and 3 is much smaller than that at En = 7.3-8.5 MeV, since at E,, < 6.5 MeV the cross sections for the nuclear reactions induced by neutrons in silicon drops

G. Doukellis et al./Nucl.

Instr. and Meth. in Phys. Res. A 356 (1995) 408-411

significantly. Thus, if data of high statistical accuracy are needed at En < 6.5 MeV, one has to make longer runs. Fig. 4 shows our data points, corrected for the incident neutron energy resolution. The line is an average of previous cross section measurements [4]. The line is shifted in energy to fit our data. From Fig. 4 we see that the agreement with previous measurements is good and that the resonance energy extracted from our data is 6286 f 15 keV. This resonance energy compares well with the accepted value of 6294 f 5 keV, reported in the compilation of Ref. [ 41.

3. Results Fig. 5 shows the “B + n cross section measured in’this work for E,, = 7.3-8.5 MeV. The energy resolution of the incident neutron beam was about 37 keV (FWHM), at the back detector. The scattering sample was metallic natural boron of 99.5% purity. The powder was pressed into an aluminium cylinder of 1.784 cm in diameter and 3.16 cm in length and 0.5 mm wall thickness. The total mass was 10.21 g. Each run lasted 10 to 15 min. The front and back detectors were placed at 6 cm and 11 cm, respectively, from the end of the gas cell (Fig. 1) . Neutrons elastically scattered into the back detector were taken into account by numerically integrating the elastic scattering cross section at forward angles, over the volume of the scattering sample and the area of the detector. The elastic scattering cross section values were taken from the literature. Inelastically scattered neutrons do not contribute, according to kinematics, to the energy region of the first four peaks (Fig. 2). For the same reason no corrections were made to the front detector for neutrons elastically scattered at backward angles, by the sample into the front detector. No dead time corrections were necessary to be made for either detector. The “B + n excitation function was measured several times that is, we returned to the same energy after measuring at many other energies. Each data point in Fig. 5 is the average of several measurements. The spread in the cross section values measured at each energy was within the statistical error. The error to the cross section due to the statistical errors was less than 2%, and it is dominating the relative error which is less than 2% for the energy region measured. The absolute error is influenced by a 2% uncertainty in the corrections for the elastically scattered neutrons into the back detector, making the overall absolute error less than 3%.

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The excitation function of “B + n measured in this work (Fig. 5) is part of a series of experiments [ 1,5,6] to determine the resonance parameters of states in “B, at excitation energies & = 10000-l 1 200 MeV. The error to the dam points in Fig. 5 is the statistical error and the lines are resonance fits. The numbers labeling each resonance are excitation energies in the compound nucleus “B, in keV. We see that we were able to resolve individual resonances in a region of closely lying states in 12B and to obtain spectroscopic information for states at high excitation energy. The resonance parameters extracted from the fit, i.e. resonance energy, width, and resonance strength, were used together with the parameters extracted from resonance fits to the previously measured [ 61 “B (n, cy)sLi excitation function, to determine the reaction rate for the astrophysically important reaction ‘Li(cy,n)“B [l].

4. Conclusions The advantage of the present system over other systems is that it is simple, compact and does not require complicated electronics (like neutron-gamma discrimination or time-offlight systems). It is a low background system with good monitoring of the neutron flux and thus it can be used to obtain data of high accuracy. Its disadvantage is that below 5.5 MeV incident neutron energy, the yield of all nuclear reactions induced in silicon is too low and thus the present system is not suitable for measurements at E, < 5.5 MeV.

References [ 11T. Parade& G. Doukellis, G. Galios, S. Kossionides. X. Aslanoglou, P. Descouvemont, S. Schmidt and C. Rolfs, (unpublished). [21 G. Doukellis, T. Paradellis and S. Kossionides, Nucl. Instr. and Meth. A 327 ( 1993) 480. [31 H. Kitazawa et al.. JAERI Eval. March 1988. [41 Ajzenberg-Selove, Nucl. Phys. A 449 (1986) 1, [51 T. Pamdellis, S. Kossionides, G. Doukellis, X. Aslanoglou, P Assimakopoulos, A. Pakou, C. Rolfs and K. Langanke, Z. Phys. A 337 (1990) 211. 161 T. Paradellis. G. Doukellis. G. Galios, S. Kossionides. X. Aslanoglou, P. Descouvemont, S. Schmidt and C. Rolfs, Proc. Int. Symp. on Nucl. Astrophys., Nuclei in the Cosmos, Karlsruhe, Germany (1992) p. 181,