factors in the amount of trailing material. The relative percentages of the adsorbed substances may indicate the cause of this adsorption. The percentage of trailing material for sodium gluconate is considerably greater than the percentages for the other substances, using both the solvent mixture and water. The same is true as to the amount of material which could not be eluted from the strip. This cannot be due alone to the carboxylate anion (- COO-), since the percentages for sodium glucuronate are not nearly so large, and are about equal in value to the percentages for the sugars, glucose, and arabinose. A possible explanation is that of all the compounds used, only sodium gluconate forms no ring struc-
ture, b u t exists as an open chain. For this reason, i t may have greater freedom in cross bonding with the cellulose fibers. Isherwood and Jermyn (6) indicate that the configuration of the hydroxyl groups influences the movement of sugars in chromatography. Possibly the adsorption is due at least in part to hydrogen bonding. Further research, now in progress, may clarify the nature of this adsorption. LITERATURE CITED
(1) Bate-Smith, E. C., Westall, R. G., Biochim. et Biophys. Acta 4 , 429 (1950). (2) Burma, D. P., Banerjee, B., Sci. and Culture (Calcutta) 15, 363 (1950): C.A. 45, 3735b (1951). (3) Frush, H. L., National Bureau of Standards, private communication.
(4) Glueckauf, E., Trans. Faraday SOC.
51, 34 (1955). (5) Hordis, C . K., Kowkabany, G. S . , ANAL.CHEM.30, 1210 (1958). (6) Isherwood. F. A.. Jermvn.' AI. A.. ' Biochem. J.'48, 515 11951)." (7) Kowkabany, G. K., Advances in Carbohydrate Chem. 9, 312 (1954). (8) Martin, A. J. P., Synge, R. L. M., Biochem. J . 35, 1358 (1941). (9) Mayer, S. W., Tompkins, E(. R., J . Am. Chem. SOC.69, 2866 (1947). (10) Rockland, L. B., Dum, &I. S., Science 109, 539 (1949). (11) Said, A. S.,A.I.Ch.E. Journal 2 , 477 (1906). RECEIVED for review July 31, 1961. Accepted December 13, 1961. In part, Division of Carbohydrate Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961. Taken from the dissertation presented by R. A. Schwane in partial fulfillment of the requirements for the degree of doctor of philosophy, The Catholic University of .kmerica.
Activation Analysis for Oxygen and Other Elements by Helium-3-Induced Nuclear Reactions SAMUEL S. MARKOWITZ and JOHN D. MAHONY Lawrence Radiation Laboratory and Department o f Chemistry, Universify o f California, Berkeley, Calif.
b A simple, rapid activation analysis has been performed for oxygen in beryllium, thorium, and Mylar samples. The ultimate sensitivity is in fractions of a part per billion. Milligram amounts of total sample are sufficient. The method can be used to analyze for other elements and for specific nuclides. More than one element can b e determined in the same activation. The sample need not be destroyed. Because of the low binding energy of the He3 incident projectile, many He3 nuclear reactions are exoergic, permitting radioactive products to be formed with large cross section from a beam o f low kinetic energy. The absolute excitation function has been determined for the production of FIB from 0l6reaction with He3 ions of kinetic energy up to 31 m.e.v. The induced radioactivity was measured b y calibrated end-window proportional counters and N a l scintillation spectroscopy. Brief bombardment of a 0.001-inch thorium foil with 0.01 pa. of He3 gave sufficient F1* radioactivity from 0 l 6 impurity to distinguish easily the 0.51 -m.e.v. annihilation radiation from the nuclear y-rays emitted b y thorium and its daughters. Methods for eliminating or minimizing interferences are discussed. Nuclear reactions of lowenergy He3 ions with nuclides up to Ca4*, product half lives, and reaction Q values are listed.
A
and absolute method for detection of oxygen and other elements over the concentration range of 1 p.p.b. to 100% has been advanced. Only a few milligrams of total sample material are required. The key feature of this method is the use of He3 ions accelerated to comparatively low kinetic energies to induce nuclear reactions which give radioactive products. These products, which decay mainly by emission of positrons or negatrons, can be detected by conventional nuclear techniques; the radioactivities may also be distinguished by half lives. More than one element can be detected simultaneously in the same activation. This depends upon the half lives and decay schemes of the radioactivities induced in the given sample. Activation analysis in general has been described in the literature, and review articles are available (I, 9. 14). I n this paper, we discuss the unique possibilities involving the use of He3 ions and the specific analysis for osygen in various metals and compounds. Unique Behavior of He3 Ions. Because of the low binding energy of the He3 nucleus, namely 7.7 m.e.v., many of the simple nuclear reactions which i t undergoes are esoergic. For reaction has example, the OL6(He3,p)F18 a Q value of +2.0 m.e.1'. I n contrast, the 016(He4,p)F19reaction has a Q = -8.1 m.e.v. The binding energy of the SIMPLE, RAPID,
He4 nucleus is 28.3 m.e.1'. This means that, thermodynamically, a zerokinetic-energy He3 will produce a He3,p reaction in 0 1 6 if it penetrates the Coulomb barrier of the 016, whereas a He4 ion must have a t least 8.1-m.e.v. kinetic energy to just barely produce a He4,p product. To produce the same product, namely Fly, via 010(He4,d)F18 reaction requires a t least 16.3-m.e.v. He4 ions. The deuteron, Ivhich has a binding energy of 2.2 m.e.17.. is a commonly accelerated ion which will produce exoergic d , p reactions and some exoergic d,n reactions. The high neutron background surrounding deuteron accelerators, holvever, induces n , y reactions that give the same product as d,p reactions, thus complicating the analysis. I n contrast, no such complication is present with the use of He3. Also d , p or n , y products usually decay by negatron emission or by electron capture, thus preventing convenient detection of annihilation radiation which results from positron decay. Proton reactions are usually endoergic; kinetic energies of a t least 5 m.e.v. are required to make p,n reactions, the exact energy depending on the specific target and product. I n most cases, low-energy protons induce only p,n reactions, thus sharply limiting the number of possible products t h a t can be detected. VOL. 34, NO. 3, MARCH 1962
329
Table I.
Nuclear Reactions of He3 Ions of Kinetic Energy Up to 8 M.E.V.
Reaction He3, d He3, H3 He3, n He3, 2n He3, d He3, H3 He3, n He3, Ha He3, n He3, d He3, a He3, 2 a He3, n He3, 2n He3, 2p He3, H3 He3, d He3, H3 He3, a He3, n He3, He3, H He3, a n He3, n He3, P He3, d He3, a He3, 2 a He3, n He3, 2n He3, d He3, H3 He3, a n He3, 2n He3, P He3, 2p3 He3, H He3, a p He3, n He3, 2p He3, H3 He3, a He3, a n He3, P He3, d He3, a He3, 201 He3, a p He3, n He3, d He3, H3 He3, a n He3, 2n He3, P He3, 2p He3, H3 He3, a p He3, n He3, 2p He3, H3 He3, a He3, 2a He3, a n HeS, n HeS, P He3, d He3, a He*, 2 a He8, a p He3, n He', 2n He3, d
Half Life
of Product 53.6 d 53.6 d 20.4 m 20.4 m 20.4 m 19.1 5 10.1 m 20.4 m 72.1 s 10.1 m 20.4 m 53.6 d 124 a 72.1 s 5568 y 10.1 m 124 e 72.1 E 10.1 m 66 s 7.4 8 124 B 10.1 m 1.6 8 110 m 66 8 124 s 20.4 m 17.7 s 1.6 8 110 m 66 a 124 8 17.7 s 10.7 8 29.4 B 110 m 7.4 8 22.8 s 10.7 s 17.7 s 110 m 66 8 2.58 y 22.8 s 17.7 8 124 s 110 m 11.9 s 2.58 y 22.8 8 17.7 B 11.9 9 15.0 h 40.2 8 2.58 y 10.7 s 7.6 s 15.0 h 11.9 s 2.58 y 110 m 22.8 s 1.7 s 8 x lo6 y, 6 . 7 E 7.6 8 11.9 8 17.7 a 2.58 y 4.1 s 1.7 8 8 X lo6 y, 6 . 7 8
Q Valuea of Reaction, (M.E.V.) +0.1 +0.9 +7.6 +0.7 +3.2 -3.6 +10.2 -2.0 -1.2 -5.8 $1.9 -5.7 $7.1 -6.1 f0.5
+0.5 +I .7 -5.2 f10.0 $5.0 -5.2 -2.8 -0.8 -3.0 f2.0 -4.9 $4.9
-5.3 fO.6 -7.1 +O.l -2.8
+0.8 -3.6 +6.9 -3.8 -1.7 -0.8 S7.6 -1.1 -3.3 +lO.l +1 .o $5.8 -3 . O +3.7 +0.3 -2.7 +6.6 +1.2 -3.5 -3 . O -3.8 +8.0
-2.5 -2.9 -2.9 $6.3 -0.8 -4.1 +8.2 -0.3 -3.4 +3.2 s 5 . 9 , +5.7 -3.2 +4.0 -5.6 -3.4 +6.1 -4.1 + 0 . 8 , +0.6
(Continued on page 331)
330
ANALYTICAL CHEMISTRY
The use of He3 as the incident particle is to be preferred because He3 usually produces neutron-deficient radionuclides with a high cross section. Such species usually are positron emitters, and detection of either the positrons or the resulting annihilation radiation (0.511-m.e.v. y-rays) is advantageous. For example, if the oxygen content of a radioactive sample (such as thorium, uranium, or plutonium) is desired, the 0.51 I-m.e.v. annihilation radiation from the 110-minute F18can be detected by scintillation counting and pulse-height analysis without appreciable interference from the other radiations in the sample. Because many of the He3 reaction products are so neutron-deficient and relatively short-lived, they quickly decay to form a radioactive daughter of longer half life, rather than a stable nuclide. The measured formation cross section is thus enhanced because there are two reactions leading to the same product. For example, O16(He3,n) produces 1.6-seconds Ne18 which decays t o 110-minute Fl*. The F18 is also directly produced by O16(He3,p) reaction. This enhancement cannot, in most cases, take place for low-energy proton, deuteron, or alpha irradiation. I n summary, the He3 ion offers distinct advantages as an incident particle for reasons based upon energetics, formation cross sections, versatility, and convenience of product detection. Possible Elements for Analysis with He3 Ions. Table I lists some simple nuclear reactions of He3ions with various nuclides. It is apparent from the many positive Q values (exoergic reactions) or relatively low negative Q values that analyses can be carried out for many elements. Specifically, the various isotopes of a n element that may be present in a given sample may be determined with a He3 beam of low kinetic energy. Each of these nuclides forms radioactive products, and detection of the radiations depends on product half life, interferences, and other factors. Solids, liquids, or gases may be analyzed. This analysis will not per se identify the chemical form of the unknon n-Le., ion, atom, compound etc.-for the analysis is for specific nuclei. Energy of the H e 3 Beam. The maximum cross section for a particular reaction depends on the He3 energy. Greater sensitivity is obtsined a t higher cross sections. The cross sections, which should be esperimentally determined, approximate within factors of two or three the geometric cross sections of the target nucleus for He3 energies a t the peak of the excitation function. If the He3 energy is low, only simple reactions take place and interferences are minimized. On the other hand, the energy must be high enough
to overcome any negative Q and the Coulomb barrier for the target nucleus, The Coulomb barrier restriction can, however, serve to advantage because no element heavier than calcium produces any interfering radioactivities. The fraction of the incident kinetic energy t h a t goes into center-of-mass motion of the products must also be considered, as this raises somewhat the effective threshold for reaction. For O16analysis, the excitation function as shown in Figure 1 indicates that the maximum cross section for F18 formation is a t EHe3= 7.5m.e.v. The nuclides and their reactions together with the product half lives listed in Table I are those which should undergo a nuclear reaction with a He3 ion of kinetic energy of about 8 m.e.v. The element of highest atomic number in Table I is calcium; a t this element, the reaction cross sections are relatively small because calcium corresponds t o the element a t which the Coulomb barrier for He3 ions is about 8 m.e.v. Table I is expected to serve as a convenient reference to determine possible interferences. Half lives that are about 1 second or greater are listed for those radioactivities which may be formed from 8-m.e.v. He3 reactions with nuclides up t o Ca48. At energies greater than 8 m.e.v., more complex reactions could of course be induced, and the number of possible elements for analysis would be increased. The arbitrary limitation of the energy to 8 m.e.v. was mainly for three reasons: a t this energy very many of the "simple" reactions would have appreciable, if not large, cross sections; the use of low-energy beams decreases the probability of producing a given radioactive product from extraneous reactions; He3 ions a t this relatively loir kinetic energy may be obtained by acceleration in a small cyclotron a t low power input and comparatively low cost. If a cyclotron were to be built specifically for He3 activation analysis a t 8 ni.e.v., H p would be 350 kilogausscm., which for a 15-kilogauss field gives 9.2 inches for the radius. Such a small cyclotron iyould allow analysis only for elements up to Z = 20 (calcium) because of Coulomb barrier restrictions on the cross sections. It nevertheless would be extremely useful. Depth of Penetration. An 8-m.e.v. He3 ion has a range of only 14 mg. per sq. cm. (approximately 0.002 inch) in aluminum and 31 mg. per sq. cm. (approximately 0.001 inch) in lead. Thus, the depth of penetration is a few mils, depending on the host or bulk material in a given sample. It is therefore possible to analyze surfaces for impurities such as oxygen, carbon, or others by controlling the energy of the incident beam by appropriate absorbers or by varying the cyclotron probe radius.
Target Nuclide Mg*6 Mg26 Mgz6 Mgzo Mg26 Mgf8 Mg26 A 2 8b
Al28
Al$8 Al26 Alp7
Ala7 Alp7
AI27 All'
Aln Si" Si" Si" Si28 Si" Si29 SizQ SiZQ Si29 Si a Si Si 30 Si81) Si" P3l
P3l P3l
Pal Pal Pal S32 S32
S32
Sa2 S32
sa* 533 533
5'8
S34 S34 S34 534
S"
s
88
S38 S" S36 S" S38 Cl" C1" C1" C1" C1S c135
C1" C1" ClS7 cia7 C137 Cla7 Cla7 Cln C1" C1" c1n C1"
Reaction He', H3 He3, a n He*, 2n He3, P He*, 2p He3, H3 He3, ap He3, d He3, a He3, 2 a He3, an He3, n He3, 2p He3, H3 He3, a He*, 2a He3, an P
He3, d He3, a He3, 2 ~ 1 HeS, cup He*, n He3, d HeS, H3 He3, a n He3, 2n He', P He3, 2p He3, H3 He3, up He3, n He3, 2p He3, Ha He3, a He3, 2a He*, an He', P He3, d He3, a He3, 201 He3, a p He*, n He8, d He3, H3 Hea, an He*, P He3, 2p He3, H3 He3, a p He3, 2n He3, P He*, 2 p He3, H3 He3, a He3, 2 a He3, a p He*, n He*, P He3, 2p He3, 3p He3, Ha He3, a He*, 2a He', a n He3, 2n He', P He*, 2p He*, H3 He3, a He3, 2 a He3, 3 a He*, a p HeS, a 2 p He', a2n
Table 1. (Continued) Half Life Product Nuclide of Product "6 7.6 a Mg'3 11.9 a 4.1 a Si27 Al28 2.3 m 9.5 m Mg27 4 2 8 , M2Sm 8 X 1 0 8 6~ . ~7 s 15.0 h Kaz4 Si27 4.1 5 Ala5
Na21
Al"
P29 A28
Sit7 A 1 2 6 Al28rn
N& A1 P" P29
Si27 Mgza
Al26, A1m s 3 1
P" P 2 9
Siz7 s 3 1 P32
si31 P" A128
Cl'* P32 S3l
P"
AP,AlPa9 C134,Clah C133 531
Sin P"
Ar" Cla4, C1*h Cl38 s 3 1
C1M
S"
C1s4, Cl*h P32 Ar37 C1" s37 c 1 3 6 36
S Si31
P34
I137 Ar37
Cl%
S"
Ar 36 c1*4,c i * b Pa cis'
K" Ar3Q
c1a
Arm
ClSS P 82 AIS SB P*4
cia"
~184,
7.6 s 22.8 8 2.1 8 4.5 8 2.3 m 4.1 a 8 X 106y, 6 . 7 5 2 . 5 8 ~ 7.6 s 2.6 m 4.5 8 4.1 5 11.9 8 8 X 108y, 6 . 7 5 2.7 s 2.6 m 4.5 5 4.1 s 2.7 s 14.2 d 2.6 h 2.6 m 2.3 m 2.8 s 14.2 d 2.7 s 2.6 m 8 X 108y, 6 . 7 s 4.5 E 3 2 . 0 m , 1 . 5s 2.8 s 2.7 s 4.1 9 2.6 m 1.8 8 32.0m,1.5s 2.8 a 2.7 s 3 . 1 x 105y 87 d 3 2 . 0 m, 1 . 5 s 14.2 d 35.0 d 37.3 m 5.0 m 3 . 1 X 1O6y d 87 2.6 h 12.4 s 1.2 s 35.0 d 3 . 1 X 106y 87 d 1.8 5 32.4m, 1 . 5 s 2.6 m 2.8 8 7 . 7 m, 1.0 s 265 y 1.0 8, 37.3 m 35.0 d 3.1 X 106y 14.2 d 2.3 m d 87 12.4 s 32.4m11 . 5 s
Q Values of Reaction, (M.E.V.) -4.3 -2.9 -5.0 +8.3 -1.3 -3.8, - 4 . 0 -2.6 t2.0 $9.2 $0.3 -7.6 +6.6 0.0 -4.8 +7.5 -1.8 -4.4 $6.3 -2.8 $3.4 -5.6 -4.8, - 5 . 0 $4.0 +0.1 -5.0 -5.1 -6.7 $7.5 -1.1 -4.3 -2.4 +3.4 $0.2 -5.5 +8.3 -2.9, -3.1 -3.6 +6.1, +6.0 -3.2 +5.5 -2.5 -0.9 $3.3 -0.3, - 0 . 4 -5.6 -2.6 $7.2 -0.7 -5.4, --5.6 -0.3 -0.9 $6.8 -3.3 -1.2 $10.7 +2.3 -3.6 $2.7 +9.6 +0.9 -7.1 -6.0 +8.0, $ 7 . 9 $1.2 -4.8 -4.2 +9.1 -1.6 -0.8 $10.3 $2.6 -7.3 +2.2 $0.3 -2.3, -2.5
(Continued on page 332)
VOL. 34, NO. 3, MARCH 1962
331
Table I. (Continued)
Target Nuclide Ar36 ArS6 Ar36 Ar36 Ar36 Ar36 Ar 38 Ars Ar 88
Ar@
Arm Ar 40 ArM Ar 40 Ar 40 Ara Ar" K39 K39 K39 K39 K39 K39 K39 K40 b K" K" K41 K41 K41 K41
K41 K41 Ca@ Ca" Cam Ca" Ca42 Ca42 Ca42 Ca42 Ca42 Ca43 Ca43 Ca43 Ca43 Ca43 Ca44 Ca44 Ca4' Ca" Ca44 Ca" Ca46 Ca46 Ca4B Ca46 Ca46 Ca46 Ca48 Ca48 Ca48 Ca48 Cae Ca48 Ca48 Ca48 Ca48
Reaction He3, P He3, 2 p He3, d He3, CY He3, 2 a He3, a p He3, 2p He3, H3 He3, He3, a p He3, 2n He3, P He3, 2p He3, a He3, 2a He3, 301 He3, a p He3, n He3, P He3, H3 He3, a He3, a p He3, a2p He3, a n He3, 2n He3, d He3, an He3, n He3, 2p He3, H8 He3, 2a He3, 3a: He3, ap He3, 2p He3, CY He3, 2a He3, a:p He3, n He3, P He3, d He3, a: He3, 2a: He3, n He3, 2n He3, d He3, H3 He3, an He3, 2n He3, P He3, 2p He3, H3 He3, 2 a He3, a p He3, P He3, 2p He3, d He3, H3 He3, He3, 201 HeS, a:p He3, P He3, 2p He3, d He3, H3 He3, a He3, 2a He3, a p HeS,a2n
Product Nuclide Ks
Ar n
K" Ars S3' C134, Clah Ar39 K 38
Ar 37 (3136
Ca41 K42
Ar41 Ar 39 SS
Si31 c 1 m Sc41 Ca41 Ca39 K" Arm
C1" K" SC" Ca41 KB Sc43 K4* Ca41 C1" P32
Ar 39 Ca41 Ca39 rlr"
KB Ti44
sc44, Sc4b Sc43 Ca41 Ar 37 Ti 45
Ti44 Sc44, Sc44m sc43 Ca41 Ti * Sc6, S c h Ca45 sc44, Sc4b Ar39 K42
Sc 48 Ca47 Sc47 SC", sc48" Cag Ar41 K44 Sc" Ca49 Sc49 Sc" Ca47 Ar43 KM
Cafi
Half Life of Product 7 . 7 m, 1 . 0 s 35.0 d 1.2 1.8 2.7
6 6
8
32.4 m, 1.5 s 265 y 7 . 7 m, 1 . 0 s 35.0 d 3 . 1 x 105y 1.1 x 105y 12.5 h 110 m 265 y 87 d 2.6 h 1 . 0 s, 3 7 . 3 m 0.9 8 1.1 x 1067 1.0 s " 7.7 m, 1 . 0 s 35.0 d 3 . 1 x 105y 1.2 8 0.9 8 1.1 x 105y 7.7 m, 1 . 0 s 3.9 h 12.5 h 1.1 x 106y 3 . 1 x 105y 14.2 d 265 y 1.1 x 106y 1.0 8 1.8 5 7 . 7 m, 1 . 0 8 103 y 3 . 9 h, 2 . 4 d 3.9 h 1 . 1 x 105y 35.0 d 3.1 h 103 y 3 . 9 h, 2 . 4 d 3.9 h 1 . 1 x 106y 3.1 h 83.9, 19.3 s 164 d 3 . 9 h, 2 . 4 d 265 y 12.5 h 44 h 4.7 d 3.4 d 83.9 d, 19.5 s 164 d 110 22.0 1.5 8.8 57 44 4.7 ? ? 164
m m
m m
m
h
d d
Q Value" of Reaction, (M.E.V.) $6.2 $1.1 -3.6 $5.3 -0.7 -1.3, -1.5
-1.1 -5.9. -6.2 +8.7 -0.1 -1.1 $7.6 -1.6 $10.7 $3.7 -4.4 0.0 +2.2 $9.0 -6.6 $7.5, +7.2 $2.2 -6.7 -5.2
-5.6 $3.4 -0.4, -0.7 $7.5 -0.2 -0.4 +3.8 -3.6 -2.6 $0.6 $4.8 -2.3 -0.7, -1.0 $6.0 $6.9, + 6 . 6 -0.6 $9.1 $2.5 $7.5 -2.0 $1.2, $0.9 -2.2 $1.1 -3.7 $7.9, $7.8 -0.3 -3.7, - 3 . 9 -3.9 -1.6 $9.0 -0.4 +3.0 -1.4, - 1 . 5 f10.2 0.0 -2.0 $8.5 -2.6 ' $4.1 $0.2 +10.6 -3.9 -5.3 -7.5
Reference (6). When reaction was not listed in Nuclear Data Tables, the Q values were calculated from masses and decay energies in ( 2 , 4, 7). * Because of the long half-life of the target nuclide, the activity produced by the He3 reaction may be considerably greater than that detected by direct counting of the same quantity of the nuclide.
332
ANALYTICAL CHEMISTRY
Samples can be made of such thickness so that either a "thin target" or "thick target" cross section is determined. Any variation of cross section with energy (because the He3 ions lose energy as they traverse the target material) can be taken into account. The limited depth of penetration need not confine this method to surface analysis, because the sample itself can be treated (by slicing, cleaning, or etching) to bring the "interior" within range of the He3 beam. Calculation of Percentage Content. The absolute disintegration rate Do of a product radioisotope produced by a n accelerator bombardment of length 7 at constant intensity Z is given by
DO= n l u [ l - exp( -0 693 7/Tlr?)] ( I ) where disintegration rate of product a t end of bombardment, in disintegrations per minute = number of target atoms per TI sq. em. of the nuclide being determined Z = average beam intensity, in incident ions per minute u = cross section for the reaction, in sq. em. per target atoniincident ion 7 = length of bombardment, in minutes T1!2= half life of product, in minutes
Do
=
The intensity of charged particles in a n accelerated beam is usually measured in units of the beam current, in microamperes, striking the target. Beam current in microamperes is converted into He3(++)ions per minute by a factor based on the unit of negative or positive charge-Le., one positive charge = 1.602 X coulomb. Therefore, 1 pa. of He3(++) = 1.873 X 1014
He3(++)per minute The cross section u must be measured for each particular reaction as a function of incident ion energy if a n absolute analysis is desired. Such an excitation function for O*6(He3,p)F18 O16(He3,n)Se18-+F1*is shown in Figure 1. I n this case, the short-lived (1.6 second) Ye1* decays to form F18 so the sum of the production cross sections for each reaction is measured. The intensity Z is determined by means of a calibrated Faraday cup. The disintegration rate, Do, a t the end of the bombardment is determined by the relationship
+
where A. is the product activity, in counts per minute a t the end of the bombardment, and ODC is the over-all detection coefficient, defined as Ao/Do. The ODC therefore includes counter efficiency, geometry, decay scheme,
1000
I
1
I
10
20
30
I
i
0 He3
Figure 1.
energy
(Mev)
Excitation function for production of
F18
from Ole Ordinate represents sum of cross sections
OlB(He3,n)and
absorption, scattering, and any other counter factors. It is measured independently, or estimated, for each product nuclide and for each detection arrangement used. Because the analysis does not destroy the sample, the chemical yield is 100%. The quantity m, the mass per sq. em. of those target nuclei being determined, is given by m = nA/6.025 X l o z 3grams per sq. cm.
(3)
where A = the mass number. The quantity M , the total or bulk mass per sq. em. of the thin target, is obtained simply by weighing a known area of starting material. Finally, 100 m / J l = % by weight of the ‘‘unknown” nuclide. A relative analysis may be carried out by the simultaneous bombardment of some standard and a n unknown in the same beam. The product activities can be counted under identical conditions. This eliminates the need for determination of absolute cross sections, beam intensities, and over-all detection coefficients. Small corrections may be necessary to allow for different beam energies and the resulting cross-section variation between standard and unknown. The beam intensities, if two successive irradiations are performed, can be relatively monitored. From Equations 1 and 2, the following simple relationship can be derived provided I , u, ODC,and 7 are the same for both standard and unknown: 4 s ) = n(std) X Ao(r)/Ao(std) (4)
0l6 (He3,p) reaction
Here n(std) is kno\vn; the ratio of product activities is to be measured. Other techniques of bombardment and product detection, such as side-by-side foil activation and simultaneous counting of the radiations from standard and unknown, can lead to convenient simplifications. Sensitivity. The sensitivity depends on t h e lowest activity t h a t can be dletected above background, the beani intensity, t h e reaction cross section, t h e length of bombardment, a n d t h e half life of the detected product (see Equation 1). If t h e length of bombardment is many times the half life (“saturation bombardment”), then 1 - exp( -0.6937/T1iz) approaches unity, and the sensitivity can be considered independent of the prodiict half life. For most of the examples in Table I, the half lives are sufficiently short so that such a n approximation can be met, if really necessary, in practice. If positron detection is carried out, for example, with conventional proportional-counter instrumentation, a disintegration rate Do of 10 disintegrations per minute can be determined. If the cross section for production of the radioactivity is 400 nnillibarns (that is, 400 X sq. em.), as it is for F18 from 0 ’ 6 at EHe3 = 7.5 m.e.v., and if a beam of only 1 pa. of He3 ions is used, then n = 1.34 X 1011 atoms 014per sq. em., and m = 3.55 X lo+ gram of 0l6per sq. em. Thus, in a 0.0005-inch A1 foil (3.55 mg. per sq. em.), for example, 1 X lo-’% by weight of 0l6or 1 p.p.b. is detected. The He3
cannot, of course, produce activity a t a depth greater than its range in the host material, so the example here is one for a “thin” target in which no beam particles are lost as the target is traversed. If the cross section varies with He3 energy over the thickness of the target foil, a n average cross section may be calculated from the excitation function and appropriate range-energy relationships. I n practice, internal cyclotron beam currents may reach 1000 pa., but few materials can withstand the resulting high temperatures without melting (1000 pa. of a 7.5-m.e.v. beam stopping in a substance deposits 7500 watts of power). If the area is small, the rate of heat transfer in many materials is too slow to prevent melting unless targetcooling techniques are used. The application of cooling techniques and the irradiation of a large area of foil will therefore permit the sensitivity to be extended by another order of magnitude (possibly orders, depending on the specific circumstances). (The internal circulating beam of alpha particles a t the Crocker 60-inch cyclotron a t Berkeley may reach 1000 pa. The external deflected beam is about 1000 pa., and subsequent collimation brings the intensity donn to approximately 25-pa. Targets can be water-cooled.) The sensitivity. therefore, is about 1 p.p.b. only if the beam is limited to 1 pa. of He3; a t 25 pa., 0.04 p.p.b. could be detected if the target were properly cooled. Interferences. PRODUCTION OF OBSERVEDRADIOACTIVITY FROM EsTRAKEOUS SOURCES. T h e cross sections of reactions listed in Table I vary with He3 energy. It should be possible, by a proper choice of He3 energy, to exclude some, if not all, of the unwanted contribution from extraneous sources to the radioactivity being observed. For example, it is desired to analyze for OI6 in aluminum by 016(He3,p)F1* reaction. The Q of the .4127(He3,3ct)F1S reaction is - 10.4 m.e.v. Therefore, this reaction cannot take place a t E H e 3 < 10.4 m.e.v. The peak in the OIfi cross section is a t 7 . 5 m.e.v. Bombardment a t this energy will not, hon-ever, exclude F’*production by any Sa23(He3,2a()F1* and F19(He3,a)F1Sreactions, whose Q values are -0.3 m.e.v. and + l O . l m.e.v., respectively. To correct for such F1* contributors, measurement of the above reaction cross sections may be necessary. It should be possible, in the same bombardment, to determine both the 0l6and F19content by two different reactions; the Olfi by (He3,p)F18and the F19by (He3,n) Nazi. The Na21has a 23-second half life, and it could be separated by decay-curve analysis from the F18 (110-minutes) and other very VOL. 34, NO. 3, MARCH 1962
333
short-lived activities produced from the 10,000 AlZ7by 7.5-m.e.v. He3 ions. PRODUCTIOS OF RADIOACTIVITIES WHICH DIFFERFROM DESIREDPRODUCT. I n some cases, impurities will produce radioactive products, which, while not identical to the desired product, decay with similar half lives or emit similar radiations thus complicating the detection techniques. I n these cases, various detection systems may be used to enhance the over-all detection coefficient of the desired radioactive product; decay-curve analysis may also be sufficient. Examples are: the use of gamma-ray pulse height analysis to “separate” (or simultaneously determine) two emitters of similar half life but differing gamma-ray energies, the detection of 20-minute C11 and 110minute Fl8, both positron emitters, by decay-curve resolution. These techI I I niques would not necessitate destruction 0 2 0 40 60 80 io0 of the irradiated sample. If the desired product half life is sufficiently long (at C h a n ne1 least a few minutes), a simple chemical Figure 2. Gamma-ray spectrum from thorium foil before and after He3activation. separation may suffice to remove unwanted radioactivities of different elePeak at 0.51 m.e.v. results from annihilation of positrons from F1* in source, which is a sandwich of thorium fail between two pieces of copper 577 mg. per sq. cm. thick. Duration of bombardment ments. I n this situation, the sample is was 14 minutes and beam current was only 0.010pa. usually dissolved and correction made for losses during separation-i.e., “chemical yield ” Keighboring radioisotopes in the light-mass region generally have sufficiently different half the energy of the beam and promote uniform foil. The carbon and hydrogen lives or radiations so that their resoluheat transfer from the Mylar. The did not interfere with the F18detection; tion presents no great problem. Mylar foils were protected from contam4 hours after bombardment, the 110Precision and Accuracy. An estiination with recoil F18 produced in the minute FU was the only nuclide demate of the precision, based upon aluminum [by A127(He3,3cu)F1sreaction tected, the 20-minute Cl1 formed by the general experience with counting techor from impurities] by placing a similar C1*(He3,a)Cl1 reaction having decayed niques, indicates t h a t t h e reproduciMylar foil between the target foil and to a very low level. bility should approach +5% or less the aluminum to serve as a recoil IRRADIATION. The length of the in straightfomard cases. T h e accucatcher. Any loss of F18by recoil out of bombardments varied between 10 and racy depends on whether relative or the Mylar itself was compensated by 25 minutes, and the average beam curabsolute determinations are carried out. recoil gain from the adjacent Mylar rents were about 0.010 to 0.100 pa. of (Knowledge of the excitation functions, foil. He3‘++’. These irradiation conditions beam energies and intensities, and overRADIOBCTIVITY DETECTIOS. After produced convenient amounts of FlSfor all detection coefficients is important bombardment, the positron-emission detection. for the absolute method.) Estimates of rate of the Mylar foils was measured The total charge received by the errors indicate that accuracies of f10% with end-window gas-flow proportional Faraday cup was measured by an inteshould be possible, the per cent errors counters. The proportional counters, grating electrometer. This electrombeing standard deviations. which used methane gas, had a backeter was calibrated by passing a known ground of about 8 to 9 c.p.m., and the current through it for a measured time stability and plateaus were checked just prior to bombardment. The curOXYGEN ANALYSIS with a Cl36 standard source. The samrent source was a thermally insulated ple foils were mounted on l/,sinch 1.019-volt standard cell and a 1.00 X aluminum cards with double-faced 108-ohm precision resistor. The conI n this section, the specific method Scotch tape and covered Kith 0.9 mg. ditions of intensity and scaling were the for OI6 analysis is described. The disper sq. em. of Videne, a clear plastic foil. same as during the actual bombardment. cussion applies, in many cases, to the They were counted at reproducible The Hilac accelerated singly charged general use of the method for other He3, which was stripped to form He3(++) geometries in a conventional shelf aselements. sembly. at 31 m.e.v. by passage of the beam Determination of Cross Sections. The F18half life in many samples was through a 0.00025-inch aluminum foil. TARGET.The cross section for forma109.7 minutes, which differs somewhat The beam of He3(++’ ions was then tion of 110-minute FI8 from Ole by from the value of 112 minutes listed in bent magnetically and allowed to (He3,p)F18 plus (He3,n)Se1s -+ F18 the Table of Isotopes ( I S ) . The disinimpinge on the target after collimation reactions was determined by bombardtegrations are 97yo by p+ emission, the through a 3/*-inch diameter aperture. ment of 0.00025-inch (0.93 mg. per sq. remainder by electron capture (5). The maximum beam energy (8) was cm.) stacked foils of Mylar with 1The maximum positron energy is 0.65 31.2 m.e.v., and the full width of the to 31-m.e.v. He3 ions at the heavy-ion m.e.v. ( l a ) . energy distribution a t half maximum linear accelerator (Hilac). Mylar, a OVER-ALLDETECTION COEFFICIENT. was approximately 2%- The target polyester, contains 33.27, oxygen, The over-all detection coefficient, ODC, was water-cooled. Aluminum foils in 62.7y0 carbon, and 4.1y0 hydrogen. It was determined as follows: The positron the target stack were used to degrade is a convenient form of oxygen in a
4 I
334
ANALYTICAL CHEMISTRY
counting rate of a sample of F18 produced by the reaction of 0 I 6 with He3 in 0.00025-inch Mylar was determined with a 4 n proportional counter. The foil used was covered on both sides with a thin layer (approximately 40 pg. per sq. cm.) of aluminum to make it conducting. Activities induced in the aluminum were insignificant. The correction to the counting rate for self-absorption of positrons in tht’ Mylar was determined by an empirical absorption curve. This correction R‘XS 1.7y0. A correction obt,ained independently (IO)in a similar manner for negatrons in WIVS plastic was approximately 2% a t comparable thickness. The positron rate divided by 0.983 to correct for self-absorption and by 0.97, the p+ branching ratio, yields the absolute disintegration rate of the F” in the Mylar. This same source TWS then mounted in standard fashion and counted on various shelves of the end-window proportional counters. The observed counting rate (corrected for decay) a t the proportional counter divided by the absolute disintegration rate is equal to ODC. It was, for example, 0.127, on Shelf 4 (about 0 6 inch from a 1-inch diameter window). Geometry, backscattering, absorption, and decay scheme factors are thus included in ODC. EXCITATION FUNCTION. The cross sections were computed from Equations (1) and (2). The energy of the He* beam a t the appropriate foil position in the stack was determined from rangeenergy relations (3, 11). The range of a He3(++)ion of energy 3E was equal to 3/4 of the range of a proton of energy E. The excitation function is shown in Figure 1. Any resonances would presumably be smoothed out because the cross section represents the sum of two reactions, He3,n and He3,p, and because the energy spread of the beam was approximately 2% a t full energy. Oxygen Analysis in Thorium. Thin foils of thorium (0.001 inch) of thickness 35 mg. per sq. em. and 1-inch diameter were mechanically cleaned and mashed in a nitrogen atmosphere. They were then sealed, irradiated, and counted in a 0.001-inch polystyrene envelope, so that oxygen was excluded from the surroundings. The results of absolute analysis by He3 reactions t o give FIE were 0.61 f 0.06%. The thorium samples were analyzed a t the Lawrence Radiation Laboratory, Livermore, by the vacuum-fusion technique. Thorium was fused in a graphite cruci-
ble, and CO and COZ were measured. Their results, 0.35y0 by weight, differ from ours, O.Sl%, by more than the estimated experimental uncertainties. Determination of 110-minute F1*was accomplished by means of a calibrated proportional counter which detected the F1* positrons plus the negatrons from the natural radioactivities in the thorium (the constant negatron “background” was subtracted), and a 3 X 3inch KaI(T1) scintillation counter and 100-channel pulse-height analyzer which I o5
I
I
signal-to-background ratio could have been greatly enhanced. if desired, by a factor of about 100 or more. This illustrates the use of the method to detect 0l6 (and by extension other light nuclides) in a host material which is itself radioactive. Oxygen in Beryllium and Mylar. The oxygen content of 0.001-inch beryllium foil was found to be about 3%. (This analysis has not yet been checked by the vacuum-fusion technique because of present experimental difficulties with that method.) After the FIB excitation function had been established, a sample of Mylar was treated as an unknown. Absolute analysis gave a result of 337, oxygen, n-hich agreed with the known composition of Mylar within experimental error. ACKNOWLEDGMENT
The authors thank Jack Fraser, Lawrence Radiation Laboratory a t Livermore for analyzing the thorium samples for oxygen. LITERATURE CITED
1 l7CO
19CC 2l3C 23CC
v
33
93CC C 5 0 3
me 3‘ d a y
Figure 3.
Decay of full-energy peak (0.5 1 rn.e.v.1.
Area of that part of gamma-ray spectrum, corrected for Compton background from gamma radiation emitted b y thorium daughters, was determined as function of time. Half life corresponds to that of 1 10-minute F’*
were calibrated for annihilation radiation. The pulse-height spectrum is displayed in Figure 2. This indicates the 0.511-m.e.v. full-energy peak from F18 positron annihilation in the source is greater in intensity than the r-ray background from the thorium daughters a t about 0.51 m.e.v. by a factor of about 30, and the sample shown has decayed through 2.4 F1* half-lives (268 minutes after bombardment). The decay of the full-energy peak, which shows that the Bf are from F1*,is given in Figure 3. The duration of bombardment was only 14 minutes a t a beam intensity of merely 0.010 pa.; thus the
( 1 ) Atkins, D. H. F., Smales, A. .4., Advances in Inorg. Chem. Radiochem 1,315 (1959). (2) Atomic Energy of Canada, Ltd., Appendix, Atomic Energy of Canada, Ltd. Rept. CRP-690, March 1957. (3) Bichsel, H., Phys. Rev. 112, 1089 (1958). (4) Cameron, A. G. W., Can. J . Phys. 35, 1021 (1957). (5) Drever, R. W. P., Moljk, A., Scobie, J., Phil.Mag. 1,942 (1956). (6) Everling, F., Koenig, L. A., hlattauch, J. H. E., Wapstra, A. H., “Nuclear Data Tables. Part I.” U. S. Atomic Energy Comm., 1960. ‘
( 7 ) Friedlander, G., Kennedy, J., “Nuclear and Radiochemistry,” p. 415, Wiley, New York, 1955. (8) Hubbard, E. L., Baker, W. R., Ehlers. K. W.. et al., Rev. Sci. Instr.
32,621 (1961). ’ (9) Meinke, IT. W., ANAL. CHEV. 32, 104R (1960). (10) Pate, B. D., Yaffe, L., Can. ,J. Chem. 33,929 (1955). (11) Rich, M., Madey, R., University of
California Radiation Laboratory Rept.
UCRL-2301, March 1954. (12) Ruby, L., Richardson, J. R., Phys. Rev. 83,698 (1951). (13) Strominger, D. H., Hollander, J. hl., Seaborg, G. T., Rem Xodern Phys. 30,585 (1958). (14) Thompson, B. il., AEZAL. CHEM. 33,583 (1961).
RECEIVED for review December 7, 1961. Accepted January 8, 1962. Work done under the auspices of the U. S. Atomic Energy Commission.
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