Cambridge, Mass.. 1961. (12) H. D. Beckey, lnt. J. Mass Spectrom. /on. Phys., 2, 500 (1969). (13) Field Desorption Users Workshop, Biochemistry Department, Michigan State University, East Lansing, Mich., March 1974. (14) Field Desorption Users Workshop, Chemistry Department, University of Illinois, Urbana, Ill., December 1974. (15) R. A. Laine, N. D. Young, J. N. Gerber, and C. C. Sweeley, Biomed. Mass Spectrom.,1, 10 (1974). (16) C. C. Sweeley, B. D. Ray, W. I. Wood, J. F. Holland, and M. I. Krichevsky, Anal. Chem., 42, 1505 (1970). (17) H. U. Winkler and H. D. Beckey, Org. Mass Spectrom., 6, 655 (1972).
(18) R. A. Hites and K. Biemann, Anal. Chem., 42, 855 (1970).
RECEIVEDfor review July 21, 1975. Accepted November 10, 1975, This work was supported in part by a postdoctora1 fellowship (to J N G ) from the Smith, Kline and French Laboratories and a research grant (RR-00480) from the Biotechnology Resources Branch, National Institutes of Health.
Multielement Charged Particle Activation Analysis with X-ray Counting J. R. McGinley and
E. A.
Schweikert’
Center for Trace Characterization, Deparfment of Chemistry, Texas A&M University, College Station, Texas 77843
Energy dispersive x-ray counting of radioactive species produced by charged particle activation was studied as a nondestructive multielement trace analysis method for elements of Z 1 26. Experimentally determined thick target yields and interference-free detection limits are presented for proton and deuteron activation products of 37 elements. pg for Mo to 3.4 These detection limits range from 1 X pg for Rh for a 6 pA-hr irradiation with 20 MeV protons. The application of this technique is illustrated with data on the simultaneous determination of up to 26 elements in NBS standard glass samples at the 500-, 50- and 1-ppm levels using 20-MeV proton activation.
T h e aim of this study was to expand the scope of charged particle activation analysis by examining the possibilities of complementing y-ray spectrometry with nondispersive delayed x-ray counting. T h e use of x-ray counting was first reported in neutron activation analysis by Shenberg et al. ( I ) and Pillay e t al. (2). More recently, Mantel et al. extended this work by surveying a large number of x-ray emitters resulting from ( n , y ) reactions (3, 4).With the exception of a note in conjunction with this project (j),no work has been reported, so far, on the analytical possibilities offered by charged particle activation followed by x-ray counting. An interesting observation in this connection is t h a t most of the radionuclides of medium and high 2 elements produced by charged particle reactions decay principally by internal conversion or electron capture and are thus predominantly x-ray emitters. T h e advantages of nuclear activation followed by nondispersive x-ray spectrometry have been discussed elsewhere ( 5 ) .T h e main features are: the direct relationship between t h e x-ray energy and the atomic number of t h e pertinent element; the relatively simple structure of x-ray spectra (in comparison with y-ray spectra); the availability of radioactive decay rates as an additional criterion of identification. These advantages must be weighed against possible limitations arising from self-absorption and enhancement effects common to all x-ray techniques. An additional limitation proper to nondispersive x-ray counting of radioactive samples arises when p activity is present, which results in increased background. This study focused on the feasibility of nondispersive x-ray counting following proton and deuteron bombard-
ment as a means of multielement, nondestructive trace analysis of medium and high 2 elements. Topics examined include: evaluation of activation reactions and detectors; interference- free detection limits for proton and deuteron activation products of 37 elements; application of the technique illustrated with d a t a on multielement determination in NBS SRM Glass samples a t the 500-, 50-, and 1-ppm levels using 20-MeV proton activation.
EXPERIMENTAL Irradiation. All irradiations were carried out at the Texas A & M
University 88-inch sector focused variable energy cyclotron. Nominal irradiation energies were 20 MeV for both proton and deuteron bombardment. In order to preserve sample integrity, the maximum beam intensities used were 3 NA per cmy. Samples. Thick target yields and interference-free detection limits were determined by irradiating pellets of approximately 2-mm thickness and containing known amounts of the element of interest. When necessary, reagent grade graphite was used as a binding material. Three standard reference glass samples from the National Bureau of Standards (SRM 610, 612, and 614) were analyzed using this method. These samples consisted of 72% SiO2, 12% CaO, 14% NalO, 2% A1203, and 61 trace elements nominally doped at the 500-, 50-, and 1-ppm levels respectively. Counting. Two detectors were used in this work: a Si(Li) of 28.3-mm2 surface area, 3-mm active depth, 174-eV FWHM at 5.9 keV, and a thin Ge(Li) of 1984-mm2surface area, 7.25-mm active depth, 238-eV FWHM at 5.9 keV. Output signals were processed through a Canberra spectroscopy amplifier and fed into a 1024 channel analyzer. Quantitation. Quantitation of SRM 610 was done by comparing the peak area of the x ray from the nuclide of interest with the same peak area from a standard. The standards consisted of 70% SiO2, 10% CaO, 12% NaO, 2% A1203, and contained six of the elements of interest in known concentrations. A sufficient number of these standards were prepared to include all of the elements sought. Copper monitor foils (25 km thick) were used to normalize sample and standard irradiation conditions. After the concentrations of the elements of interest had been determined in SRM 610, all other samples were compared to this material for quantitation.
RESULTS AND DISCUSSION Activation Reactions. A comprehensive survey was made in order to determine the reactions of interest by proton and deuteron activation. T h e criteria for selection were as follows: Simple, low threshold, nuclear reactions; target nuclides having high natural abundance; product nuclides decaying primarily by electron capture or, in the case of isomeric transitions, having a high internal conversion to y-ray emission yield. ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
429
\
, 50
25
1
75
100
125
ENERGY (Kev)
Figure 1. Comparison of Si(Li)and LEPS efficiency
Almost 400 suitable reactions were found with product nuclides of 4 sec It 1 / 2 I120 days. Among these, only those reactions satisfying the following two requirements were considered for further study: (a) principal mode of decay (i.e., close to 100%) leading to emission of one or more x rays; (b) production of the nuclide of interest with a bombarding energy 5 20 MeV. Virtually all the elements between 26 I2 I83 are accessible by one or more nuclear reactions. Considering t h a t the x-ray emitting radioisotopes belong to medium and high 2 elements with relatively high Coulomb barriers for 3He and 4He activation (e.g., -10 MeV for 2 = 26), experimental work was concentrated on proton and deuteron activation. Detector Selection. T h e performance of a detector must be evaluated in terms of a broad objective: high resolution and high efficiency over the entire range of x-ray energies of interest (6.4 t o 89.8 keV). T h e detector should have low efficiency for the detection of B particles and for photons outside of the energy range. As neither of the detectors available for this work, a Si(Li) and a thin Ge(Li) detector (LEPS), met all these criteria, a compromise had t o be made. T h e absolute efficiency curves of these detectors are shown in Figure 1. T h e Si(Li) detector has excellent resolution (174-eV F W H M at 6.4 keV).and reasonably good efficiency for x rays between 4 and 50 keV. However, the low efficiency of the Si(Li) detector above 60 keV would make it difficult to detect K x rays from elements with 2 > 75. A further disadvantage of the Si(Li) detector is its long time constant (in our case: T 1 msec a t 59.6 keV). As a result, pulse pile-up, which causes peak broadening, occurred if count rates exceeded 1000 counts/sec. An additional limitation was imposed by the sensitivity of the Si(Li) to p particles. Thus, in practice, the Si(Li) was easily overloaded if the matrix was an intense B emitter. It should be noted t h a t while p particles can be screened out with an absorber, this causes absorption of x rays of interest and thus decreases sensitivity. T h e L E P S detector does not have as high a resolution as the Si(Li) detector and is sensitive to higher energy photons. However, the L E P S provided a more satisfactory per-
-
430
formance in terms of efficiency, could tolerate much higher count rates than the Si(Li) detector, and was not as sensitive to /3 particles. Consequently, all subsequent analytical work was done with the LEPS. Detection Limits and Interferences. Data from proton and deuteron activation in a combined total of 39 elements are presented in Table I. Proton activation shows broad elemental coverage and a capability for trace analysis with detection limits for 33 of the 34 elements studied ranging from gg t o 1 pg. Similar trace levels can be detected with deuteron activation for 10 of the 13 elements studied. These detection limits are for interference-free conditions, calculated according to the definition given by Currie for the case of a “zero blank” (6).A compilation of possible nuclear interferences affecting the reactions studied is given in Table 11. Several observations can be made regarding sensitivity and selectivity: a) the magnitudes of the interferences are in most cases small (O
-2.8 -11.2 -1.8 0.0
Interference-freec detection limits, pg
Fe Ka (6.4) Ga K a ( 9 . 2 ) As Ka (10.5) Se K a ( 1 1 . 2 ) Sr Ka (14.1)
>O
2.8 days
Thick targetb yield, c p m l p g
1.0 x
lo-,
>O
1 7 days 38.9 hr 1 3 7 days 4.4 hr 19.6 hr 3.37 days 2.5 hr 1 2 0 days 96 min 5.1 days 5.3 days 4.8 hr 37 min 1 0 . 3 hr 1.8 hr
-1.1 11.8
-1.7 10.5 -2.6 -1.2 0 -1.7 -3.2 -4.1 -10.5 8.04 -1.2 11.8
10-3 10-1 10-3
lo-'
>O
0.3 days 8.2 days 7 0 days 56.4 hr 2.2 hr 64.0 hr 41.0 hr 3.2 hr 3.0 days 3.1 hr 39.0 hr 64.1 hr 5 2 . 1 days 66.9 min 6.2 days 35.0 hr 4.5 hr 1.1hr 16.0 min 25.0 min 2.9 hr 4.0 hr 2.8 days 2.5 hr
-1.7 -9.5 -3.7 -1 0.5 -1.7 -1.9 -2.7 -3.7 >O
-3.6 -2.8 -1.5 -7.6 -3.3 -1.6 -1.6 0 -4.4 -4.3 >O
-10.6 O >O
>O >O >O
Os K a , , , (61.4; 63.0) Ir Ka,,, (63.3; 64.9) Ir K a , , , (63.3; 6 4 . 9 ) Pt KO,., (65.1; 6 6 . 8 ) Au K a , , , (67.0; 68.8) Tl K a , , , (70.8; 72.8) Pb K a , , , (72.8; 74.9) Pb K a , , , (72.8; 74.9) Br K a ( 1 1 . 9 ) Br K a (11.9) Pd K a ( 2 1 . 1 ) Sb K a (26.3) Te Ka:( 2 7 . 3 ) CS Ka:( 3 0 . 8 ) Ba Ka (32.1) Gd K a , , , (42.3; 43.0) Dy K a , , , (45.2; 46.0)
1.5 X 8.5 2.5 X 1.7 x 1.7 6.8 5.8 X 6.6
lo-' 10-1
10-1 lo-, lo-,
lo-' 10' 10' 10'
lo-'
2.4 x 4.5 x 5.8 x 2.2 x 7.8 x 1.5 x 9.7 x 6.6 x 8.6 x
10-I 10-3 10-' 10-3 10-4 10-4
10-4 10-4 10-3
10-3
lo-,
10-4 10-4
10-4 10-3
lo-* lo-' 10-3 10-4 10-2 10-2
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2 , FEBRUARY 1976
431
Table I (continued) Q-Value. Reaction0
Half-life
X-ra) detected (energv in k e V )
MeV
Thick targetb Sield, cpm/pg
Interference-freec detection limits, pg
6.4 X >O Dy K a , , , (45.2; 46.0) 1 . 9 x 10' 38.0 min ' 63Dy(d,n)'6 4 H ~ -3.8 Tl K a , , , (70.8; 72.8) 6.1 1.1 x 10-4 52.0 hr 20'Tl(d,2n)Z03Pb >O Pb Ka,,,(72.8; 7 4 . 9 ) 1 . 2 x 10' 3.1 x 10-3 66.9 min 203Tl(d,n)z04mPb -6.7 Pb K a , , , (72.8; 74.9) 7.0 9.2 x 10-5 6.24 days z06Pb(d,n)Z06Bi 142Nd(d,dn)141Nd 2.5 hr Low yields 1 4 9 S m ( d , n ) 1 5Eu 0m 1 2 . 8 hr I 1 Eu(d,p ) I 5 2 mELI 96 min >O 0 Incident particle energy: 20 MeV p+; 20 MeV d+. b Yields corresponding t o the following conditions: irradiations of 5 min at 1 pA on pure targets (natural isotopic abundance) measured at t o on the nuclide listed. C Based on a 3-pA irradiation for 3 hr or 1 half-life of the product nuclide (whichever is shorter) and a count time of 1 half-life of the product nuclide. Detection limits were calculated using the "zero-blank" case of Currie (6).
>YJ
Table 11. Evaluation of Interfering Reactions Giving Long-Lived X-ray Emitting Nuclides Q-value, MeV
Reaction
Fe(p,n)' Go 69Ga(p,n)69Ge '5As(p,n)75Se 80Se(p,n)80m Br s5Rb(p,n)*5m Sr "M~(p,y)~~Tc 94Mo(p,211)~'Tc 'Mo( p,y ) 9 S T ~ 95Mo(p,n)95T~ '03Rh(p,p')103n1 Rh
'
Pd ( p,y ) I O Ag 04Pd(p,2n)'"Ag Ia4Pd(p,n)' O'Ag "'Ag(p,n)' '-Cd 1 I OCd (p ,n ) I 'In ' 'OCd(p,y ) I I 'In "Cd(p,n)l ' I In 1 "Cd( p,2n)' I 'In 1 3 mSn 1131n(p,n)l ' I 51n(p,p') I I s m Sn O2
I
' ' SSn(p,y)'I9Sb ' I9Sn(p,n)' I9Sb ' 'OS,( p, 2n)' I9Sb 2'Sb(p,n)'"Te 'Cs( p, n ) '' Ba 139La( p,n)' "Ce I
1
140Ce(p,2n)'39Pr I 4 2 Ce( p,n)' 2Pr
"'Pr(p,n)' 4iNd
' Eu(p,n )' Gd I 5 3 E ~ ( p ,) p I s3mEu ' ' Gd ( p,n )' 5Tb 51
I
lS6Gd(p,n)'j6Tb IGaDy(p,n)lbom Ho '"Dy(p,2n)lbomHo 1 6 5 H o ( p , p n ) 1 6Ho 4m
-5.4 -3.0 -1.7 -2.7 -2.0 >O
Interference
58Ni(p,2pn)s6Co -'Ge(p,pnrgGe 76Se(p,pn)'Se *'Br(p,pn)*OmBr 86Sr(p,pn)85mSr 96Ru(p,aY3Tc
>O
-13.6 0 >O
-13.4 -5.1 -2.2 -4.7 >O
-2.8 -11.2 -1.8 0 >O
-1.4 -7.8 -2.1 -1.3 -1.1
-11.8 -1.7 -2.6 -1.2 0 -1.7 -3.2 -4.1 -10.5 -8.0 -1.2 -1.7 -9.5 -1.7
'--Hf(p,n)::iTa '-'Hf(p,n) Ta 1-9Hf(p,2n)'78Ta I 8 2 W( p,n ) I Re ' ,'W( p, 2n)' 82Re I9"0s(p,n)"'Ir
-1.9 -2. 7 -8.8 -3.7 -9.8 -3.2
>O
Ru(p,2 ~ ) ~ j T c 9 8 R ~ ( p)'jTc ,a ~ ""'Pd( p,2p)' 0 3 Rh I O 5Pd(p, 2pn)' O 3 I n Rh '"6Pd(p,a)'03n' Rh 106Cd(p,~)103Ag
96
06Cd(p, 2pn )' '"Ag "Cd(p,2n)' "Cd "'Sn( p,2pn)l ' 'In I ' 31n(p,2pn)" 'In I I ZSn( p,2p)' I 'In "4Sn(p,a)" 'In 114Sn(p,pn)'13mSn I 6Sn(p,2p)lI 5 m Sn I I 4Cd(p,? ) I 5 m Sn "'Sb(p,p2n)' I9Sb 1 Te( p, 2p ) I I 9Sb I "Te( p , a ) ' I9Sb I Te( p,pn ) ' I Te ' 34Ba(p,pn)'"l Ba I4'Ce(p,pn)' 39Ce I4'Pr(p,2p)'"Ce 39pY 141Pr(p,p2n)l I 43Nd( p,2p )' 42Pr '44Nd(p,2pn)142Pr 1 45Nd( p,a ) I 42Pr 14zNd(p,pn)141Nd ' 52Gd(p,pn)'51Gd 5 4 G d ( p , 2 p ) ' s 3Eu m 1 56Dy(p,2p)155Tb ' 'Dy(p,2pn )' 5 6 Tb ' 62Er(p,2pn)' ' O m Ho I
I
' 63Dy(p,y
432
Ir
-19.5 -1 0.7 -10.4 -9.4 -11.2
O
-7.6
7.3
5
>O
-8.7 -15.8
-1
>O >O
O
-9.2
I O
'Cd (d,a n
)I O
Ag
' I9Sn(d,n)'ZoSb ' "Sn(d, 2n)l *"Sb ' 22Te(d,a)l *"Sb
Q-value, MeV
-8.3 -15.0 -7.9 -8.3
>O 6.76 -5.0
25
-5.6
35
>O
>O
obtained by t h e comparator method using pellets with known contents as standards. Several elements t h a t were expected t o be present (i.e., Rh, P d ) could not be detected, because of the short half-lives of their product nuclides which decayed away before t h e sample could be counted. Spectra from this sample taken 1 hr, 12 hr, and 60 hr after irradiation are shown in Figures 2 A , B , a n d C, respectively. T h e sample was irradiated for 15 min at 0.5 PA. All count times were 20 min. T h e very high count rates obtained under these conditions from t h e nuclides of interest (typically 500 cpm at t o for concentrations between 300 and 400 ppm) show t h e potential sensitivity of this technique. Analysis of SRM 612. T h e results of t h e analyses of S R M 612 are given in Table IV. Once again, there is good agreement between experimentally determined concentrations and the known concentrations. Twenty-six elements were detected following a 30-min irradiation a t 0.5 PA. T h e longer irradiation time and higher beam intensity needed for this sample produced more matrix activity. As a result, this sample could not be counted until 4 to 6 hr after irradiation. Typical count time for a sample was 300 min. Analysis of S R M 614. T h e results of the analysis of S R M 614 are given in Table V. In this case, only seven elements were detected. T h e low levels of t h e trace constituents in this sample required long irradiations (1-2 hr) a t high beam intensities (1-2 PA). Thus, even 2 to 3 days after irradiation, the background remained high (typically 150 cpm per channel). This long lasting high level background prevented detection of a number of trace elements which intrinsically should be detectable at t h e 1-ppm level with t h e irradiation conditions used. Errors. T h e elemental concentrations given in Tables I11 and IV represent the averages for t h e number of determinations specified on S R M 610 and S R M 612. Included in these data are t h e standard deviations based on t h e individual determinations. Data for S R M 614 (Table V) are from a single determination. Among the factors affecting the precision of t h e technique are t h e relative count rates, the presence or absence of adjacent x-ray or y-ray peaks a n d t h e intensity of the background radiation. On samples S R M 610 and S R M 612, a relative precision of -5 to -15% was achieved. T h e accuracy of the method is illustrated by t h e good
1
3-5 10
O
Ba( d,dp )' 3 4 m Cs -8.5 36Ba(d.a)134m Cs >O >O I "Ce(d;aIi)' 33La ' 3 2 Ba( d,n )' 33La >O ' 'ODy(d,n)' d l Ho >0 ' 62Er(d,dp ) I 'I Ho -6.4 ' '4Er(d,o n )' '' Ho >0 I 63Dy( d,n)' "Ho >O I 6sHo( d,dn)Id4Ho -8.0 '66Er(d.a 6 4 H ~ >O Er (d;a n) ' ' Ho >O * 0 3 T l ( d, 2n )* 03Pb -3.8 204Pb(d,dn)2G3Pb -8.2 203Tl(d,n)2G4mPb >O 204Pb(d,d')204mPb >O Magnitude of interference is calculated as (for example) (cpm 56Co/gNi/cpm 56C0/gF e ) X 100. 133Cs(d,p)'34n' Cs
M a g n i t u d e of interferencea, %
15
