Reading University Reading, United Kingdom
I
Neutron Source for Student Use
Small laboratory neutron sources based on (a,n) and ( y , n) reactions have been in use for many years. Recently
small amounts of 252Cf have become available and, since this nuclide decays largely by nuclear fission, it forms a convenient source of neutrons with a half-life of 2.6 yr. The element was availahle in the form of short lengths of wire 33 mm long by 0.95 m m diameter. Most users have designed their irradiation equipment with a large mass of sample surrounding the source, so that virtually all the neutrons produced traverse the sample h u t the neutron flux is not uniform.l.2 An alternative approach is to place several sources in a cylindrical array, so that the neutron flux within the cylinder is approximately constant. In our case only six sources were availahle, and these were arranged with their long axes parallel and forming a hexagonal cylinder. The central axis of the cylinder will he called the z-axis below. Design and Construction of Source Holder Since the uniformity of the sources was stated to he +1590, i t was decided not to attempt to obtain a neutron flux more uniform than this figure. The theoretical flux variation was calculated along two axes perpendicular to the z-axis, namely-x-axis, joining opposite vertices of the hexagon (8.66 cm apart), and y-axis, joining opposite sides of the hexagon, perpendicular to the x-axis. The sides were 7.5 cm apart. The theoretical results are shown in Figure 1, from which i t can he seen that in a concentric cylinder of radius 1.7 cm, the fluxes along both x- and y-axes virtually coincide and the flux should he 20% higher a t the margin of the cylinder than it is a t the center. The flux variation along the z-axis was also calculated (Fig. 2), and shown to he 1090 higher a t the center of the cylinder than a t either end. No corrections were made for neutron reflection or end effects. Both absolute flux and flux variation were shown to be remarkably insensitive to the raising or lowering of individual sources. Given that the sources each contained 1.9 fig 252Cf, with a neutron emission rate of 2.4 x 106 s-*fig-', the ahsolute neutron flux a t the geometrical center of the array was calculated to he 1.02 x 105s-'em-2. The sources themselves were mounted in 9 mm diameter perspex rods, 30 em long, drilled and tapped at one end. Each source was inserted into the hole in the rod by remote control, after which the perspex plug was screwed into place to hold the source in position (Fig. 3). The source rods were held in position by inserting them in perspex guide tubes (o.d. 1 2 mm, i.d. 9 mm). The guide tubes were cemented in a hexagonal array around a large central perspex guide tube (length 50 cm, 0.d. 1.5 cm, i.d. 6.3 cm). The central guide tube, which was closed by a watertight plug at its base, was held in position along the main axis of a cylindrical galvanized steel bin, 44 cm in diameter, by means of wwden locating rings. About 70 kg of molten paraffin wax was then poured into the bin and allowed to solidify, to act as a biological shield, leavinga cavity in the central guide tube. The cavity was plugged by the sample tube, which was made as follows. A perspex tube, 65 cm long and 6.3 cm in outer diameRicci, E., and Handley, T. H., Anal. Chem., 42,378 (1970). 2Gage, S. 3.. Draper, E. L., Bouchey, G. D., and Day, R. R., U.S. Atomic Energy Commission Report CONF-710402, II,I-196, (1971). 682 / Joumal of Chemical Education
Figure 1. Neutron flux across horizontal section of sample cavity. A calculated, x-axis: B = calculated, y-axis: C =experimental, x-axis.
=
Figure 2. Neutron flux along vertical axis of sample cavity. A = calculated: B =experimental.
ter, was filled with paraffin wax apart from a central cylinder 3.1 cm high, 5.1 cm in diameter and 75.6 cm3 in volume. This cylinder was delimited by perspex discs above and below, and access to it was obtained by a rectangular hole. 3.7 cm X 4.5 em, through the side wall. The upper end of the sample tube was drilled to take a 1.5-cm wwden rod perpendicular to the axis of the tube, the rod acting as a handle. The handle was restrained by strings attached to the bin, so that the sample tuhe could easily be lifted to insert or remove samples into the cylindrical cavity, but could not be removed completely from the guide tube. This design ensured that the sources were always surrounded by at least 20 em of paraffin wax, even when the sample tuhe was raised. As shown in Figure 4, when the sample tube was lowered the sample cavity was aligned to coincide with the hexagonal ring of sources. Radiation Hazards It was found that radiation dose rates in excess of 0.75 mrem/hour could be measured a t distances less than 2 m from the bin. The latter was therefore surrounded with 20 cm of concrete and kept in a locked r w m . With these precautions, i t was found that radiation doses outside the room were negligible, and that provided that no student remained in the room for more than 1 hr/mo he would not
Figure 3. Expanded view of source rod (not to scale)
Activities of Various lsotooes Sample
Radioactive Nudide
HalfLifelmin
Specific ActiviLy/s-18-1
need to be designated a s a radiation worker, i.e. his annual radiation dose would not exceed 1.5 rems. Students may only use the room, which is a Radiation area, under supervision. The average time needed to load or unload the source is 10 s. Costs and Availability The costs of materials, not including machinists time, used to make this equipment were a s follows: steel bin, $10; paraffin wax, $20; perspex tube and rod, $20; and concrete blocks (36), $50; total, $100. To this must be added the costs of buying, renting, and handling"2Cf. The current price of this nuclide is about $10/wCi. Californium for educational work may be loaned for the cost of shipment from the International Atomic Energy Agency in Vienna, Austria, who supplied my sources. U S . organizations should consult their Atomic Energy Commissions Materials Loan Program (Agent; The Manager, Savannah River Operations Office, P.O. Box A, Aiken, S. Carolina 29801). Calibration of the Source The source was calibrated by irradiating a number of 0.5-g discs or wires of indium, fixed in known positions, for about 54 min. After a waiting period of 10 min to allow short-lived isotopes to decay, the indium-116 was counted using a NaI/T1 scintillation counter. The specific activities 'of the indium samples, after correction for decay and backmound. are shown in Fieures 1and 2. " It can be seen that the horizontal flux variation across the x-axis aerees with the theoretical calculations and is only *3%. The vertical flux variation is +9%, in the opposite sense to that predicted by calculation. As yet, few experiments have been carried out to determine the absolute or relative fluxes of fast and thermal neutrons in the sample cavity, hut the cadmium ratio is about 8. Uses in Student Experiments A number of elements were activated in polyethylene containers for one half-life and shown to give measurable activities, listed in the table. Specific activities are experimental values uncorrected for counting efficiency. Student Experiments Investigation of Production and Decay of Vanadium 52 Weigh accurately 1.00 gofvanadium turnings into8polyethylene capsules, labelled A-H. Activate A for 1.9 min, remove, and count
Figure 4. Neutron source assembly with dimensions in cm. Vertical cross-section above: horizontal crass-section below.
100 s as rapidly as possible, noting the time pefiod between removal from the source and the beginning of the count of the nearest second. Repeat with B-H with activation periods of 3.8, 6 , 10, 15, 20,30, and 40 min. Plot a decay curve of log (counts s-I) against time. Correct all counts for background and decay, and calculate the specific activity of samples A-H in counts sslg-I at the end of their activation period. Plot these specific activities against time period of expasure to neutrons, to obtain an activation curve for
Determination of Manganese in Pyrolusite Weigh out accurately about 5 g pyrolusite and 2 g manganese into polyethylene pill boxes. Activate the two samples together for about 30 min. Remove the samples and count each for about 10 s. Repeat counts after 2.6 and 5.2 hr to confirm that the radioactivity is decaying with the half-life of 56Mn. This method is non-destructive and samples may be reused after 24 hr. Calculate the Mn content of pyrolusite. Determination of Silver in Chlorargyrite Weigh out accurately about 5 g chlarargyrite and 5 g silver into nolvethvlene ill boxes. Activate both sam~les toeether for 2.4 , mill. nnn -tart a wpuatch a i the iampl,-. are rmwwJ Crmnt mmpk and stnndsrd ahernatel, for 10-s periods nr apprurmateI" 2;-s intervals for the next 15 min. C w r c r t snrnples for hnr*. ground. Plot log (counts ss') against stop-watch time and attempt to tit the curve obtained assuming two components with half-livesof 24.4 sand 2.4 mi". Calculate the Ag content of chlarargyrite. The method is non-destructive, and samples may be reused after 30 mi". 7
~
.
Volume 52, Number 10, October 1975 / 683
Determination of Sodium in Glass Weigh out accurately about 5 g of crushed soda glass, 5 g of crushed borosilieate glass, and 5 g of anhydrous sodium carbonate into three polyethylene pill boxes. Activate overnight and count all three samples for 1000 s next day. If a gamma spectrometer is available, search for elements other than sodium in the activated glasses. Calculate the sodium content of each glass. N. B. Pyrex glass has a very low sodium content, and can be qualitatively distinguished from soda glass by cheap, simple monitors after activation in this way; the high boron content of Pyrex may also reduceits activation potential. Determination of Tungsten in Complexes Weigh out accurately 1-2 g complex and 1 g tungsten metal into two separate polyethylene pill boxes. Activate overnight or for 24 hr. Count the samples for 1000 s and calculate the tungsten mntent of the complex. Note: Bromine or iodine will also activate and interfere, if present.
684 / Jourmlof Chemical Education
Summary T h e design of a neutron source using about 12 g Californium-252 i s described. Samples of volumes u p to 75 m l can b e activated in a neutron flux of about lo5 neutrons cm-%-I which is uniform t o *lo%. T h e cost of the aource mounting and shielding was about $100. T h e source can be used bv students to activate a t least 12 different elements. to investigate activation a n d decay curves of radioactive isotopes, and t a perform non-destructive activation analysis for gram amounts of some elements. Acknowledgment
I wish to thank Mr. K. R. Gowers for calculating the theoretical neutron fluxes, and for help with constructing the source; I would also like t o thank Dr. S. A. Katz for many helpful suggestions.
