2128
Anal. Chem. 1980, 5 2 , 2128-2132
Evaluation of a Boron-Filtered Epithermal Neutron Irradiation Facility Ernest S. Gladney,” Daniel R. Perrin, John P. Balagna, and Charles L. Warner University of California, Los Alamos Scientific Laboratory, MS 490, Los Alamos, New Mexico 87545
Natural isotopic B is used to fabricate a neutron filter for epithermal activation analysis, which is permanently installed near the reactor core. This system has pneumatic transfer capablllty for the study of activation products with short half-lives. The analytical properties of this facility are compared to Cd and Cd -k B epithermal activation procedures and demonstrated through the analysis of NBS fly ashes for 33 elements. Detection limits are given and interferences from several sources are discussed.
T h e utility of epithermal neutrons for nuclear activation analysis has been well established. Applications to the assay of geological and biological materials have been widely explored (1-7). T h e principal advantage of using epithermal rather than thermal neutrons for activation analysis is that for many elements t h e ratio of the neutron activation cross section to the cross section for the major elements commonly found in rocks (e.g., Na, Al, Ca, K, Fe, Mn, Sc, P) is enhanced. This results from the fact that most stable isotopes of the more abundant elements follow t h e so-called l / u law (activation cross sections inversely proportional to neutron velocity) throughout the neutron energy spectrum, while many of the less common elements have strong cross-section resonances in the epithermal region. Nuclear reactors provide the most common epithermal neutron source. In order to maximize the ratio of epithermal t o thermal neutrons in a reactor irradiation, one needs to position the sample near the reactor core and surround it with a material that has a very high thermal-neutron capture cross section. While Cd and B are most often used, NaCl(6), MnClz (6),and B + Cd (5, 7-9) have also been investigated. For this study natural B was chosen because it is readily available, i t has a high thermal neutron cross section, it is very refractory, and its neutron cutoff energy (280 eV) has been shown to be much higher than t h a t of Cd (0.4 eV) (8). Cadmium, while an effective neutron filter, has the further disadvantage that samples must usually be encased in foils or boxes which emerge from the reactor highly radioactive and must be handled in a hot cell. This does not readily permit study of short-lived isotopes. We have eliminated this problem by permanently installing our B filter within the reactor shielding and using pneumatic transfer of samples in polyethylene rabbits. Although B-polyethylene rabbits have been investigated for high-energy neutron activation ( I O ) , we felt that installation of a fixed facility was superior. This paper presents the design, fabrication, and analytical evaluation of such a boron filtered epithermal activation facility. Its capabilities will be compared to those of Cd and Cd + B filtered facilities.
EXPERIMENTAL SECTION The epithermal irradiation facility was designed to fit into the upper through-port of the Los Alamos Omega West Reactor (OWR) from the south face of the reactor shield. This port is a 6-in. diameter stainless steel tube which passes tangential to 0003-2700/80/0352-2128$01 .OO/O
the west face of the reactor core slightly above the core horizontal centerline. The rabbit facility is centrally located in the port tube by graphite sleeves and the port shield plugs and terminates at the core vertical centerline (Figure 1). The shield material consists of a mixture of 50% (by volume) powdered elemental crystalline boron and 50% aluminum (“boral”) hot pressed into aluminum sleeves. Due to the overall length limits of the hot press process, it was necessary to fabricate the shield in five pieces. The pieces are welded together to provide the necessary overall length of the shield. The boral powder is pressed to 92-95% of theoretical density and the shield wall is 2.54 cm thick. This provides about 2.3 g/cm2 of boron, with an estimated filter cutoff energy of 280 eV (8). To ensure proper cooling of the sample and the neutron shield, the boral powder was hot pressed into the aluminum sleeves to achieve good thermal contact. The sleeves are welded together to form the inner wall of the cooling water jacket and the surfaces are machined to provide a helical water flow path. Two thermocouples were positioned in the inner aluminum piece of the shield adjacent to the rabbit irradiation position to monitor temperatures during operation of the facility. The concept of the advantage factor (AF) for elements irradiated in Cd-filtered epithermal neutron fluxes has been developed and used by Brune and Jirlow ( I I ) , Steinnes ( I ) , Kucera (7), and Randa ( 5 ) . The AF is calculated by comparing the actual epithermal cross section of a given element to its theoretical l / u cross section. The calculated data from Steinnes ( I ) and measured values of Kucera (7) are included in Table I. Since the exact neutron cutoff energy in our facility is not known, an experimental method had to be devised to compare a boron-filtered system to the Cd-filtered system of Steinnes (1). The permanent installation of the neutron filter near the reactor prohibits the conventional evaluation by means of filtered and unfiltered irradiation at the same location. Since Sc has no epithermal neutron resonances below 2 keV, it serves as a convenient base line for comparison of relative induced activities. We decided to develop an enhancement factor (EF) by a double normalization procedure defined as follows:
where (X), and (X), are the induced activities per gram per unit irradiation time of element X in our epithermal facility and a thermal irradiation position, respectively, and (%)E and (SC)T represent similar activities of Sc per gram per unit irradiation time. A similar double normalization scheme was employed to a limited extent by Cesana et al. (9) to explore the performance of a Cd + B filter for determination of C1, Br, and I in biological materials by instrumental epithermal neutron activation analysis (IENA). Pure elements or compounds were dissolved in water or mineral acids and 50-wg quantities were individually pipetted onto filter paper disks. These were irradiated in the same position in both thermal and boron-filtered epithermal fluxes and counted on large Ge(Li) detectors coupled to 4096-channel pulse-height analyzers. Peak areas were determined by computer and corrected to equal neutron fluences. These corrected activities were used in eq 1 to calculate the EFs reported in Table I.
RESULTS AND DISCUSSION Hanna and Al-Shahristi (6) have also reported measurements of “advantage factors” for eight elements relative to @ 1980
American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
3
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980 ~
y1
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EPlTHERUCL M d T Q O N FILTEF(-.
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Figure 1. Top view of the OWR showing the various irradiation ports and the location of bora1 epithermal neutron filter. N, S, E, and Ware
compass directions and TC are thermal column ports. Na in a boron-filtered IENA system. When their data are renormalized to Sc, excellent agreement between their data and our work is obtained for all elements except I, as shown in Table I. This apparent discrepancy may be related to I loss during longer irradiations. Our E F data may be conveniently compared with those of Steinnes by noting that the Steinnes AF for Sc is 1.0. This means that a t the Cd cutoff energy AF = EF. Any net gain in relative activation of an isotope by using the higher neutron energy cutoff of B will be reflected by E F > AF. Such a result indicates a net improvement in detection limits in the boron-filtered system. The reverse result, E F < AF, indicates t h a t the isotope has a resonance for neutrons of energy 0.4-300 eV and that the use of the boron-filtered system yields a poorer detection limit for that isotope in our system relative to a Cd covered irradiation. Three groups of elements emerge from comparison of our EFs and Steinnes' Cd-filtered AFs. There is a definite sensitivity advantage for B-filtered IENA for elements which show EF/AF > 3.0 (K, Mn, Fe, Cu, Zn, Ga, As, Br, Rb, Mo, Ru, P d , Cd, Te, I, Ba, Ce, Nd, Gd, Tm, Hf, Ir, Pt, Th, and U). Little sensitivity difference is observed between Cd- or B-filtered systems for elements which have EF/AF in the range of 0.5-+3.0 (Na, Al, V, Cr, Co, Se, Ag, Sb, Cs, La, Sm, Ho, T a , W, Re, Au, and Hg). The third group of elements shows a definite sensitivity advantage for Cd-filtered IENA with EF/AF < 0.5 (In, Eu, Tb, Er, and Yb). The real analytical advantage of the B-filtered system is for the determination of elements in this first group and possibly for those with high EFs for which comparative AFs are not available (Ge, Sr, Zr, Sn and Os). A mixed Cd-B filter has been proposed as perhaps the ideal filter material for general IENA applications (5, 7-9). However, comparison of our EFs with those measured by Randa (relative to Sc) using a Cd-B filter (Table I) demonstrate that our B-filtered system is superior. In fact, Randa's EFs are consistently within a factor of 2 of those calculated by Steinnes ( I ) or measured by Kucera (7). Kucera's measurements of actual detection limits for four biological reference materials in both Cd and Cd + B filtered systems reveals only slight improvements of 20-40% for Cd and Mo determination using the Cd B filter. Twenty-two other elements exhibited equal detection limits or actual loss of sensitivity of up to a factor of 6 for the Cd B filter relative to Cd alone. National Bureau of Standards fly ash standard reference materials (1633 and 1633a) were chosen for demonstration of
+
+
analytical capability and detection limits, since these silicates are well characterized (12, 13). Samples of 200 mg were encapsulated in small polyethylene vials and irradiated in polyethylene rabbits for 1 min or 7 h for the determination of elements shown in Table I1 with decay times of 1 6 0 min or 1 1 day, respectively. Samples were counted with large Ge(Li) detectors coupled to 4096-channel pulse-height analyzers for varying periods a t the different decay times shown in Table 11. All y-ray spectral data were reduced off-line on a P D P 11/04 minicomputer. These activation and data reduction procedures are described in greater detail elsewhere (14). The mean plus or minus one standard deviation values shown in Table I1 are for three replicate determinations on each material. For comparison, certified NBS values are provided if available; otherwise, comparative literature values from a recent literature survey by Gladney (13) are given. Excellent agreement is observed for most elements between B-filtered IENA and NBS or comparative values. For SRM 1633 only Br, Si, and Sr show significant disagreement. The problems with Br measurements in fly ash have been discussed in greater detail elsewhere ( 1 5 ) , and our value is in good agreement with one of the two sets of data identified in Gladney and Perrin (15). The Si data summarized by Gladney (13) are highly scattered and tend t o cluster in two groups, one with a mean of 22-23% Si. Interference of 511-keV annihilation radiation with the =Sr line sharply degrades the quality of Sr data. Strontium determinations should be based only on 87mSr.For SRM 1633a, significant disagreements are observed only for Co, Mn, Mo, and Rb. There are insufficient data from non-NBS sources to speculate on the reasons for these discrepancies. The detection limits shown in Table I1 are calculated by using the standard deviation in the background in the region immediately above each line. These data represent detection limits for a real silicate matrix, not the ideal, interference-free detection limits frequently reported in the literature. In IENA, the prominence of (n,p), (n,a), and (n,2n) products, which offer improved opportunities for determination of elements such as F, Ti, and Si (16),also presents serious interferences with the determination of some easily activated elements (e.g., Sc, Na, and Al). The large EFs for several elements with very complex y-ray spectra (e.g., Ta) also increase direct y-ray interference problems. These interferences have received some discussion in the literature (1-7). Table 111 gives our evaluation of these interference problems in silicate matrices. The contribution from each interference was evaluated by running single element standards of the elements involved. So, for Na determination, Mg and Al standards were run simultaneously with an Na standard and the 1368-keV line observed from the Mg and Al standards was standardized as though it had arisen from the =Na (n,y) 24Nareaction. All liquid standards used in this fashion were checked for impurities by atomic absorption, and no significant levels of contaminants were found. The fractional contribution from the interference (last column, Table 111) was calculated for SRM 1633 assuming a 1.0-g sample and multiplying the apparent concentration from interference (third column, Table 111) by the mean concentrations of interferent elements in SRM 1633 from Gladney's compilation (13). The apparent concentration is divided by the sum of the actual and apparent concentrations to yield the fraction of the observed elemental concentration arising from the interference contribution. For Na, Mg, Al, Cr, and Yb the interference contribution is so large that the accuracy and precision of the data are badly degraded. We have not reported these elements in Table I1 for this reason. The interference contribution to Ga, Sc, and Se is substantial, but manageable, and only the precision of the data is affected as shown in Table 11. The remainder (Fe, Co, and
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
2131
Table 11. Elemental Concentrations in NBS Fly Ashes by Instrumental Epithermal Neutron Activation Using a Natural Boron Neutron Filtera
nuclide
way energy,
measd
keV
decay time
Fi 0
(ref)
Ag AS
ll0rnAg 76As
Ba
'"Ba
658 559 497 268 662
20 days 1 day 20 days 1 day 2 min 6 0 min 5 days 1 5 min 1 5 min 20 days 20 days 20 days 20 days 20 days 20 days 1 day 20 days 1 day 5 days 6 0 rnin 5 days 20 days 20 days 20 days 20 days 20 days 20 days 1 5 rnin 20 days 60 min 20 days 20 days
< 0.6 60.4 i 0.08 2900 i 200 2720 i. 80 2 7 5 0 + 140 2 6 0 0 + 170 6.5 i 0.2
0.25-1.3 (13) 61 t 6 (12) 2700 i 1 7 0 ( 1 3 )
4.5 i 0.6 154 t 8 38 i 2 8.7 t 0.3 2 t 2 5.8 2 0.3 5.9 i: 0.2 43+1 8.1 i 0.1 1.83 i 0.05 79i. 6 513 i 1 5 28 i 1 60i 2 84 i 6 117 k 6 7.7 i 0.5 26i 2 9 t 2 23.5 t 0.5 1510 * 60 1260 i 30 1.8 c 0.2 2.0 t 0.1
4.6 i 0.4 (13) 1 5 2 t 11 ( 1 3 ) 38 ( 1 2 ) 8.6 i 1.8 ( 1 3 ) 2.6 t 0.2 (13) 6.2 i 0.4 (13)
24 i 1 7000 i 1 0 0 11.5 i 0.5 230 i 30 5.0 i 1.0 204 i 1 3 221 i 1 6 340 t 50 290 r 20
24 (12) 7200 e 720 ( 1 3 ) 11.6 3 0.2 ( 1 2 ) 214 i 8 (12) 5.5 i 2.6 ( 1 3 ) 210 i 10 ( 1 2 )
element
13smga
is7rnga
Br
'%Ba 82Br 80Br
Ca (%)
49Ca
Ce co cs Eu Fe (%)
I4'Ce
Ga Hf K (%I La Mn Mo
9
Nd
I4'INd
Sr
9
58Co
86Rb I2?3b
MSC 75Se 29
AI
Ta Tb
85Sr 87rnSr ls2Ta 160Tb
Th Ti
47sc
U V
w
Zn Zr
166
776 616 3083 145 1173 795 1408 1099 834 8 34 481 1525 3 28
60Co
'-3 I5'Eu 59Fe 54Mn 72Ga Is1Hf 42K lMLa 56Mn
Ni Rb Sb sc Se Si (%)
a
NBS SRM 1 6 3 3 certified or comparison value
IS3Pa 239Np 5 2 v
1 8 1 W
69mZn 65Zn 95Zr 97Zr
1811 ~
1~ 4 0 531 811
1078 1691 889 265 1273 514 388 1221 962 + 966 312 160 228 1434 686 438 1115 7 57 743
20 days 5 days 5 days 7 min 1 day 1 day
20 days 20 days 1 day
