Dioxide at Room Temperature," paper presented at the Ninth Informal Conference on Photochemistry, Ohio State University, Columbus, Ohio, 43210, September 1970. Merer, A. J., Discussions Faraday Soc. 35, 127 (1963). Mettee, H . D., J . Chern. Phys. 49, 1784 (1968). Mettee, H. D., J . Phjs. Chem. 73, 1071 (1969). Okuda, S., Rao, T. N., Slater, D. H., Calvert, J. G . , Ibid. 73, 4412 (1969). Otsuka, K., Calvert, J. G., J. Amer. Chem. SOC.,93,2581 (1971). Pitts, J. N., Jr., Adcan. Enciron. Sci. 1, 289 (1969). Prager, M. J., Stephens, E. R., Scott,-W. E:, Ind. Eng. Chem. 52.521 (1960). Rao,'T. N., Calvert, J. G., J. Phys. Chern. 74, 681-5 (1970). Rao, T. N., Collier, S. S., Calvert, J. G., J . Amer. Chem. Soc. 91,1609 (1969a). Rao, T. N., Collier, S. S., Calvert, J. G., Ibid. 91,1616 (1969b) Renzetti, N. A., Doyle, D. J., Intern. J . Air Pollut. 2, 327 (1960). Shirai, T., Hamada, S., Takahashi, H., Ozawa, T., Ohmura, T., Kawakami, T., Kogyo Kagaku Zasshi 65,1906 (1962). Sidebottom, H. W., Badcock, C. C., Calvert, J. G., Rabe, B. R., Damon, E. K., J . Atner. Chern. Soc. 93 (13) (1971a). Sidebottom, H. W., Badcock, C. C., Calvert, J. G., Reinhardt, G. W., Rabe, B. R., Damon, E. K., Ibid. 93, 2587 (197 1b). Strickler, S. J., Howell, D. B., J . Chetn. Phys. 49, 1947 (1968). Timmons, R . B., Photochern. Photobiol. 12, 219 (1970).
U. S. Dept., of Health, Education and Welfare, "Air Quality Criteria for Sulfur Oxides," Public Health Service, Consumer Protection and Environmental Health Service, NAPCA, Washington, D. C., 1969; NAPCA Publication no. AP-50. Urone, P., Lutsep, H., Noyes, C. M., Paracher, J. F., "Static Studies of Sulfur Dioxide Reactions in Air," presented before the Division of Water, Air, and Waste Chemistry, 155th National ACS Meeting, San Francisco, Calif., April 1968. Urone, P., Schroeder, W. H., ENVIRON. Scr. TECHNOL. 3, 436 (1 969) ,----I'
van den Heuval, A. P., Mason, B. J., Quart. J . Roy. Meteorol. Soc. 89. 271 (1963). Warneck,' P. J.-, ccA Technology Division, Bedford, Mass., private communication, 1968. Wilson, W. E., Jr., Levy, A., J . Air Pollut. Control Ass., 20, 385 (1970). Receiced for reciew Januarji 4 , 1971. Accepted May 10, 1971. The authors gratefully acknowledge the support of this work through a research grant from the National Air Pollution Control Administration, USPHS, Arlington, Va. During the period of this research, one of us (CCB) held a post-doctoral fellowship froin the National Air Pollution Control Administration, Consumer Protection and Encironmental Health Sercice, PHS.
COM MUN ICATION
Proton Activation Analysis for Trace Elements Sidney Fiarmanl and G a r y Schneier Departments of Physics and Chemistry, University of Kansas, Lawrence, Kan. Trace element concentrations in water samples have been determined from analysis of y-ray spectra produced by bombarding the samples with a 4-MeV proton beam from a Van de Graaff accelerator. Concentration levels in the parts per billion range for most light elements Z 7 25 seem t o be detectable. Some measured representative values are: P, 28 ppb; N, 190 ppb; S, 37 ppb; F and Na, 0.1-1.0 ppb. ~~~~~~~~
~
T
he method of identifying trace isotopes from their X-ray and y-ray spectra is proving t o be very sensitive (Bull. Amer. Phys. SOC.,1971; Johansson et al., 1970). Both kinds of spectra are generated when a several million-electronvolt (MeV) beam of charged particles, impinging upon a thin layer of material, strips off atomic electrons and collides with atomic nuclei. The X-rays are produced from the de-excitation of the excited atoms, while the y-rays come from the excited nuclei. Whereas X-ray analysis is currently feasible for elements with a charge greater than 11, the y-ray technique is better suited for the lighter elements (Bull. Amer. Phys. Soc., 1971). This paper will describe some measurements of trace element concentrations in a water solution by use of protoninduced reactions and y-ray detection-proton activation analysis. In this work, a Van de Graaff accelerator is used t o produce a proton beam at a well-defined energy equal to the resonance energy of a specific strong nuclear reaction. Generally, the strongest reactions are inelastic proton scattering followed by
Table I. Detectable Concentration Levels in Light Elements Estimate of lowest detectable concentration Reaction E, (MeV) I- (keV) levels, ppb 1.03 0.991 2.567 2.664 1.747
168 89 40 48 0.075
0.1-1 . 0" 0.1-1 . o 0.1-1.0 3 200 . . .
0.899 3.045 0.846 0.874
2.2 8.0 40.0 5.0
1.327 1.458 2.400 2,930 1.365 1.652 3.58 3.102 3,096 2.808
2 8 7.5 0.47 50.0 1 1 53.0 95 20.0 0.34
190" 500 0.1-1 .O" 0.1-1 .O" 0.1-1 . oa 0.1-1.0 0.1-1.0' 31' 10 15" 22" 3T 50
Present address: Dept. Physics, Stanford University, Stanford, Calif. 94305. To whom correspondence should be addressed. Volume 6, Number 1, January 1972 79
m \
GOLD D I S K
DETECTOR
I
solid-state detector (ORTEC Catalog 1002, Oak Ridge, Tenn.) [Ge(Li)]. Signal pulses from the Ge(Li) were amplified, shaped, and then stored in a 1024-channel pulse-height analyzer. A %!o source was used to calibrate the y-ray energy spectra.
PROTON BEAM
r
Phosphorus The 31P(p,a,y)2*Si*(1.78 MeV) reaction was used to detect phosphorus at a proton energy equal t o the resonance energy at Ep = 3.102 MeV. The target was bombarded with a beam of 2 pA for 50 min. The y-ray spectrum is shown in Figure 2. If we assume that a phosphorus peak twice the size of the statistical fluctuation in the background could be measured-Le., equal to 2 d N , where N is the average number of counts in a background channel-then it is possible to detect a peak 550 times smaller than the one shown here, which would correspond t o a concentration level of 28 ppb. If the y-rays from sulfur were not present, the background near the phosphorus peak would have been less, lowering the detectable concentration level to 22 ppb. The factor which limits the minimum concentrations detectable by this method is the y-ray background due to the other trace elements. One can minimize this effect by using a very thin target, but one for which the proton energy loss will be no less than the energy resolution of the proton beam: about 2 keV. If the background has a sharp minimum within the half-width of the resonance (20 keV in this case), it would be advantageous to collect data there.
f
PR E AMP
MA1 N
'
*'
AMP
1024
.'
>~ C H A N N E L ANA LY Z E R
Nitrogen Of the possible reactions with nitrogen, the largest resonance occurs with the l5N isotope (only 0.37% of the naturally occurring abundance). The reaction used was 15N(p,a,y)12C*(4.43 MeV) at a proton energy of 0.899 MeV. The target was bombarded for 1 hr at 15 PA. Figure 3 shows the y-ray spectrum. The broadening of the y-ray peaks is due to Doppler effects and the kinematics of the reaction. Because of the small (0.37%) isotopic abundance of I5N,the lower limit of nitrogen detection was 190 ppb. The possibility of using a '*N(p,p',y) reaction to improve on this value is being investigated. Sulfur The sulfur from Na2S04in the standard solution was detected at a proton energy of 3.096 MeV. A beam intensity of l
Electronic and Physical Setup The proton beam was stopped by the gold disk (Figure 1) and the y-rays were detected by a lithium-drifted germanium
3' P
Ep = 3.102 MeV
t
Figure 2. y-Ray spectrum at the resonance energy of a 31P(p,a,y)28Si*(1.78 MeV) reaction. Also shown are 23Na(p,a,y) and 3 2 S ( ~ , 7-ray ~ , ~ )peaks
Iq
1
:
" 5
0
80 Environmental Science & Technology
23Na
m
6
32
Ep =0.899MeV
aoo
YO0
Figure 3. y-Ray spectrum at the resonance energy of a I6N(p,a,y) W*(4.43 MeV) reaction. Also shown are two of the three lgF(p,a,~)peaks (the detection mechanism of these higher energy ?-rays produces three peaks)
-
I9F
(I)
I-
I
2 3
n
0
u
-&
1
I
500
,.,:.. ,,.,.' ..,. .\,.\.: ~~.~.......'.".',.~'. >$ ,.
I
'
~
65 0
CHANNEL
I
800 NO
23N, "P
Figure 4. y-Ray spectrum at the resonance energy of a 3aS(p,p',y) reaction
CHANNEL NO.
pA for 2 hr on a target made from a few drops of water was used. The lower limit of detection extracted from the data in the spectrum shown in Figure 4 was 37 ppb. Fluorine and Sodium Large y-ray yields from F and Na were also observed and these elements could be detected in the 0.1-1.0-ppb range. In fact, for the resonances near 3.0 MeV, the intense y-ray flux from the 23Na(p,a(,y)and 23Na(p,p',y) reactions limited the beam current, thereby precluding faster collection times. Application to Air Samples The standard bubbler used in the West and Gaeke (1956) SO2 detection method pumps 300 liters of air through 50 ml of solution. This corresponds to a gain in solution-to-air concentration of 8-i.e., 10 ppb in air corresponds to 80 ppb in solution, based on 100% efficiency in the scrubber. A convenient scrubber to use is hydrogen peroxide which oxidizes the sulfur dioxide to sulfuric acid. Adding sodium chloride converts the H2S04to NasSOa and HCI, and the Na2S04can then be evaporated onto a gold disk. The same procedure can be used to prepare a nitrogen target from NO2 since,
and HNOI
+ NaCl
-P
NaNO8
+ HC1
Future plans include determining the ultimate sensitivity
of this technique for the elements listed in Table I from measurements first on standard solutions, then on actual samples of water and air. Acknowledgment We would like to thank R. L. Middaugh for making many helpful suggestions. Literature Cited Bull. Amer. Phys. SOC.,16, 545 (1971), Abstracts DJ-4, DJ-5. Johansson, T. B., Akselsson, R., Johansson, S. A. E., Nucl. Instrum. Methods, 84,141-3 (1970). West, P. W., Gaeke, G . C., Anal. Chem., 28,1916 (1956). Receioed for review May 13, 1971. Accepted August 1, 1971. Supported in part by the U.S. Atomic Energy Commission. Volume 6 , Number 1, January 1972 81
