Prompt Gamma Activation Analysis of Hydrogen Daniel M. Downey and Thomas Alwr Sandy West Virginia University, Morgantown, WV 26506 In recent years the availability of small 252Cfradioisotopic neutron sources has made possible the teaching of lahoratory experiments in activation analvsis in radiochemistrv courses a t colleges and universities i i t h o u t routine access reactors (1-3I.l One area that has not received much attention, however, is activation analysis based on the measurement of the prompt gamma rays emitted from samples during neutron irradiation. The increasing interest in this technique for process stream analysis ( 4 ) , borehole logging (5), and in vivo clinical analysis ( 6 )suggests the technique may merit closer attention in the instructional laboratory. Additional topics that are associated with the techninue that further merit its inclusion a s a laboratory experiment are the visunlization of aamma s ~ e c t r aand calibration of the multichannel analyzer, effects-of geometry on the energy spectra (efficiency, resolution, and peak ~ositian).countine statistics, net peak area determination, nuclea; electron?cs, and density determination.
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Theory
The capture of a thermalneutron by the nucleus of a given element gE to give the product A f-i Emay he represented. hv. the equation $E
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+ +,In
A+iE+ Q
(1)
where Q is the binding energy release by the reaction. Some of this energy appears as gamma radiation emitted almost simultaneouslv upon the capture of the neutron. These prompt or capture gamma rays are of discrete energy and indicative of the target nucleus. In traditional instrumental neutron activation analysis (INAA) the sample is removed from the irradiation chamber and the decav of the radioactive product nucleus is monitored for both qualitative and quantitative analysis. However, the prompt gamma rays emitted during irradiation may also be monitored for analysis. Such prompt gamma activation analyses (PGAA) may therefore he conducted for elements which do not yield radioactive products amenable to INAA, such as hydrogen, carbon, iron, and sulfur. A disadvantage to PGAA is that the analysis must be conducted with the detector subject to a large background radiation and therefore signal to background (SIN) ratios are low. Another disadvantage is that large volume detectors are required to measure efficiently prompt gamma rays which are typically of much higher energy than the gamma rays emitted by radioactive decay. Listings of the prompt gamma rays from the various elements may be found in refs 7 and 8. The determination of the hydrogen content of samples serves as a useful procedure for illustrating PGAA as hydrogen emits during neutron irradiation 2.23-MeV capture gamma rays in high yield, which are readily detected with the common 2- X 2-in. NaI(T1) detectors found in teaching laboratories. In obtaining PGAA spectra of hydrogen the observed countrate, C, which is peak area collected per unit time for the 2.23-MeV peak, is given by
c = N d .I n.
.-,
(2)
where N is the number of target atoms, u is the thermal neutron capture cross section (cm2), 6 is the neutron flux 178
Journal of Chemical Education
-. Figure 1. PGAA irradiation apparatus. (1)252Cfneutron source. (2) inadiation post. (3) water extended polyester neutron shielding. (4) lead gamma ray shielding,(5)sample bonle, (6)Nal(T1) detector, (7)higlwoitage power supply cable, (6)signal to amplifier/multichannel analyzer. (ncm-%I), I is a multiplicity factor arising from the number of gamma rays emitted per neutron capture, and 7 is a counting efficiency factor. The numher of target atoms,N, is given by the sample size. The cross section, a, which is a measure of the probability of neutron capture and the multiplicity, I,may be considered as physical properties of a given nucleus. The neutron flux, 6, is dependent on the size of the source. The counting efficiency r) is a function of three other factors where G is a geometry factor giving the radio of the average numher of gamma rays incident on the detector to the total numher of 2.23-MeV gammas emitted in capture. The second factor, r, is an efficiency factor for total counts per incident gamma ray to the detector. In other words, it takes into account all of the counts which are due to photoelectric, Compton, and pair-production effects. The factor f , however, takes care of the counting efficiency of the photoelectric effect and gives an indication of the fraction of total counts that are observed in the photopeak. The r a n d f factors can he increased by increasing the size of the sodium iodide detector, but at an increased cost. The 2- X 2-in. detector was thought to he a good compromise between cost and efficiency for hydrogen PGAA determinations. The r and f factors are fixed a t a eiven enerev. ".therefore the onlv wav to increase efficiency, 7, is to maximize the geometric factor, G. Since the flux incident on the samnle is also a function of the geometry and as the values of u and I may be considered constant, then the geometry and sample size may he varied in order to maximize countrate. The maximum countrate is desirable so that analyses may be conducted in a minimum time. Obviously, the closer the source is to the sample and detector, the higher the flux and the countrate. Unfortu-
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UO to 10 ua Of % 'f may, be obtained ~- without ... c b m--a for &ma----tionai'purposes-from the University Loan Program of the Savannah River Laboratory. For additional information, contact M. L. Toole. DOE. Savannah River Operations, P.O. Box A, Aiken, SC 29801. ~
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nately, t h e 2 W f source background increases as the detector is moved closer; therefore the SIN ratio decreases. For a particular source-sample-detector-shieldingconfiguration, the optimum irradiation geometry (Fig. 1) must he found experimentally. The number of hydrogen atoms, N, is given by
where ME is mass of hydrogen, A is Avogadro's number, and (A.M.)n is atomic mass of hydrogen. Therefore, the larger the number of atoms, the higher t h e countrate. I n orde;to obtain couutrates significantly above background with the small source and detector. i t is necessarv to use relativelv large sample sizes. By choosing appropriate organic solvents or mixtures of solvents with water. a calibration curve of countrate and % hydrogen can be constructed for the analysis of unknowns. Since
where Vis the sample volume, p is the density of the sample, and %H is the percentage of hydrogen in the sample, then substituting eq 3 in eq 1gives
When volume is held constant, only the density and percent hydrogen will vary from sample to sample, and
Thus the calibration curve will consist of a plot of the counts obtained for the 2.23-MeV hydrogen peak for a given time interval divided by the sample density versus the percentage of hydrogen. The %H for unknowns may then be readily determined by extrapolation. As the density of a sample is also an unknown value, an additional calibration curve must be obtained by measuring the density of the known materials by gamma gauging. I n this procedure the attenuation of gamma radiation emitted from a long-lived radionuclide, such a s t h e common 137CsP37MBa source, by samples is measured and a plot of relative counts versus density generated. T h e density of an unknown material is then determined by extrapolation.
Experimental Materials An irradiation apparatus should he constructed prior to the lahoratory meeting by the instructor as shown in Figure 1. For this discussion, the neutron source was a 4.5-pg 252Cfsource obtained from the Savannah River Laboratory. The detector system was a 2X 2-in. NaI(T1) detector powered by a Canberra MDL 3100-01 HV power supply with signal led from the PMT base into a Canberra Series 30 Multichannel Analyzer (MCA) with X-Y recorder and data p r i n t o ~ tThe . ~ detector should be readily removable from the apparatus. The energy spectra obtained with this system were calibrated with the 0.667, 1.173, 1.33, and 2.50-MeV gamma rays of '37Csl'37MBaand 60Co gamma sources. A single-channel analyzer with amplifier, timer, and scaler could he substituted for the MCA, but the results would he meaningful if the student could see the spectra and determine net peak areas hy calculation from the printouts. Polyethylene bottles filled with acetone, toluene, acetic acid /water mixtures, methanol, ethanol, isopropanol or other solutions were used as standards and samples. Sample containers were 1000mL polyethylene battles ohtained from Fisher Scientific Co. (Nalge 2002). Procedure Caution:The students should be warned of the hazards of252Cf as
there is potential hiologieal damage from both neutrons and gamma radiation. The irradiation apparatus should he setup in an isolated area and the students should spend a minimum tinie changing samples. The electronics may he located some convenient distance from the source by using long cables to the detector. All personnel should wear the standard film badges or pocket dosimeters for the gamma radiation exposure monitoring and TLD's or BFs neutron survey meters must also he used to determine neutron exposure rates. Place the gamma ray sources in the sample position near the detector and while aeeumulating a spectrum, adjust the amplifier gain to put the 2.50-MeV sum peak about 314 acrosa the available channels; e.g., for the 1024 channel analyzer in about channel nuinber 800 (Note I), record the channel number far each peak, and prepare a graph of peak energy versus channel number. From this graph determine the channel number for the 2.23-MeV hydrogen peak. Move the positions of the calibration buttons and determine the shift in channels of the ~ e a kin s the soectrum. Return the detector to t i e irradiationa~~aratus and collect the ,, hackgnjund for thr "'Cf source for 10 mi"' I'rinr our rhe data and record unX-Ychnrt paper.Olrtarn threesrnndnrdsand nn unknown and cuunt each one for three 10-min intervals prrming out the data andlor recording the spectrum on X - Y chart paper each time." Remove the detector from the irradiation assembly to an area of low background, Place a LWs1137MBa source in a fixed position 12 cm in frontof the face of the detector and place each standard and unknown, in turn, between the source and the detector again obtaining three counts for each! Again printout and record the data.' ~
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Data Reduction The areas of the net full energy peaks (FEP) for the 2.23-MeV hydrogen and the 0.662-MeV 1 3 7 C ~ P ~ ~gamma ~ B a rays should be determined by the method of Bauer et al. (9).A plot of the FEP for the '37Cs1'37MBagamma ray versus density of the standards may he used to determine the density of the unknown. A plot of the FEP for the prompt gamma ray of hydrogen for each standard corrected for density versus the %H in each standard may then he used to determine the %H in the unknown. A statlatical evaluation should he performed to determine the quality of the results by the method given in Choppin and Rydherg (10). The typical SIN should he determined using the background counts for 252Cf.The students should he asked to consider the effect of the Compton contribution of the higher energy carbon peaks to the FEP for hydrogen. RBsuIts and DIscu881on Examination of the irradiation apparatus in Figure 1will reveal that the experiment can he setup with a minimum effort on the part of the instructor. The base of the apparatus is the water extended polyester filled bucket in which the 4.5-fig 252Cfsource was shipped from Savanhah River. A layer of lead bricks was built over the source to reduce background with a hole left directly above the access port of the bucket. The 2- X 2-in. NaI(T1) was mounted a t a rieht angle from the opening and shielded with additional lead. Larger detectors gave better results. but onlv data ohtained with the smaller dktector ate presented heresince this size is commonly found in teaching laboratories. Samples were irradiated by simply removing the polyethylene plug and being placed over the opening in a fixed geometry. The flux a t point "A" in Figure 1 was found to be about 3 X lo2 n.~m-~.s-'. With this simple design the neutron source was not removed from its original container and radiation hazards were minimized. Exposure rates a t any point one meter from the source did not exceed 0.42 mR.h-'. The coutrihuThe principles and operation of a multichannel analyzer with Nai(TI) detector should be fully explained in the prelab lecture. The calibration is best doneaway from the irradiation apparatus due to the high 252Cfbackground. In order to reduce dead time, me instructor should set the lower level discriminator as high as reasonable. Only the region of interest at the 2.23-MeV peak should be printed Out.
These counts should be for the minimum time interval necessary to obtain good counting statistics, usually one or two minutes, for the 0.667-MeV peak. Volume 63 Number 2
February 1986
179
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