^ s X MDDC - 972 (LADC - 409) UNITED STATES ATOMIC ENERGY COMMISSION A NEUTRON DETECTOR HAVING UNIFORM SENSITIVITY FROM 10 KEV TO 3 MEV by A. O. Hanson J. L. McKibben Los Alamos Scientific Laboratory This document is reproduced as a project report and is without editorial preparation. The manuscript has been submitted to The Physical Review for possible publication. t Manuscript Date: February 11, 1947 Date Declassified: May 29, 1947 Issuance of this document does not constitute authority for declassification of classified copies of the same or similar content and title and by the same authors. Technical Information Branch, Oak Ridge, Tennessee AEC, Oak Ridge, Tenn., 5-2-49--850-A3671 Printed in U.S.A. PRICE 10 CENTS A NEUTRON DETECTOR HAVING UNIFORM SENSITIVITY FROM 10 KEV TO 3 MEV By A. O. Hanson and J. L. McKibben ABSTRACT A neutron detector having approximately uniform sensitivity from a few kUovolts neutron energy to a few million volts energy is described. The arrangement known as a long counter con- sists of a paraffin cylinder about 10 inches outer diameter x 12 inches long surrounding a long boron proportional counter. Sensitivity curves are given for two of the best arrangements. The response is flat over the above range to about 10%. LONG COUNTERS A neutron detector which has a uniform efficiency for neutrons of widely different energies has many advantages for certain types of measurements. A large water bath containing slow-neutron de- tectors in some form fulfills this requirement and has been very useful in determining the number of neutrons emitted by various neutron sources.^ The examination of the number of slow neutrons as a function of the distance from the source in such a water bath gives additional information regarding the energy of the neutrons.^ But there are many experiments where the use of a large water bath is either awkward or gives erroneous results due to effect of the degraded neutrons reflected from the bath into the experimental setup and yet a detector having a uniform sensitivity to neutrons is required. In order to achieve a high efficiency in a detector of reasonable size an attempt was made to find a suitable arrangement of paraffin surrounding a boron detector. The analogy with the water bath ex- periment suggested that a long boron counter embedded in a block of paraffin would have a counting rate which would not depend much on the energy of the neutrons. The fir.st detector constructed con- sisted of a boron-lined ionization chamber 20 cm long surrounded by a cylinder of paraffin 20 cm long and 17 cm in diameter, this was used with its axis pointed toward the neutron source. Preliminary tests on the sensitivity of this counter showed that the sensitivity was very nearly the same from neu- tron energies of from 0.4 Mev to 2 Mev; these tests served to encourage the development of counters along the same lines. This type of detector will be referred to as a long counter. The theoretical treatment of the sensitivity of this type of counter is quite complicated and has not been worked out. There are, however, certain qualitative arguments which may help in under- standing the behavior of these counters and which may serve to suggest further improvements. Let us examine first an arrangement in which a long thermal-neutron detector is imbedded in a large (semi-infinite) slab of paraffin and in which neutrons of various energies are incident upon this slab in the direction parallel to the axis of the detector. The neutrons entering the paraffin wDl be slowed down primarily by the hydrogen atoms to thermal energies and some of these neutrons will be captured by the central thermal-neutron detector and will be recorded as counts in some manner. If the neutrons have a very high energy, the mean free path of these neutrons is initially large and therefore will be slowed down an appreciable distance from the front face of the slab. After a number of collisions, the mean free path will be reduced to such an extent that these neutrons have a veiy MDDC - 972 [ 1 2 ] MDDC - 972 small chance of escaping out of the front face of the slab. This is not the case, however, for neutrons having energies of the order of 100 kev or less since in this case the mean free path of the neutrons does not change appreciably as the neutrons approach thermal energies.' These neutrons would there- fore have a much larger probability of escaping out of the surface of the semi-infinite slab as com- pared to the high-energy neutrons. This effect is partially compensated for by the fact that the less energetic neutrons would need to make fewer collisions before becoming thermalized, but the effect would be such that the detection efficiency for high-energy neutrons would be several times greater than that for very low-energy neutrons. If the sensitivity for low-energy neutrons is to be approximately the same as that for neutrons of high energy, some modification of this idealized arrangement must be made. Therefore the suc- cess of the first long counter must be ascribed to a fortunate choice of the dimensions of the paraffin block such that the increased probability of escape of high-energy neutrons compensated in some de- gree for the escape of low-energy neutrons from the front face. The effect of the size of the paraffin cylinder has been investigated roughly and is indicated in a later section of this report. Furthermore, the size of the central cavity made by the detector is perhaps important although no systematic investi- gation of its effect was made. It will be shown, however, that the introduction of additional holes in the front face affected the low-energy sensitivity of one of the counters appreciably. DESCRIPTION OF COUNTERS Two counters were used quite extensively and wUl be described here in detaU. The first consisted of a central BFj proportional counter 1 inch in diameter, which had an effective length of 8 inches. This tube was surrounded by paraffin cylinders 12 inches long and 6, 8, and 12 inches in diameter. The counter with the 8 inch paraffin cylinder is shown in Figure 1. The proportional counter protrudes slightly from the front face of the paraffin block but is protected from direct thermal neutrons by means of a cadmium shield. The proportional counter is supported by means of ceresin wax in the center of an aluminum tube which also serves as an electrical shield. The body of the counter was a 1/32 inch wall brass tube and was soldered to Kovar-glass seals. The central electrode consisted of a 10-mil Kovar wire. The 1/4-inch intermediate electrode was used as a guard ring and was connected to ground. The counter was filled with enriched BF3(80 percent B^") to a pressure of 25 cm Hg. With -2700 volts on the outer shell the proportional counte^" gives a gas amplification of about 10. The signal was further amplified by means of a model 100 linear amplifier^ with a R-C time constant of y microseconds and the pulse was counted by means of a model 200 discriminator and scale-of-64 circuit.^ Except in the cases where impure BFj were used, owing to leaks in the tube or fUling system, the bias curves (counting rate against minimum pulse height recorded) were such that a change in the bias voltage by a factor of two in either direction would not change the counting rate by more than 5%. The counting rate was not affected by the radiation from an unshielded 500-mg radium source used at a distance of 40 cm from the counter. The other counter, which wUl be referred to as the shielded long counter, is shown in Figure 2. The principle modification being that an additional paraffin and boron shield is used so as to make it less sensitive to neutrons which have been scattered about the room. The proportional counter is similar except that it was 10.5 long, 1/2 inch in diameter, and was filled with BFj to a pressure of 40 cm. For most of the measurements made with these counters it was convenient to use a matched pair of the counters in the arrangement shown in Figure 3. In addition to increasing the sensitivity of the overall system such an arrangement minimizes errors due to exact positioning of sources. SENSmVITY CURVES The data on the sensitivity of the counters to neutrons of various energies were obtained by 3 methods, namely: MDDC - 972 [3 HIGH VOLTAGE (-2,700 VOLTS) m^ GUARD Ring CONNECTED ! TO GROUND "^^ CADMIUM CAP ALUMINUM TUBE ^FOR SHEILDING AND SUPPORT Figure 1. 8 inch OD long counter. CASE OF .050" SHEET IRON REMOVABLE ALUMINUM CYLINDER V HIGH V0LT4GE- TO PREAMPLIFIER ^M^y ^<^^.>.->A-^^.;.,- 4- CERESIN ■//////,■ GLASS 6 HOLES CENTERED ON 3^ " CIRCLE -CADMIUM CAP -16|' Figure 2. Shielded long counter. 4 ] MDDC - 972 1) By comparing the counting rates in the counters due to various radioactive neutron sources whose total neutron yield had been compared by some other method such as the water batli technique. Photo-neutron sources used were Sb-Be and Y-Be. Alpha neutron sources used were P0-BF3' and Ra-Be. The energies of these sources were taken to be 0.023, 0.16, 2.2, and 5 Mev, respectively. While the energy of the photo-neutron sources should be well defined the alpha neutron sources give a spectrum and the values given represent only average energy. The assignment of an average energy to the neutrons from Ra-Be is especially dubious since the energy spectrum of these neutrons extends out to about 14 Mev. The fraction of neutrons below 0.1 Mev, however, is estimated to be less than 10%.'' 2) The degradation of the energy of the neutrons from a given source by surrounding the source with spheres of graphite and heavy water. A graphite sphere, 24 cm in diameter, was used which had the effect of reducing the average energy of the neutrons by a factor of about 2 . The heavy-water sphere, 20 cm in diameter, served to reduce the average energy of the neutrons by a factor of 4 or more. Since neither graphite nor heavy water absorbs neutrons appreciably, the number of neutrons emerging from the sphere would be the same as that emitted from the source. A change in the counting rate with the sphere around the source was therefore taken as a measure of the change in the sensitivity of the counter to the modified spectrum of neutrons. The use of DjO was, of course, limited by the fact that any source having sufficiently high energy gamma rays would give rise to photo-neutrons from, the deuterium. In the case of yttrium it was found that the number of photo- neutrons from deuterium due to the high-energy ray (~2.8 Mev) was only 3% of that due to the Be and hence could be accurately taken into account. 3) The use of homogeneous neutrons of known energy from the Li (p,n) and D (d,n) reactions. In these experiments the flux of neutrons into the counters were determined by counting the fissions occurring in a standardized sample of uranium 235. The energies of these neutrons are accurately known, and the flux measurement are considered to be reliable so that these points should be quite significant. The summary of the data on the first counter with 6-, 8- and 12-lnch cylinders are shown in Figure 4. Since very few data were obtained with the 6 inch cylinder, the curve is sketched in only to indicate the general trend of the sensitivity curve as the size of the paraffin cylinder is reduced. It is seen that the 8 inch cylinder gives the best approximation to a uniform sensitivity over the region shown. Other tests with neutrons absorbable by cadmiimi indicated that the sensitivity of the counter to thermal neutrons was about 70 on the scale used in- Figure 4. The sensitivity of this counter would be somewhat effected by the arrangement in which it is used. For most of the tests described here the counters were used as a matched pair as the arrangement shown in Figure 3 so as to reduce errors due to the location of the sources. This pair of counters was used in the center of a room approximately 15 by 20 feet at a height of about 50 inches above the floor. In spite of pre- cautions to keep all other material as far from the counters as possible about 15% of the counting rate in the counters was due to scattered neutrons when a Ra-Be source was placed at a distance of 1 meter from the front face of the counters. The absolute sensitivity of this counter was such that it would give about 1 count for every 10^ neutrons emitted from a source placed at a distance of 1 meter from the front face. The shielded counter is less sensitive to scattered neutrons by a factor of about 3 and hence largely eliminates this objectionable feature of the 8 inch long counter. The sensitivity of this counter to high-energy neutrons is considerably increased owing to the large mass of paraffin in the shield and the sensitivity to low-energy neutrons is therefore relatively low. The use of holes in the front face, however, increases the sensitivity to low-energy neutrons to a sufficient extent so that the re- sponse curve of the counter is about as good as that of the previous counter. The effect of these holes is clearly indicated in the sensitivity curves shown in Figure 5. It seems probable that the sensitivity of this counter would remain fairly constant up to energies greater than 5 Mev but there has been no MDDC - 972 [5 PARAFFIN CYLINDER BF3 TUBE PREAMPLIFIER -SPHERE -NEUTRON SOURCE 120 CM. — TO 200 CM. Figure 3. Experimental arrangement with a pair of 8 inch OD long coimters. 2 3 4 5 NEUTRON ENERGY (MEV.) Figure 4. Sensitivity of 8 inch OD and 12 inch OD long counters. The X's and circled X's represent points obtained with Li7 (p,n) and D (d,n) neutron sources. Although the curves are not continued beyond 3 Mev the general trends of the curves above this energy are indicated by the points obtained with the Ra Be source. MDDC - 972 120 100 > 1 ^80 z LLl 60 1 8-1" HOLES IN FACE .-^OLID -