H. W. Nass. 'R. S. Maddocka and W. W. Meinke? J n vcrs ty of M cn.qan Ann
Arbor
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I I
Neutrons for Small Laboratories
In the small college or university laboratory, as well as in the small industrial laboratory, low-level nentron sources can he versatile tools to augment an educational or analytical program. Until recently, however, the principal neutron sources available were the expensive, long-lived radium-heryllium sources with high levels of accompanying gamma radiation or shorter-lived polonium-beryllium sources of 140 day half life. Fortunately, less expensive and less bothersome sources of useful neutrons are now available for an investment of only a few thousand dollars. A general outline detailing the present availability and cost of different types of neutron sources has appeared in THIS JOURNAL,^ while another article4 describes simple neutron activation experiments. This paper will summarize our experimental studies with two m a l l neutron sources while determining their potential usefulness for teaching and research.
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Figvre Karnan neutron generdor Figure 2. Kamon neutron generator lcvtaway view of shield). ond shielding orrembly. Karnan Neutron Generator
A Kaman Model NT 60-8 pulsed neutron generator5 was studied to determine its usable flux, useful life, and versatility. This unit consists of a 4411. diam. X 18-in. long generator tube and a 20-X 12-X 8-in. deep control chassis connected by two cables which can be extended to 30 ft. The control chassis houses a pulsing high voltage power supply while the cylindrical generator tube contains the generator, pulse transformer, ion source, and "VacIon" pump. The unit is designed to have an output of lo7 neutrons/pulse and a guaranteed tube life of 20,000 pulses. The pulse rate is variable fmni 1 to 10 pulses per sec and the unit may be operated continuously at 4 pulses per sec or lower. The investigations reported here were made a t 4 pulses per sec a t 4250 volts. Present address: Union Carbide Nuclear, Tuxedo, New York. $Present address: Analytical Chemistry Division, National Bureau of Standards, Wa~hington,D. C. WISE, E. Pi., J. CHEM.EDUC., 39,, A771 (1962). ' VORRES,K. S., J. CHEM.EDUC.,37,391 (1960).
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Journal o f Chemical Education
Shielding. The generator was set up in the corner of a laboratory on a sturdy bench with the control chassis a t one end and the concrete shield housing the generator tube a t the other (Figs. 1 and 2). The generator tube is mounted upright in the center of the shield and should be kept upright to maintain the transformer oil a t the proper level. The shield was constructed of standard 4-X 8-X 16-in. solid concrete construction blocks. The side walls were approximately 8 in. thick on the sides facing the room walls, the front, and the bottom, while next to the hood the wall was approximately 12 in. thick. The outside cave dimensions were 38 in. high, 36 in. wide, and 40 in. deep with 4 in. clearance around the generator tube inside the cave. A small port in the front of the shield used for insertion of samples was closed by two concrete blocks when irradiations were made. Health Monitoring. Conventional neutron survey meters are not reliable when used to determine the number of neutrons from a pulsed source; they register only one neutron per pulse regardless of the number of neutrons actually produced. Health monitoring, therefore, was accomplished with copper foils to detect fast neutrons and indium foils to detect thermal neutrons. The copper and indium foils were plawd OII thc outsido of th; xhield, and the indwrd wtivity nxs connted in u Nal(TI) scintillation n d l crystill. \\'it11 a uew gcnernror tube, thr coppel foils showed a fast neutron yield of 10-20 n cm-"ec-I while the indium foils showed about 50 n seer1. Thus even with this simple shielding the hazard is at tolerance or below for 40-hr continuous operation. Since in normal operation these tubes would be run for only a few hours continuously, the hazard is minimal. Table 1.
Foil No.
Variation of Fast Flux With Foil Position-Kaman Neutron Generator
%
Distance from target center (em)
Positiona
Bottom end on odge of tube Bottom end below center
a
See Figure 3 far schematic showing foil positions. 100% equal to 1.6 X 10% n e m ' sec-'.
For prolonged periods of operation additional concrete blocks might be necessary. Neutron Output and Flux. The "thermal" neutron flux was determined with indium foils. Three configurations were used: (1) indium foil only, placed directly on the tube face (because of the design of this particular "old type" tube the tritium target is 1 in. from the tube face; i.e., there is in. of plastic and in. of oil between it and the tube face); (2) indium foil with no para& above but with 5 in. of paraffin beneath; and (3) indium foil with '/? in. of paraffin above and 5 in. of paraffin beneath. The results showed that with no external moderator the "thermal" flux was about 950 n cm-2 sec-'; with no paraffin above and 5 in. of paraffin below, about 3.3 X lo3 n ~ m sec-I; - ~ and with paraffin above and below, about 5.3 X lo3n cm-%sec-I.
Table 2. Copper Foil Activity From Kamon Neutron Generator Measured With Various Detectors
Counter
Counts per time background
Count time (min)
Counts above hackground
Multi channel eamma. spectrometerBeta proportional Well counter ( 7 ) Low-level heta
1390 300 700 9
20 5 5 10
1910 1660 3751 991
All counts normalized to 1 X 10' fast n cm-= sec-'. *Integrated peak area of the 0.511 Mev gamma only " Counting chamber window: 10 mg/em2.
Table 3. Comparison of Count Rates for Various Elements Irradiated With Kaman Neutron Generator
Rh V In Cub Ag Br" AP Pb b
SLCTIDI VlW
Figure 3.
5.1.'
,"lo4
2.10.
3.d
4 110'
1
NUMBER OF PUL9EO USED
Figure
4.
2.3 4.4 3.7 54 9.9 2.3 6.3 9.5 2.3
5 5 5 5 5 5 5 5 5
5 3 3 5 3
3 3 3 3
62418 c/10 min 1250 c/4 min 2361 c/5 min 62418 c/10 min 2478 e/5 min 998 c/3 min 5282 4 5 min 7966 c/2 min 1076 e/3 rnin
27.2 13 8 13.3 29.2
34.1 19.4
22.6 0.32 1 . 5 -
Counts normalized to 1 X 105 n em-%sec-' fast flux. Reactions using fast neutrons are underlined.
Kamon nevtmn generator tube; flux monitor position%
The "fast" neutrons produced were measured by the s3Cu (n, 2n) W n reaction with copper foils. A new generator tube was pulsed for 5 min a t 4 pulses/sec, resulting in a fast neutron yield of about 1.6 X 106n cm-% sec-I a t the tube face. Table 1 shows the fast flux distribution about the generator a t positions indicated in Figure 3 normalized to 1.6 X 10' n ~ m sec-' - ~ as 100%. Activity of both indium and copper isotopes was measured in a calibrated 1 3 / j n . X 2-in. NaI(T1) well crystal attached to a decade scaler.
d
4.6Lib 17.9 34.1 4.31"
a
Sib
--
c/min/ 10 mg'
Kamon neutron generator; flux as a function of tube life.
Useful Tube Life. Figure 4 shows the fast neutron flux as a function of the number of pulses of our tube up to around 45,000 pulses (the manufacturer guarantees
20,000 pulses). For the first several hours the fast flux a t the generator tuhe face was about lo5 n cm-% sec-I. As previously mentioned, the tritium target is 1 in. (2.54 cm) from the end of the tuhe face. The fast flux - ~ By exa t the tube face is 1.6 X 105 11 ~ m sec-'. trapolation, it should be close to 106n cm-? sec-' for fast neutrons a t 1 cm. If by redesign, then, as in newer tubes, the distance between the target and sample would be reduced to 1 cm or less, the flux could be increased by as much as a factor of ten. Experimental Activations. Two different sample holders were used to determine the sensitivity of several elements. A paraffin block with a hollowed out cavity on the top for the sample was used for thermal neutron irradiations, while an open wood frame skeleton was used for fast neutron irradiations. The irradiation holder was placed on a L'scissor-type" platform and raised into position under the accelerator tuhe. Copper monitor foils for each run were placed on the side of the tube and then corrected using the factors of Table 1 to the fast flnx a t the tube face. All runs were normalized to a fast flux of 1 X 105 n cm-a sec-I. The samples were counted in a 13/+-X 2-in. iYaI(T1) scintillation well crystal attached to a decade scaler. Other counting systems were tried, including a multichannel gamma-ray spectrometer with 3-X3-in. NaI(T1) crystal, a thin window proportional counter: and a low-level heta counter; none of these systems produced count rates as high as the well counter system. Tahle 2 summarizes this for copper-62. Volume 41, Number 3, March 1964
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Several different elements were investigated using both thermal and fast neutron reactions. All samples were counted in the well counter and followed to background; the resulting decay curve was then resolved for the appropriate isotopes. Approximately one gram of pure element or oxide was used in these experiments. Table 3 summarizes the experimental results shoving some elements which give a useful counting rate. Neutron Howitzer
A Nuclear-Chicago "Keutron Howitzer"'" was obtained by our Chemistry Department for use in course work and in preparation of small amounts of tracers for laboratory experiments. The characteristics of this unit as well as its optimum operating conditions were studied.
this size of source is of interest. The AEC will permit lease of only a total of 5 curies of Pu-Be to one lahoratory. Encapsulation of a Pu-Be source costs ahout $1000, however, whether the source is 1 or 5 curies. We decided to obtain all our allowable Pu-Be in one source. It was specified that this source fit the Howitzer diameter of 1.02 in.; as a result, this 5-curie source is approximately 3 times longer than the standard l-curie source (l-curie source: 1.02 in. diam. X 1.445 in. long; 5-curie source: 1.02 in. diam. X 4.425 in. long.) The longer neutron source causes special geometry problems by spreading out t,he neutron field over a larger area and somewhat defeats full utilization of the larger amount of plutonium. The first experiment was designed to determine the effect of the neutron source position in the vertical tube. Table 4 shows the results of a series of irradiations performed with indium foil as a monitor wherein the source was moved up and down from the "normal" irradiation position for a l-curie source. Source position is important in the Howitzer, and optimum performance with this 5-curie source requires a different position from that recommended by the manufacturer. At the optimum geometry, tests were run to determine the optimum amount of paraffin to be used in front and behind the sample being irradiated. Silver foil was used as the detector to permit shorter irradiation. Table 5 summarizes these results. Table 4.
Figure 5.
Variation of Activity with Source Position in Neutron Howitzer (5-curie Pu-Be)
Neutron Howitzer
Distance from normal position (em)
Table 5.
,-n,ir-
-
Figure 6. Neutron Howitzer (rchemoticl.
Moderator behind sample
(4
The "Neutron Howitzer" shown in Figures 5 and 6 is an aluminum barrel-shaped vessel 22l/2 in. in diameter by 35 in. high filled with paraffin. A central vertical storage tube holds from 1-5 curies of plutoniumberyllium sources. Perpendicular to the central vertical storage tube are two ports in which samples may be activated reproducihly or other equipment set up to utilize the neutrons from the source. A 5-curie plutonium-beryllium source was obtained from XUMEC.' The rationale behind procurement of Kamm Nuclear Corporation, Colorado Springs, Colorado. 'Nuelenr-Chicago Corporation, 333 E. Howard Ave., Des Plaines, Illinois. 'Nuclear Materials and Equipment Corporation, Appolo, Pennsylvania.
158 / Journal o f Chemicol Educofion
Relative activity (c/min mp)
Variation of Relative Activity Using Different Amounts of Moderafor in Ports
Moderator between sample and source (rm) 0 1 2 4 16
From Tables 4 and 5 the optimum irradiatiou condition was determined to be a positioning of the neutron source 3.6 cm below the "normal" with paraffin moderator 1 cm in front of the sample and 16 cm behind. Several activation experiments were then performed under these optimum conditions to determine activity levels for different elements. The samples were
irradiated for 5 min and counted in the scintillation well crystal. All counts were corrected for decay to the end of the irradiation. Table 6 compares the results of these irradiations with those made with the Kaman accelerator previously discussed and shows the Howitzer comparing quite favorably with the generator. The thermal flux of this generator averaged > lo3 n cm-a sec-' (from a moderated 1.6 X lo5 n ~ m sec-' - ~ fast neutron source) while the Howitzer 5-curie source should produce > lo4n cm-= sec-'. Newer generator tubes have been Table 6. Comparison of Activity Produced by Neutron Howitzer and Komon Neutron Generator (5 min irradiation)
Element
* In
Al
Rh Dy
Half life (min) 54 2.3 3.7 2.3 4.4
138
Type of sample 5 mil foil 5milfoil oxide Smilfoil 2milfoil oxide
Weight Sample aotivity of (c/min 10 mg) sample Gener(g) Howitzer ator 0.5 0.5 400 5 0.6 0.3
255 144 120 12 63 207
29.2 19.4 13.3
. .. . ..
13.8
improved so that they produce fluxes quite comparable with the Howitzer. The actual flux value (about 2.5 X lo4 n cm-2 sec-1) found for the Howitzer is lower than that given in several published tables based on a 1-curie source extrapolated to 5 curies. The reduction in flux is probably due to the geometric difference between the two Pu-Be sources.
Volume 41, Number 3, Morch 1964
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Summary
A neutron generator or Howitzer would surely be a worthwhile addition to any laboratory interested in low-level tracers for experiments or analysis. Each has its particular advantages: the pulsed generator with its on-off control and production of 14-Mev neutrons for short periods of time (1-15 min), the Howitzer with its lower energy neutrons in a continuous production for longer period experiments. With the advent of these commercially-available,lowcost neutron sources, the small school or laboratory need no longer be limited to the use of long-lived, sometimes costly radioactive isotopes. The small neutron generator costing $3000 to $7000 with its replaceable generator tube costing about $500 each is primarily suited for short (20 sec to 15 min) irradiations with either fast or slow neutrons. The Howitzer costing about $1200 plus $1200 for a 5-curie plutonium-beryllium neutron source can be used for many types of nuclear investigations requiring irradiations of a wide time range. Excellent counting units are now available with either Geiger, proportional, or scintillation detectors for about $2000. These new advances make it possible for a laboratory to set up equipment for nuclear investigations requiring neutrons for a cost of between $4000 and $9000. Acknowledgments
The authors wish to thank Akiro Tani and Guido Romero who assisted in some of the measurements. This work was partially supported by the U. S. Atomic Energy Commission.
