Sensitivities for Photon Activation Analysis with Thick-Target, 1 10-MeV Electron Bremsstrahlung Enzo Ricci Analytical Chemistry Division. Oak Ridge National Laboratory. Oak Ridge Tenn 37830
Recent reviews ( I , 2) have described active progress in photon activation analysis (PAA). In particular. during the past few years, several authors have developed useful specific methods and determined elemental sensitivities. for a variety of bremsstrahlung production conditions (38 ) . Yet, most of this work involved the use of relatively thin electron targets (2-6m m thick) and electron energies below 40 MeV. Only two groups reported photon-activation yield determinations with bremsstrahlung from 70-72 Me\' electrons (7, 8). and none beyond this electron energy. The Oak Ridge Electron Linear Accelerator (ORELA) was designed with the main purpose of producing neutrons; these are used to determine cross sections of interest to the Liquid Metal Fast Breeder Reactor and other programs. The ORELA can accelerate electrons up to energies of 150-160 MeV. but normal operation-particularly during our experiments-has been at or near 110 MeY. Because of ORELA's main application. the electrons-whose energy is seldom lowered below 100 MeVare made to impinge on a massive (3.03-cm thick) t a n t a lum target. T h e beam power can easily be raised to a maximum of 50 kW-approximately equivalent to a 350 p A current-but a beam of 30 kW or less is frequently used. as was the case in our work. T h e beam is pulsed. and to reach a 30- to 3O-kW power a t 100-140 Me\.', typical conditions are PO0 pulses per second of 20- to 50-nsec duration and 10- to 1.3-A peak intensity. These somewhat rigid operation conditions-quite different from those characterizing most PAA experiments to date--govern bombardments a t the ORELA4. Also. economic efficiency reasons call for simultaneous use of the accelerator for both PA4 and neutron physics experiments. Consequently. two important tasks became clear early in our work. First. to design a pneumatic system that would convey the sample-with the accelerator on, and while being used for other experiments--to a position where it could be cooled and bombarded homogeneously with ORELA's brernsstrahlung. Second. to determine experimentally PAA sensitivities and interferences for ORELA'S conditions. and to put them in perspective with values from other authors (3-8). in light of differences between the bremsstrahlung photon-energy distributions involved. For the first task. we designed a n original system (9). which we describe in detail and assess elsewhere ( 1 0 ) . This paper concerns experiments and discussions relative to the second task. W. S. Lyon. E Ricci, and H H . Ross. Anal Chem. 44, 438R (1972). G. J Lutz. A n a / . Chem , 43, 93 (1971). Y . Oka. T . Kato. K Norriura and T. Saito. Buli. Chem. Soc Jao 40, 5'75 ( 19 6 7 ) . G. J Lutz. Anal Chem , 4 1 , 4 2 4 (1969) J. L. Lebrun and P. Albert. B u l / SOC Chirn. F r , 3, 1020 (1969) C . Engelmann, F r e r c h R e o t . CEA-R-4072 (1970). T Kat0 and A. F Voigt. J . Radioanal. Chem . 4, 325 (1970). T. Kat0 and Y . Oka, Ta/an!a. 19. 515 (1972) E. Ricci. T. H. Handley, and M . G. Willey. "Fluid Supported Sample Holder for Homogeneously Irradiating Samples." U.S.Patent No. 3,549,492 ( D e c . 22. 1 9 7 0 ) . E. Ricci. N u c l . lnstrum Methods. in press.
B R E M S S T R A H L U S G P H O T O N ENERGY DISTRIBCTION ' The uncommon characteristics of bremsstrahlung production a t the ORELA pose two important questions at the theoretical or semi-empirical level. T h e first one refers to the influence of photon angular distribution-for both intensity and energy-on bombardment homogeneity; this has been studied in detail in another publication ( 1 0 ) . T h e second question involves t h e influence of the bremsstrahlung energy spectrum on PAA sensitivities and interferences: we will attempt to answer it, in light of the scarce theoretical and experimental d a t a available for ORELA's operating conditions. Briefly. our PAA pneumatic transfer system (9. IO) consists of a spherical aluminum rabbit (5.1-cm diameter) that carries four Z-cm3 sample capsules to a bremsstrahlung photon irradiation chamber. The gamma radiation is originated by ORELA's intense electron bombardment of a 3.03-cm thick tantalum target consisting of ten parallel, vigorously water-cooled tantalum plates of 1.8- to 6.8-mm thickness. When the sphere arrives a t the irradiation chamber. it floats there. on a n air upstream, a t 15.0 cm past the center of the tantalum target in the direction of the electron beam: the air also provides cooling and induces spinning. thus facilitating homogeneous bombardment of the samples. When a sample is irradiated with bremsstrahlungfrom a beam of electrons of energy Eo impinging on a heavy-element target-it is exposed to a photon spectrum which includes all gamma-ray energies h u p to Eo. If a nuclear reaction is induced in one of the sample elements by this radiation, a t each photon energy k the reaction yield is proportional to the cross section u ( h ) . given in cm2. T h e total yield in reactions per second is: then ( 1 2 , 12).
Y( E,,) =n
'
.Y( E,,.h , d h ) d h
(1)
where n is the number of atoms of reacting element per is cm2 of sample seen by the bremsstrahlung. and AV(EO,/~) the spectrum function-i.e.. the number of photons of energy h, per unit energy k , which enter the sample per second. This function can thus be explicitly written as
& Y i E , , . l= ?) F ( E , . k ) I ( E , , . h / k (2) where the function I ( E o , h ) ! h is proportional to the bremsstrahlung photon energy distribution: F ( E 0 , h ) is a normalizing-correcting factor related to the converter and sample thickness. T h e bremsstrahlung spectrum I ( E o , h ) / k us. h. integrated over all photon angles for a very thin (1 pm or less) platinum radiator can be calculated from tables ( I f ) for electron energies Eo from 1 to 960 MeV. We have thus plotted spectra for seven Eo values (19 to 160 MeV) in
(11) A. S. Penfold and J. E Leiss, "Analysis of Photo Cross Sections," Rept. Physics Research Lab., U.Illinois (May 1958) (12) 8.I. Goryachev,Af. € n e r g y R e v 2, (31, 71 (1964).
~
~
~~~
Table I. Interference-Free PAA Sensitivities at ORELA's Analytical Facility
Element (target)
Half-life
Be B C (graphite) N (NaN03) O(Pb0) F(NaBF,OH) Na(NaN03)
53.6 d 53.6 d 20.34 m 9.96 m 2.07 m 109.7 m 14.96 h 2.62 y 14.96 h 9.46 m 14.96 h 2.50 m 6.6 m 2.31 m 3.75 m 3.43 d 16.0d 36.0 h 48.6 m 55.Om 5 3 . 5 h (eq.) 3 . 1 4 hr 45.0 m 54.0 m 2.81 d 5.5 h 52.1 h 66.9 m
Mg A1 P (red)
V(V20s) Ni Cd
Sn Ta Pb(Pb0)
Gamma-ray measured,* MeV
0.477 0.477 0.511 0.511 0.511 0.511 1 . 3 6 9 or 2.754 0.511 1.369 or 2.754 0 . 8 4 or 1 . 0 1 3 1 . 3 6 9 or 2.754 0.511 1.28 1.780 1.434 0.160 0.511 0 . 5 1 1 or 1 . 3 7 0.150 0.247 0.511 0.335 0.617 0.158 1.09 1.293 0.247 0.215 0.333 0.444 0.279 0.401 0.912 0,899
+
+
+
+ +
+
Average electron beam energy, Eo, MeV
Sensitivity or specific absolute activity, dpm/pg element'
0.813 0.138 3,996 3,878 4,750 25,930 13.6* 0.755 151 49.1* 17 .Of 8,246 137 171* 2,521 4.15 0.461 455 1,715
110 110 105 110 140 105 110 110 105 110 110 105 105 105 110 110 110 115 115 115 115 115 110 110 110 105 140 140
161
119 68.3 1,144 50.7 8 .os 72.2 777 37.5
Only most probable reactions listed. "or" between energies means both ?-rays used to calculate independent, agreeing sensitivity values; " f "means all pbotopeak areas added together to obtain sensitivity. At end of bombardment for one half-life (maximum 2 h) with bremsstrahlung from a 30-kW electron beam. Errors not listed because results are averages-for two or four irradiation capsules (see Ref. 10).Asterisk stresses neutron reactions.
Figure 1. Though ORELA's thick tantalum radiator provides clearly different conditions. it is known ( 2 1 ) that a small difference in target atomic number (73Ta to 78Pt) does not result in significant change of the bremsstrahlung spectrum. and that only the shape of its high-energy tip is affected by radiator thickness. The histogram of Figure 1-calculated from data (13) for a thick Ta target (1.912 cm)-tends to prove this. It results from a rather accurate Montecarlo calculation and shows reasonable agreement. a t low photon energies, with the thin-platinum bremsstrahlung spectrum curve corresponding t o the same electron energy. Eo = 100 MeV. (13) R G . Alsmiller and H . S . Moran, Nucl lnstrum ( 19 6 7 )
Methods 48, 109
Moreover. Figure 1 shows that at low photon energies ( h 9 20 MeV), bremsstrahlung spectra from electrons of Eo
from 25 to 160 MeV do not differ by more than a factor of 1.5, This includes bremsstrahlung from a thick target also, as the histogram shows, a t least u p to 10 MeV: in fact. though the latter is for Eo = 100 MeV, it approaches the low-&, thin-target spectra. because of attenuation of high energy photons in the tantalum converter. These facts are very important. in view of the behavior of the excitation function u ( h ) of Equation 1 for h 5 30 MeY. It is well known that this function displays a socalled "giant resonance" for most photonuclear reactions (2). The giant resonance energy for the common reactions --I e (-r.n). (;.2n). iy,pi-varies only from 20 to 13 MeV
Table 11. Comparison of Sensitivitiesa Engelmann (experimental; Ref. 6)
Element (product)
c ("e)
N ( 13N) 0 ('SO) F(I8F)
Na(22Na) Mg ( 24Na) P (3OP) Ni (57Ni) Cd(1Wd) Cd ( IlZAg) P b (*O "b) Average ratio a
A, this work," Eo = 105 to 115 MeV, dpm/iig
3,996 3,878 4,750 25,930 0.755 151 8,246 455 161 68.3 777
Eo
= 25
Eo
MeV
B, sensitivity, dpmirg
B /A
=
Eo
30 MeV C /A
1,000 1,205 1,000 7,692
0.25 0.31 0.21 0.30
7,143 3,571 5;263 15,390
275 313
0.61 1.9
642 769
1.4 4.8
0.085 0.52 i 0.63
144
0.19
65.7
=
40 MeV
D, sensitivity,
C, sensitivity, dpm/pg
1.8
0.92 1.1
0.59
1.5 f 1 . 5
D / A dpm/irg
40,000 12,500 27,780 45,460 7.11 1,679 2,564 525
10
3.2 5.9 1.8 9.5 3.7 16 0.68 6 . 3 i.5 . 2
All values normalized to end of bombardment for one half-life (maximum 2 h). Bremsstrahlung from 30 kW (-210 MA) electron beam on 30.33-mm thick
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(light to heavy elements). while the width a t half maxim u m of the resonance peak fluctuates between 3.8 and 10 MeV, with smaller values for magic-number nuclei (22, 24). Now, because cr(k) has significant values only within the range of the giant resonance, just the low-energy part of the bremsstrahlung spectrum ( k = 10 to 30 MeV) contributes to the photon activation yield in Equation 1. This. in addition to the observed similarity (Figure 1) of bremsstrahlung spectral shapes within approximately the same energy range, suggests that ORELA's PAA sensitivities should be similar to those obtained by other authors with Eo below 30 or 40 MeV, and with thin converters.
HISTOGRAM ( R e f 1 3 ) T h c k To torqet (1912crn)
CURVES (Ref I I ) Very thin Pt Torqet 1 s l # m ) Function I ( E o , k ) / k , nteqtd over 011 onqles ( E q 2 )
g
10
>
t L
z
EXPERIMESTAL
5Q
-
-1
-
+ w
-
6 z
2 -
1
2
9107
To determine interference-free sensitivities for 16 elements, n e bombarded pure substances in our new facility. As bombardment is not perfectly homogeneous in the sphere-rabbit ( I O ) . to obtain average results we placed these substances either in each of the four sample capsules. or in two of them. oppositely located in the sphere; l - c m 3 polyethylene inserts were used to hold loose powders. Irradiations ranged from 30 sec to 30 min-depending on the half-life of the product-at electron beam powers from 9.3 to 29.0 kW . We counted the radioactivity of the products with a conventional gamma-ray spectrometer, consisting of a n RCL 400-channel analyzer with paper tape readout and a 7.62 cm X 7.62 cm NaI(T1) detector: the latter was preferred to a Ge(Lij detector. t o facilitate absolute counting. We sandwiched all positron emitters between sufficient thicknesses of Lucite. to count the annihilation radiation. and measured all samples through a 1.24-cm thick Lucite beta absorber. We tried to identify-and follow the radioactive decay of-all the products induced in each of the elements irradiated that gave significant gamma photopeaks. Least-squares decay curve analysis (Z5) was applied to the results so obtained.
a
-
5-
2 -
''-0
20
40 60 80 100 120 140 PHOTON E N E R G Y , k ( M e V )
160
(80
Figure 1. Integrated-over-angle bremsstrahlung spectra for electron beams of €0 = 19 to 160 MeV impinging on thin (curves) and thick (histogram) converters
and lead-with poor neutron-activation sensitivities but great environmental significance-show encouraging results a t the ORELA. We have stressed results from neutron reactions with a n asterisk; they are characteristic of ORELA's design and not reported by most other laboratories. S o t e that the vanadium ( m y ) sensitivity is among the six highest in Table I. On the other hand, results for some reactions are low. but they are useful to estimate potential interferences in PAA. We chose 11 elements-from light to heavy in the periodic table-among our 16 sensitivity determinations. to compare our results with those obtained by other authors for Eo 5 40 MeV, and thus test our theoretical expectations of agreement. Table I1 establishes this comparison and shows that. indeed. our results generally agree with experimental ( 6 ) and calculated ( 2 , 4 ) ones. and fall between sensitivities for electron beam energies Eo of 25 and 30 MeV. Sensitivity ratios bear out this agreement with standard deviations ranging from 53 to 121%. This variation is small. considering that it amounts to a factor of' 2.2. while the sensitivity data range from 0.755 to 23.930 dpm/Fg-thus implying a factor of 34,000. Furthermore, analogous ratios between sensitivities reported by the two
RESCLTS ASD DISCUSSION Table I summarizes our results. T h e best sensitivities are for carbon, nitrogen. oxygen, and. particularly, fluorine-elements difficult or impossible to determine by neutron activation analysis-in agreement with other a u thors (3-7). Because 3oP is also induced significantly in sulfur-which often accompanies phosphorus-the highly sensitive (-,,n) reaction on the latter cannot be considered as useful. But heavier elements. such as nickel, cadmium, B. L Berman, "Atlas of Photoneutron Cross Sections Obtained with Monoenergetic Photons," U . S . A t Energy Comm Rept. UCRL74622 (1973) (15) J. B. Cumming, in 'Applications of Computers to Nuclear and Radiochemistry," G . D . O'Kelley, E d . , Rept. NAS-NS 3107 Chemistry (1963), p 25. (14)
Lutz (calculated: Ref. 2. 4id Ea = 25 MeV
Ea = 30 MeV
E, sensitivity,
dpm/rg
1,800 3,000 3,000 1,600 0.6 90 .o 3,000 210 36.4 60 .O T a target.
E /A
0.45 0.77 0.63 0.062 0.80 0.60 0.36 0.47
0.53 0.077 0 . 4 7 =t0 . 2 5
100 PA electron beam on 2 mm Pt.
F, sensitivity, dpm/fig
Eo = 35 MeV
F /A
6,000 12,000 9,000 2,500 1.5 270 12,000 600
1.5 3.1 1.9 0.096 2 .o 1.8 1.5 1.3
144 120
2.1 0.15 1 . 5 XZ! 0 . 9
G, sensitivity, dpm/rg
15,000 24,000 18,000 4,900 2.7 600 18,000 900 216 180
G/A
3.8 6.2 3.8 0.19 3.6 4.0 2.2 2 .0 3.2 0.23 2.9 i 1 . 8
100 fiA electron beam on 6 mm W.
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other authors (Table 11), yield standard deviations of a similar magnitude-e g , ratio E/B = 1.53 I 1.15 (75%). F/C = 1.31 A 1.12 (85%),G / D = 0.72 f 0.67 (93%). We should not assign great significance a priori to the fact that our sensitivities are between others' values for Eo = 25 MeV and Eo = 30 MeV. Our distance sample-converter is greater (15 c m ) , and our electron beam intensity double (-210 PA) theirs. However, these features result in opposite effects. Moreover, the general agreement of sensitivity ratios is quite important, because it implies activation trends for our system, parallel with those of thintarget, low-& facilities, for the whole range of giant resonance energies ( k = 10 to 30 MeV). T h a t is, it suggests reasonably similar bremsstrahlung spectrum shapeswithin said range-for the ORELA and conventional facilities (2, 4, 6), as we had expected from Figure 1. Table I1 shows, finally, that our sensitivities agree best
with Lutz's results. The latter were obtained from a comprehensive table (2, 4 ) which lists calculated sensitivities for 76 photon reactions on 59 elements for electron beam energies Eo = 25, 30, and 35 MeV. With reasonable approximation, the average ratios of Table I1 can be used to estimate sensitivities for our PAA system, from Lutz's table, for elements and reactions not included in our work.
ACKNOWLEDGMENT We thank J. A. Harvey and H . A . Todd for their continuous interest in our work, and ORELA's operators for smooth scheduling and conduction of our bombardments. Received for review September 12. 1973. Accepted November 12, 1973. Oak Ridge National Laboratory is operated by the Union Carbide Corporation for the U.S. Atomic Energy Commission.
Determination of Lead in Paint with Fast Neutrons from a Californium-252 Source George J. Lutz Activation Analysis Section. Analytical Chemistry Division, National Bureau of Standards, Washfngton. D.C. 20234
Several decades ago in the United States, it was common practice to paint the interior walls and woodwork of dwellings with a formulation of paint containing large amounts of lead. This has created a tragic health problem, particularly in some of the larger cities, causing lead poisoning of children in certain susceptible age groups who have ingested this lead-bearing paint which has peeled or chipped off from the walls. The detection and alleviation of this hazard depends upon reliable chemical determinations of lead in suspected areas. Currently, screening of suspected paints is oriented to the detection of those containing greater than -0.5% of lead. The accurate determination of lead a t this level is not particularly difficult by a variety of methods, but they usually require time consuming manipulations such as dissolution prior to measurement, although Rasberry (I) has studied portable X-ray fluorescence analyzers developed specifically for use in screening, in situ, wall paint for lead. A purely instrumental activation analysis, particularly if it could be accomplished with a relatively inexpensive irradiation source would appear t o have merit. It would dispense with lengthy chemical treatments and would have the potential for largely automating the determination. This paper describes the evaluation of a small 252Cf source for this determination. T h e element californium was first synthesized by Thompson et al. (2) in 1950. Californium-252 is currently manufactured by irradiating plutonium targets in a nuclea r reactor. Starting with 239Pu, the heaviest isotope available in large quantities, the production of 252Cf requires a series of 13 neutron captures. Current production is about 1 gram per year. The isotope has a half-life of 2.65 years. It decays both by N particle emission and by spontaneous ( 1 ) S. D. Rasberry, Appl. Spectrosc.. 27, 102 (1973). (2) S . G . Thompson, K . Street, J r . , A . Ghiorso, and G . T Seaborg, Phys. Rev., 7 8 , 298 (1950).
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fission. The neutron output from spontaneous fission of 1 gram of 252Cf is 2.34 X 10l2 per second. T h e unmoderated neutron spectrum of 252Cf is roughly the same as that of 235U.Relative to (N,n) isotopic neutron sources, 252Cf sources have smaller dimensions and less radioactive material. They require less space for decay helium and, for practical purposes, rarely require cooling. They are also competitive on a cost per neutron basis with other radioisotope neutron sources (3). Ricci and Handley ( 4 ) aptly considered their system utilizing this isotope for laboratory activiation analysis a "portable, maintenance-free, quasi-reactor."
EXPERIMENTAL The iiBS 252Cf facility consists of 8 sources of 75 gg each a t the time of these experiments. The sources are mounted in a 90- x 90- X 210-cm high aluminum tank filled with demineralized water. The source configuration is shown in Figure 1. The purpose of the octagonal source array is to produce a neutron flux as homogeneous as practical at the sample irradiation position. Since the stable isotopes of lead do not undergo nuclear reactions with thermal neutrons useful for analytical purposes. the sources are moved as close as possible to the central pneumatic tube to enhance t'ast neutron reactions. The fast neutron flux is approximately 5 x 10; n.'cm2-sec. Samples are packaged in a polyethylene snap cap vial of 1-cm diameter and 2.S-cm length. This is placed in a larger polyethylene vial with spacers to ensure that the sample is in the position of maximum neutron flux. A compressed air pneumatic transfer system transfers the samples to the irradiation position and return. Preliminary experiments involved irradiating and counting a few grams of lead for various lengths of time. Three nuclear reactions were detected. These are shown in Table I. The threshold of the reaction 2"O'Pb(n,2n)2"3Pbis about 7.3 MeV and the yield even with long irradiation times is inadequate for analytical purposes. The inelastic scattering reactions. however. yielded sufficient activity to warrant further investigation although, because ( 3 ) J. L. Crandall. isotop. Radiat. Techno/..70, 306 (1970). ( 4 ) E Ricci and T. H. Handley. Ana/. Chem., 42, 378 (1970).
