10BF3-paraffin neutron counter calibrated with six neutron sources

NUCLEAR

INSTRUMENTS

AND METHODS

159 ( 1 9 7 9 )

71-73;

©

NORTH-HOLLAND

PUBLISHING

CO.

I°BF3-PARAFFIN NEUTRON COUNTER CALIBRATED WITH SIX NEUTRON SOURCES Y. EYAL

Department of Nuclear Physics, Weizmann Institute of Science, Rehovot, lsrael Received 27 April 1978 and in revised form 7 September 1978 An efficient and reproducible l°BF3-paraffin neutron counter which may be conveniently used in experiments associated with particle accelerators has been constructed. The efficiency for neutrons in the energy range 23 keV to 5 MeV has been measured in detail with the aid of Pu-Be(~, n), Po-H2180(o~, n), Sb-Be(y, n ) a n d 17N sources, and with monoenergetic neutrons produced in the reactions 7Li(p, n) and 11B(oq n). The counter is almost flat in its response to neutrons of energy above 0.8 MeV.

1. Introduction A proportional counter filled with l°B-enriched BF 3 gas is an old and well known device for monitoring flux of thermal neutrons. When combined with an energy moderator such as paraffin, the above counter can serve as an efficient detector fbr fast neutrons. Indeed, a l°BF3-paraffin counter whose efficiency is almost independent of neutron e,nergy ("long counter") was already built 30 years ago by Hanson and McKibben~). A survey of the properties of long counters can be found in ref. 2. In this paper we report on the construction and calibration of a simple and efficient l°BF3-paraffin neutron counter which may be conveniently used in experiments associated with particle accelerators. In particular, the response of the detector to neutrons of energy ranging from 23 keV to ,~ 5 MeV has been studied in detail with six different neutron sources and reactions. An early version of the present counter was used in the measurement of 9Li, 16C and 17N (delayed-n emitters) formed in the bombardment of light targets with 156 MeV protons3), and for the measurement of ~6C and ~7N yields in the reaction 180+~4C (ref. 4). Recently, the present neutron counter was employed in the study of sub-barrier fusion cross sections of 180 with 12C (ref. 5). Due to the low neutron background near the target position the above excitation function could be followed easily over a cross section range of almost five orders of magnitude. A future application of the present counter is the study of interaction barriers of heavy nuclei by observing the "threshold" energies for (HI,xn) reactions.

of three I°BF3 proportional counters (procured from 20th Century Electronics, Englar,.d) embedded in a wooden box filled with paraffin and shielding cadmium sheets. Each ~°BF3 tube has a diameter of 2.5cm and 31cm active length. The external dimensions of the box are 50× 43 × 23 cm 3. The neutron sources are directed towards one of the two largest faces of the box, called herein the front face. All three proportional counters are placed parallel to the front face at a depth of 4.5 cm. The distance between neighboring tubes is 7.5 cm. In order to reduce the sensitivity of the counter to scattered neutrons, 1.5 mm thick cadmium sheets were introduced inside the paraffin at distances of 6 cm from the back face and all four side faces of the detector. The reference point for counter-source distance measurement is the center of the front face. The ~°BF3 counters were chosen to have matched avalanche gains. The output from the three counters were paralleled, and the signals, after passage through a common charge-sensitive ~\\. \\\\\\\\\\\\\\\. !ii:i ii!!iiii!iiii~#ii!!i#i !!?~!:?ii??~

Cd ~

iiiiiii!~ili)ii)iiiii } i~i!~i~i!~ii~ix.~i: Wood --~

!iili iii~i!iiiii ........

hree 'OBF3 detectors , , ~ Front face

~ i!;ii:iii:ii:ii!i:ii; Scale in cm

2. Mechanical construction and electronics A schematic view of the ~°BE3-paraffin neutron counter is shown in fig. 1. The detector consists

0

I0

20

Fig. 1. C r o s s - s e c t i o n a l counter.

view

of

the

l°BF3-paraffin

neutron

72

v. EYAL

preamplifier, were recorded by standard electronics. All signals reside well above the level of the electronic noise. The counter is not sensitive to yrays.

der with neutrons. The reactions taking place are 6Li(n, ~z)t and 180(t, ~z)17N. The irradiation was performed in the core of the Soreq reactor with the aid of a fast pneumatic transfer system.

3. Neutron sources

3.5. 7Li(p, n)7Be Monoenergetic neutrons in the energy range 0.065-1.12MeV were selected according to reaction kinematics in the bombardment of 7Li with a 3 MeV proton beam provided by the Weizmann Institute Van de Graaff accelerator. The target was prepared by the evaporation of Li metal on Cu backing inside a small scattering chamber.

3.1. Pu-Be (~z, n) The absolute strength of this standard neutron source (1.31x 104 n/s, ±4%) was determined by the manganese bath procedure6). The average neutron energy is 4.5 MeV. 3.2. 21°Po-H2180 (~z, n) This neutron source has a half-life of 138.4 d and an average neutron energy of 2.4 MeV (ref. 7). The source was prepared in the following manner: an aqueous acidic solution which contained 170mCi 21°po was evaporated slowly to nearly dryness. The residue was dissolved in 1 ml of 99% H2180 and the solvent was again evaporated. Then the residue was dissolved in a fresh quantity of 1 ml of 99% H 2180 and the solution transferred to a stainless steel capsule. The absolute strength of the source (2.58×104n/s, _+3%) was determined by the manganese bath procedure. 3.3. 124Sb-Be(7, n) This source has a half-life of 60d and emits neutrons of an energy of 23 keV. The preparation of this source involved irradiation of metallic Sb with neutrons inside the core of a reactor (Soreq IRR-1 swimming pool reactor). After several days of irradiation the sample was transferred to a zone far from the core in order to let the short-lived product 122Sb(T1/2= 2 . 8 d ) decay. Then the 1245b sample was surrounded with Be rods and placed inside a polyethylene container. The strength of the gamma activity of the source was approximately 12mCi. The absolute neutron intensity (9.26× 104 n/s, +8%) was measured by the manganese bath procedure. 3.4.

17N (DELAYED-n)

The /3 -decay of 17N(T1/2 - - 4 . 1 7 s) proceeds predominantly to unbound states in 170 and is followed by subsequent neutron emission with energies and abundance as followsS): 0.385MeV (37.9%), 1.163MeV (51.1%) and 1.675 MeV (5.8%). The weighted average neutron energy of this source is, therefore, 0.88 MeV. The 17N activity was obtained by irradiating a few mg of 90% 6Li-enriched and 99% 1SO-enriched 6LiC 1803 pow-

3.6. 11B(~, n)14N A self-supporting 99%, liB-enriched target was bombarded with a 3MeV He + beam at the Van de Graaff accelerator. The neutrons produced by the above reaction were used to calibrate the efficiency of the ~°BF3-paraffin neutron counter in the range 1.36-3.00 MeV. 4. Efficiency measurements and results

The efficiency of the l°BF3-paraffin counter for the sources Pu-Be(~z,n), Po-H2180(~z,n) and Sb-Be(y, n), situated at a distance 1 m away from the center of the front face of the detector, is shown in fig. 2. Further measurements at various source-counter distances have shown that the efficiency follows accurately the inverse distancesquare law, provided that a constant length of 14.4cm is added to the measured distance. The above additional quantity can be viewed as the effective internal " r a d i u s " of the detector. The efficiency of the l°BF3-paraffin counter to monoenergetic neutrons produced in the reactions 7Li(p, n) and llB(~z, n) was determined by compar-

40-

~" 0× 35 - j],

wZ~ ~ 25 ~ 30~L~

I~Pu-Be(a,n) ~)Sb-Be(y,n) ~Po-H2'eO(a,n) # rLi(p,n) (

c

,

-

,n)

UJ20' 5(~-

I

I

I

I

I 2 3 4 NEUTRON ENERGY (MeV)

Fig. 2. Efficiency off the l°BF3-paraffin counter as a function of energy for neutrons emitted 1 m away from the center of the front face of the detector. The solid curve presents the smoothed empirical response to monoenergetic neutrons.

~OBF3-PARAFFIN N E U T R O N

ing its response to that of a standard 3He-poly,ethylene long counter (Texas Instrument Co., USA) ,of known characteristics. Both counters were ,=entered at the same scattering angle on opposite sides of the beam axis. In order to reduce the ;amount of energy variation over the active part of the detector, the distance of each counter from the ~Larget was set to approximately 2 m. The absolute scale of the efficiency curve was then determined according to the known efficiencies of the counleers for the standard Pu-Be(ot, n) source. The resuits of these measurements, corrected for the source-counter distance of 1 m, are summarized in fig. 2. Neutrons emitted by 17N produced in the sample 6LiC 1803 were counted simultaneously by the :t°BF3-paraffin counter and the reference long counter placed symmetrically about the source and 2 m apart. The irradiation (8 s) and counting (12 s) periods were repeated in cycles until good counting statistics was achieved. The efficiency extracted from the above measurement is indicated in fig. 2. :5. Discussion and summary We have described the performance of a simple and efficient ~°BF3-paraffin neutron counter whose response to neutrons of energies in the range 23 keV to - 5 M e V was studied in detail. The solid curve drawn in fig. 2 presents the smoothed empirical efficiency of the counter for monoenergetic neutrons emitted at a distance of ] m from the front face of the detector. The distance dependence of the efficiency obeys accurately the inverse-square law. In the measurements of the counter efficiency we have utilized five well known neutron sources and reactions. In addition, we have demonstrated the applicability of the short-lived 17N delayed-n source for efficiency calibration. The advantage of having this isotropic neutron source which possesses a simple and known decay scheme with an average neutron en-

COUNTER

73

ergy of - 0 . 9 MeV is obvious. It should be noted that the efficiency for 17N obtained in the measurement described in section 4 agrees very well with the value calculated from the smooth efficiency curve and the abundance of neutrons of various energies mentioned in subsection 3.4. The response of the present l°BF3-paraffin counter is rather flat for neutrons of energy above 0.8 MeV. This feature is of importance when the energy spectrum of the measured neutrons is complex or unknown. The present counter is well suited to serve as a directional neutron detector in experiments associated with particle accelerators. It is a pleasure for me to thank Prof. I. Dostrovsky for his continuous interest in this work and for many valuable suggestions. The collaboration with the late Profs. S. Amiel and Y. Wolfson is deeply appreciated. Helpful discussions with Prof. Z. Fraenkel, Dr. D. Kedem and Dr. Y. Gurfinkel are gratefully acknowledged. I would like to thank my colleagues at the Soreq Nuclear Research Center, Yavne, for allowing me to carry out irradiations at the reactor, and for their permission to use their long counter and manganese bath assemblies. References l) A. O. Hanson and J. L. McKibben, Phys. Rev. 72 (1947) 673. 2) W. D. Allen, in Fast neutron physics (eds. J. B. Marion and J. L. Fowler; Interscience Publ., New York, 1960) part 1, p. 361. 3) 1. Dostrovsky, H. Gauvin and M. Lefort, Phys. Rev. 169 (1968) 836. 4) y. Eyal, I. Dostrovsky and Z. Fraenkel, Nucl. Phys. AI80 (1972) 545. 5) y. Eyal, Europ. Conf. on Nuclear physics with heavy ions, Caen, France (1976) Communications, p. 129. 6) y. Gurfinkel and S. Amiel, Nucleonics 23 (1965) 76. 7) A. G. Khabakhpashev, Soy. J. At. Energy 7 (1960-61) 591. 8) A. R. Poletti and J. G. Pronko, Phys. Rev. C8 (1973) 1285; F. Ajzenberg-Selove, Nucl. Phys. A281 (1977) 1 and references therein.