Multi-reaction proton activation analysis for traces of molybdenum

Allalytica Cltimica Acta, 79 (1975) 161-173 Q Elsevior Scientific Publishing Company, Amsterdam

MULTI-REACTION OF MOLYBDENUM

PROTON

ACTIVATION

V. KRIVAN Max-Manck-Institut fiir Mctallforschung Stuttgart. Laboratorium fiir Reinststoffc. 7070 Sclawtibisch Republic of Germany) (Received 21st March 1975)

-

Printed

in The Netherlands

ANALYSIS

Institut Gmiind.

161

FOR TRACES

fiir Wcrlrstoffwissenschaften, Katharinenstrasse 17 (Federal

Molybdenum is a significant impurity in several high-purity materials such as special metals, e.g., niobium, hafnium, tantalum, rhenium and tungsten, and also sometimes - depending on the origin and preparation procedure in other metals such as iron, cobalt, copper, etc. ’ There exist many chemical and physical methods for the determination of molybdenum which have been excellently compiled by Elwell and Wood [ 1 J . Reviewing these methods one can see that, for different reasons, very few of them can detect molybdenum at the lower p.p.m. range and below in metallic matrices; the detection limit often depends very strongly on the matrix. For most of the above-mentioned metallic matrices, radiochemical neutron activation analysis is the most sensitive method, despite the fact that the crosssections of the principal reactions and/or the Y-ray intensities of the indicator radionuclides are rather low. The daughter technetium radionuclides 99mTc and iolTc formed by decay from the product radionuclides of the ‘sMo(n,r) 99Mo and ‘O’Mo(n,y) ‘O*Mo reactions have often been detected. A detection limit down to about 0.1 p.p.m. can be achieved for the determination of molybdenum in difficult matrices [2-51. However, neutron activation analysis is not very suitable for instrumental performance of the analysis. Even in matrices as suitable for instrumental neutron activation analysis as niobium and aluminium, experience has shown that molybdenum cannot be determined instrumentally at concentration levels much lower than 10 p.p.m. Recently, molybdenum was included in the instrumental multi-element proton activation analysis of tantalum [6] and niobium [7] . From these two applications it is evident that proton activation analysis is, generally, a very interesting technique for the determination of molybdenum. In the work described here, the application of all relevant proton-induced reactions to the determination of molybdenum was systematically investigated at proton energies of 12 MeV and 16 MeV. The following reactions were studied: 92Mo(p,n)g2Tc, 94Mo(p,n)94gTc -+ gsMo(p,2n)94sTc, 9sMo(p,n)95mTc + 96Mo(p,2n)9smTc, 9SMo(p,n)gSeTc + 9GMo(p,2n)9sgTc, 96Mo(p,n)9GgTc + 97Mo(p,2n)96gTc and looMo(p,2n) 9gmTc. In addition to a high sensitivity, an

162

advantage of this technique is that several principal reactions yielding indicator radionuc~ides with different half-lives can be used simultaneously for the determination. This is demonstrated by using the determination of molybdenum in cobalt as an example. EXPERIMENTAL

Targets

Thick targets of pure molybdenum were used for measuring the analytical sensitivities and for standardization. Analyses were performed on cobalt sarnples of VP grade (Materials Research GmbH, Eching by Munchen, West Germany). A pre-irradiation chemical etch of the molybdenum targets was accomplished with a nitric acid-hydrofluoric acid mixture (4:l) and the etch of samples with dilute (1 + 1) nitric acid.

Irradiation

Irradiations were performed with 1% and 16-MeV protons at the Karlsruhe isochronous cyclotron. For measuring the analytical sensitivities and for s~nd~dization, the beam currents were 5-200 nA of protons for 20-60 s. Cobalt samples for analysis were irradiated with 12”MeV protons in a watercooled target holder at beam intensities of 3-5 &A for 5-60 min.

Cow ting equipment

y-Rays were observed with a 30.cm3 or a 66-cm3 Ge(Li) detector coupled with a 4096-ch~nel Northern or Nuclear Data pulse-height analyser. The energy resolutions of the two detectors were 2.3 keV and 2.1 keV FWHM, respectively, at the 1..332-MeV photopeak of ‘OCo. The peak-to-Compton ratios were 26 : 1 and 29 : 1, respectively. The low-energy y-rays were counted with an Elscint germanium low-energy detector (200 mm2 area and 7 mm depth). The output signals from the detector were passed through an Elscint CA-N;l/lC preamplifier, a Canberra 1713 amplifier and a Canberra 1764 spectrum enhancer. The total system resolution was 495 eV at 122 keV. The detector was coupled to one of the two analysers.

Nuclear data used

Recent compilations and tables were used to obtain Q-values [3], threshold energies [9f, excitation functions [lOJ , isotopic abundance in natural elements [ 111, and the decay data [12,13]. RESULTS

AND DISCUSSION

Choice 0 f analytical reactions Preliminary considerations and experiments showed that, of the various charged-particle activation techniques, proton activation analysis is most suitable for the determination of molybdenum. Proton activation gives the

163

benefit of a great choice of analytical reactions yielding indicator radionuclides with half-lives varying between 4.4 min and 61 days. In a given case, either the most suitable reactions can be chosen for the determination, or several or even all possible reactions can be used simultaneously. The latter way may be important in checking the accuracy of the determination. Only deuteron activation would offer similar possibilities with respect to the choice of analytical reactions and their sensitivities. However, the deuteron activation technique is less suitable for instrumental analysis than proton activation, because of the greater probability of activation of the matrix elements. For example, an instrumental determination of molybdenum in cobalt, which matrix was used as an example of the feasibility of the proton activation technique, would not be possible owing to the high activation of cobalt by the sgCo(d,p)6oCo reaction. The same is true for many other matrix elements. If 3He and o-particles are used, similar activation can occur, in the case of cobalt by the sgCo(3He,2p)60Co and sgCo(ar,2pn)60 reactions, respectively. In general, the activation of elements by means of deuterons, jHe and a-particles is more complex than proton activation. Proton-induced nuclear reactions suitable for the determination of molybdenum are listed in Table I along with the pertinent nuclear data. In the energy range considered here, some of the indicator radionuclides can be produced via both the (p,n) and the (p,2n) reactions, because of the polyisotopic composition of the natural molybdenum. In these cases, the (p,n) type represents the main reaction while the contribution of the (p,2n) type to the production of the indicator radionuclide is generally much lower. Sensitivdty

The measured sensitivities, expressed as disintegrations per min per p.p.m. of molybdenum for a beam intensity of 1 /IA, are summarized for six protoninduced reactions and for proton energies of 12 MeV and 15 MeV in Table II; the values given are averages of two or three runs. However, these results are valid only for matrices with an atomic number close to that of molybdenum. In other cases, because of matrix effects, the sensitivities will differ somewhat from those given in the Table. Figure 1 shows the dependence of the correction factor for 12.MeV and 15-MeV proton energies. As has recently been shown [6,7,14-171, the most interesting energy region for multielement proton activation analysis is that between 11 MeV and 15 MeV. To provide information about the dependence of the sensitivity on proton energy, data were obtained for two different energies: 12 MeV and 15 MeV. Increasing the proton energy from 12 MeV to 15 MeV increases the sensitivities differently for the individual proton-induced reactions, as can be seen from Table II. The increase varies by a factor of from 2.2 for the MO + p + gsgTc reaction to 5.9 for the iooMo(p,2n) ggmTc reaction. In cases where the indicator radionuclide is produced by both (p,n) and (p,2n) reactions, as outlined above, the contribution of the latter to the increase of the sensitivity may vary from negligible to substantial depending on the appropriate Q-values, excitation functions and

164 TABLE I Production and properties of the indicator rndionuclides in activation of molybdenum nuclear (p,n) and (p,2n) reactions Nuclear reaction

Isotopic abundance (%)

“Mo(p,n)“Tc

16.84

8.8

9.04

16.72

“Mo(p,n)‘l*Tc YbMo(p,2n)95mTc

Half-life

Threshold energy (MeV)

Major r-rays WV)

Absolute intensity (%)

4.4 min

147.9 243.7 329.3 773.1 1509.6

55.0 15.0 78.0 97.0 100.0

5.1 12.5

52.0 min

870.9 1622.0 1868.8

91.0 5.4 5.3

9.04 16.72

5.1 12.6

4.88 h

702.6 849.7 870.9 916.2

100.0 100.0 100.0 6.8

16.72 16.53

2.6 11.8

61.0 d

203.9 682.1 786.2 835.1

80.3 44.2 12.0 36.1

16.72 16.53

2.6 11.8

20.0 h

765.8 1074.1

94.0 4.0

16.53 9.46

3.8 10.6

4.35 d

773.3 812.3 850.3 1127.2

100.0 82.0 99.0 15.0

9.63

7.8

6.02 h

140.5

85.0

via

TABLE II Analytical sensitivities of different proton-induced reactions on molybdenum Reactions

“Mo(p,n)“Tc M~+p-‘~nTc MO * p - 95mTc Mo+p-~~?I’c Mo*p““‘=Tc ‘““Mo(p,2n)PVmTc

Sensitivity, d.p.m./p.p.m.

uAa

Ep=12MeV

I&=lSMoV

5.87 - 10’ 7.03 10’ 12.5 2.06.10’ 4.73 10’ 1.12 - 103

1.23 3.39 30.2 4.60 1.17 6.67

l

l

- 10” * 10’ - 10’ - 10’ - lo3

a For an irradiation time of one half-life or u maximum of 2 h, at the end of the irradiation.

165

abundances for both reactions. Activation curves for the Mo(p,xn)g‘gTc and the Mo(p,xn) g4&T~reactions (Figs.2 and 3) illustrate these circumstances. From Table II, it is evident that the g2Mo(p,n)g2Tc reaction is the most sensitive of the listed reactions. However, the disadvantage of this reaction may be the short half-life of g2T~ (4.4 min). High sensitivities of the order of 1.50

125 g Y z+

1.00

Y 8 07:

0.54

I

I

1

20

LO

60

ATOWC

WMBER

OF TCL

I

80

MATRIX

loo

ELEUZNT

Fig.1. Sensitivity correction factors for the matrix effect denum as a function of the at,omic number of the matrix

10-

in proton activation of molyb)12 MeV.(_ _ -)15 elemcnt.(-

MeV.

lo-

0.6 -

0 5

10

15 PROTON

20 ENERGY I MeVl

25

XI

L5

10 PROTON

Fig.2.

Activation

curve for the Mo(p,.rn)q6‘?c

reactions.

Fig.3.

Activation

curve for the Mo(p,xn)‘“RTc

reactions.

15 ENERGY

20 IMCWV)

25

30

166

10’ d.p.m./p.p.m. @A, can be achieved by using the MO + p + g4gT~, Mo+p+ gsgTc, MO + p + g6&Tc, and 1ooMo(p,2n)ggmTc reactions. The halflives of the indicator radionuclides produced by these reactions vary between 4.88 h and 4.35 days. The g5Mo(p,n) g5mTc reaction gives the lowest sensitivity; under certain circumstances, however, e.g. if medium or high trace concentrations must be determined and the counting can be done only after a long decay time, this reaction may be of interest. Table III summarizes primary interfering reactions which are energetically possible with proton energy up to 15 MeV. With only one exception - in the *‘*Mo(p,2n) ggmTc reaction - they are nuclear reactions induced on ruthenium. If the molybdenum and ruthenium concentrations are equal, the interfering reaction yields much lower activities of the indicator radionuclide than the. principal reaction does. This may be caused by the very low cross-sections of the given interfering reactions up to proton energies of 16 MeV, and by the low isotopic abundance of the ruthenium nuclide in question. Ruthenium may also be determined via (p+n) reactions, with ggmRh, gggRh, looRh and l”lmRh as the indicator radionuclides /18] ; thus the presence of ruthenium as possible inte’rfering element in the determination of molybdenum can be checked easily and any necessary corrections can be made. If instrumental proton activation analysis is considered, attention must be paid to interferences arising from overlapping y-rays from other activation TABLE

III

Possible primary interfering reactions Principal reaction

Interfering reaction

Threshold enorgy (MeV)

Ikotopic abundance (%I

10.6

6.61

9.7 7.4

5.61 1.87

7.4 0.0 4.9 14.6

6.61 1.87 12.72 12.62

0.0 6.6 1345

12.72 12.62 17.07

9.3 16,2 0.0 12.1 3.1

12.62 17.07 31.61 18.68 100.0

167

products. For “BTc and “aTc, data on such interferences have already been summarized [7] . The interferences for all other indicator radionuclides were investigated and are listed in Table IV. For y-energies below 1 MeV, all those y-rays were considered whose energy differs from that of the analytical y-ray by up to f 5 keV; and for y-energies above 1 MeV, up to +Z7 keV. In the cases of the 147.9-keV y-ray of g2T~, the 203.9-keV y-ray of g5mT~, and the 140.4keV y-ray of ggmT~, counting with a low-energy detector is assumed. Consequently, y-rays differing in their energy up to 4 800 eV from the energy of the analytical y-ray were considered as interferences. The half-lives of the interfering radionuclides considered were limited to the interval between 1 min and 1000 times the half-life of the indicator radionuclide. A comparison of the individual indicator radionuclides and their Y-rays shows that the possibilities of avoiding instrumental interferences are very different. Apparently the most suitable for instrumental analysis are the 140.4-MeV T-ray of ggmT~ produced by the looMo(p,2n) reaction where only zirconium can interfere, and the 778.3-keV T-ray of gGsTc produced mainly by the “Mo(p,n) reaction where only selenium interferes. In both cases, the difference between the half-lives of the indicator radionuclide and of the interfering radionuclide is great enough to permit analysis of the decay curve. In the first case, the 140.5.keV analytical y-ray and the 141.2-keV interfering T-ray can be resolved by using a good low-energy detector and a suitable computer program for peak analysis. When any of the other y-rays are used for the determination, two or more elements can interfere. At any rate, it is absolutely necessary to check if the counted y-rays are free of interference; for this purpose, a combination of procedures may be used as discussed recently [7,12] . Analysis of samples The applicability of the analytical reactions listed in Table I to the determination of traces of molybdenum is shown by using the analysis of cobalt as an example. In 12-MeV proton activation of cobalt, practically no radioactivity is produced from the matrix so that the sample can be counted immediately after the end of the irradiation, i.e., any indicator radionuclide can be counted. Table V gives the results of analysis of VP-purity cobalt obtained by using six different analytical reactions. The results and the deviations given are average values of three determinations on replicate samples. Experimental examination ensured that there were no detectable nuclear interferences; the y-rays counted were carefully checked for instrumental interferences by following their decay curves to determine the half-lives, and, where possible, by examining the ratios of peak intensities of different y-rays of the given indicator radionuclide. Only those y-rays were considered which were in any case free of interference, or could be obtained so during the period of counting. Table V also shows the limits of detection obtained under optimal experimental conditions with regard to irradiation, decay and counting time;

TABLE IV Possible instrumental interferences in the determination of molybdenum by ~-ray spectrometry r-Ray measured (heV)

92Mo(p,n)9’Tc

Interferences r-Ray

Intensity

&eW

w

reaction (T,_ = 4.4 min) 40.0 147.9 - ‘146.7

329.3

773.1

Nuclide

18ZrnTa

Halflife

16.0 m 24.0 h 9.7 h

147.1 147.7

1.7 37.0

189&

148.9

15.0

“‘*Re

324.4 325.7

11.0 95.0

“Ru “‘Ta

326.6 327.6

4.4 ?

“‘Nd “‘In

1.8 h 58.0 m

328.5

60.9

19’Au

39.5 h

331.9

25.0

“*Ta

2.1 h

333.9

4.0

“‘Eu

12.8 h

333.9

70.0

“‘Pm

770.0

?

23.4 m

773.1 776.5

?

83.2

777.4

22.4

15.0 h 35.4 h 14.6 h

196mAu

64.0 h 2.88 d 2.1 h

2.68 h

Possible

kotopic

Threshold

formation

abundance

energy

“6W(p,crn) “‘Os(p,a) ‘q6Ft(p,n) “‘Au(p,pn) ‘99Hg(p,a) ‘r’W(p,n) ‘160s(p,an) 9*Ru(p,pn) “‘Hf(p,n) “‘W(p,an) “ONd(p,pn) “‘Cd(p,n) “‘Sn(p,an) ‘9iPt(p,n) ““H&w’) “rHf(p,n) ‘“‘W(p,an) ‘soSm(p,n) “‘Eu(p,pn) “‘Gd(p,an) ’ ‘ONd(p,n) “*Sm(p,n) 60Ni(p,n) 61Zn(p,ctn) ‘““C@‘n) “‘Se(p,n) r6Sr(p,n) ‘“Zr(p,en)

28.41 41.0 25.3 100.0 16.84 26.41 1.59 1.87 27.14 26.41 5.62 0.87 0.96 32.9

0.0 0.0 2.3 8.1 0.0 3.7 0.8 10.4 2.7 0.9 7.4 6.0 7.8 3.3 2.0 2.7 0.9 3.1

10.02

27.14 26.41 7.44 47.32 2.15 5.62 22.71 26.23 48.89 1.59 9.19 9.86 51.46

;:; 0.9 2.1 7.0 11.0 4.6 0.9 6.1 12.9

TABLE IV (continued) Y-Ray measured (keV)

1509.6

Interferences r-Ray WV)

Intensity (%

Nuclide

1502.8 1507.7

6.5 7.0

s4Y

1508.0

6.7

s9rna

1508.0 1509.0

25.0 ?

1511.9

2.6

“OLIJ

1514.9

4.0

zooTI

92Mo(p,n)9’mTc reaction (‘I’,,, = 52.0 min) 870.9 866.6 ? 870.5 3.2 870.9 100.0 871.5 872.0

12.3 9.5

872.5 873.8

5.8 5.2

9’Mo(p,n)9’grTc reaction (Ty, = 4.88 h) 702.6 698.6 28.0 700.6 2.4 701.7 100.0 703.1 4.6

1 I 6rnIu

9’mM0

‘OAS

Halflife 43.0 h 54.0 m

36.0 h 17.0 h 4.88 h

160Ho s9Ge

25.0 m 39.0 h

9’hf0

a2BC 129m

Ba

s3mFe 106rn Ag

Isotopic abundance

Threshold energy

*Wp,o)

0.56 7.58 8.58 51.46 100.0 100.0 15.84 20.52 0.87 3.03 0.18 23.13 1.45

7.8 1.3 3.8 12.1 3.7 5.6 12.8 7.1 11.2 4.2 1.7 3.3 1.3

94Mo(p,n) 9aRu(p,crn) 160Dy(p,n) 69Ga(p,n) ‘OGe(p,pn) “OBa(p,pn) 92Mo(p,pn)

67.88 0.19 9.04 1.87 2.29 60.4 20.52 0.10 15.84

12.4 10.3 8.8 7.4 3.7 3.1 11.7 10.5 12.8

*‘Se(p,n) “OBa(p,pn) “Fe(p,pn) ‘06Pd(p,n) “‘&(p,pn) “OCd(p,un)

9.19 0.10 5.82 27.33 51.35 12.39

0.9 10.5 13.6 3.8 9.6 6.7

” ‘Wp,n)

“‘Sn(p,an) 4.18 m “a(p,pn) “‘Y(w) 93Nb(p,an) 66.0 s 92Mo(p,pn) 52.0 m 70Wp,n) ‘%e(p,crn) 2.0 d “OWp,n) “‘Hf(p,an) 26.1 h 2ooH&p,n) ‘O’Pb(p,an)

“Ni 13’Ce 9’Tc

i29mg,

Possible formation

2.13 h 15.5 m

35.4 h 2.13 h 2.53 m 8.5 d

S’Ni(p,pn) ’ 3’Wp,pn)

w %

w 2

TABLE IV (continued) r-Ray measured

Interferences

(keV)

r-Ray

Intensity

NJ)

(%I

703.3

849.7

15.4

Nuclide

E’Y

Halflife 14.4 h

706.3

16.0

7.7 h

707.4

31.2

4.9

846.7

99.9

77.3 d

847.3

3.6

848.4

3.2

106m 42

“Mn

8.5 d

5.6 d

850.3

99.9

96Tc

4.35 d

852.7

3.6

“‘LU

8.3 d

Possible kormation

“6Sr(p,n)

9.86

9DZr(p,an) ‘66Er(p,n) “OYb(p,an) “OCd(p,n) “%n(p,an) 56Fe(p,n) *ONi(p,an) ‘06Pd(p,n) ““&(p,pn) “OCd(p,an) ‘Q(p,u) 56Fe(p,an) 96Mo(p,u)

51.46

99R~(~,d

870.gb

870.9

91.0

674.8

6.6

95Ma(p,n)9smTc reaction (‘Z”,,, = 61.0 d) 203.9” 202.3 21.8 202.8 58.2 203.4 5.3

9JmTc

52.0 m

‘85

94.0 d

OS

‘29mh “‘Xe ‘lZLu

2.13 h 36.4 d 6.7 d

“’ Wpd *7JHf(p,an) 9’Mo(p,n) 91Ru(p,an) ‘*Wp,pn) ‘*‘Re(p,n)

204.1 205.8

14.6 3.3

191h

155.0 d 74.2 d

33.41 3.03 12.39 0.66 91.66 26.23 27.30 51.35 12.39

‘*‘Hf(p,a) ‘92Wp,n) ‘93Wp,pn) ‘q51%(p,Q)

Threshold energy 6.1 12.9 3.8 2.1 7.4 4.8 5.6 11.8 3.8 9.6 6.7

83.76

5.6

91.66 16.53 12.72 14.31 0.18 9.04

13.3 3.8 0.0 2.7 1.7 5.1

1.87

1.4

1.59

8.3 1.8

37.07

’ 3oBa(wn) 0.10 ‘J71(p,n) 100.0 14.31 ’ ‘WP,n) ‘76Hf(p,(rn)

177mLU

Isotopic abundance

5.20 35.24 41.0 62.7 33.8

10.5 1.5 4.1 1.8 0.0 1.8 7.8 0.0

r-Ray measured (keV)

582.1

835.1

Interferences r-Ray (keV) 579.3

Intensity (W 14.0

579.4

2.8

580.6

4.8

582.0 584.0 584.4 585.9 831.8 832.4

9.0 6.6 ? 12.3 ?

833.6 833.6

2.8 5.9

833.9

76.6

833.9 834.8

100.0 99.9

1.3

Nuclide

100

Tl

Halflife

26.1 h

“BC s6Y

56.0 h

I IOh

4.9 h

166

Ir

“Br “OPm 06Y 129mg, %a

“As

72Ga “Mn

14.6 h

15.0 h 56.0 h 2.68 h 14.6 h 2.13 h 9.4 h 26.0 h 14.1 h 312.5 d

835.7

4.4

a6Y

14.6 h

839.8

4.6

“‘LU

8.3 d

67.0

‘“Nb

14.6 h

‘ooMo(p.2n)99mTc reaction (6.02 h) 141.2 140.5”

Possible formation

looWp,n) “‘Pb(p,crn) “Se(p,n) *Wp,n) ‘“Wp,en) “‘Cd(p,n) “‘Sn(ppn) ‘*60s(p,n) “Wp,n) “ONd(p,n) *%r(p,n) 9oZr(p.an) ““Bahw) ““znhw) ‘“Wwn) 27GdP,n)

‘%e(p,an) ‘Q(pw) “Wp,n) ‘*Mn(p,pn) “Pe(p,a) ‘Wp,n) 9oZr(p,an) “‘Yb(p,n) “‘Hf(p,a)

Isotopic abundance

Threshold energy

23.13 1.48

3.3 1.3

7.58

2.2

9.86 51.46 12.39 0.66 1.59 7.58 5.62 9.86 51.46 0.10 27.81 20.52 27.43 9.02 7.76 2.38 100.0 2.19 9.86 51.46 14.31 0.18

6.1 12.9 4.8 7.4 4.6 2.2 0.9 6.1 12.9 10.5 6.0 10.2 5.2 10.4 7.9 2.2

6.1 12.9 2.7 0.0

51.46 9.04

7.0 9.1

a Counting with a low-energy detector is assumed. b Interferences given for the 870.9-keV ~-ray of the 94mTcas indicator radionuclide must also be added.

10.4 0.0

z w

172 TABLE V Results of analysis of VP-purity cobalt for molybdenum induced reactions Analytical reaction P1Mo(p,n)9’Tc Mo+p-r9”rCTc Mo + p .-. 9’mTc MO + p -a vSeTc MO + p -b “6Brlrc ‘““M0(p,2n)~~mTc

Detection

Concentration determined

limit

(p.p.m.)

(P.P.rn.)

87.6 87.0 86.1 93.4 89.1 91.1

9.2 1.1 6.2 0.09 0.1 1.2

f 7.1 +- 4.2 * 6.9 * 3.8 + 3.9 f: 5.2

obtained by various proton-

these limits were calculated by reducing the peak intensities in the r-ray spectra of the cobalt sample to the minimal detectable values by means of the “working expressions” of Currie [19] . As can be seen, the limits of detection of the individual analytical reactions, even of those giving very similar absolute sensitivities (Table II), differ considerably. The high limit of detection of the g2Mo(p,n)g2Tc reaction is caused mainly by the high Compton background in the counting of 4.4-min g2T~; this background originates predominantly from 23.4-min “Cu produced by the boNi(p,n) reaction from the nickel present in the sample at a concentration of 570 p.p.m. Moreover, counting could be started only after 1.5-2 half-lives of g2Tc had elapsed after the end of the irradiation. Similarly, the presence of nickel and other impurities yielding relatively short-lived radionuclides, e.g. chromium, tin and zinc, are the reason for the relatively high limits of detection for the MO + p + g4sT~ and reactions. In the MO + p 4 gsmTc reaction, the limit of looMo(p,2n) ““Tc detection is determined mainly by the low saturation factor which arises from the long half-life of gsmTc (61 d). Thus, for cobalt samples of this grade of purity, the best sensitivity for the determination of molybdenum is achieved “gTc and the MO + p 4 gGgTc reactions. From Table II it is bytheMo+p-+ evident that all considered reactions give much better sensitivities if radiochemical separation of the indicator radionuclide is performed. Conclusions Molybdenum can be determined with high sensitivity via five proton-induced principal reactions. One of the important features of this multi-reaction activation technique is the possibility of checking the accuracy of the analysis with regard to nuclear and instrumental interferences as well as with regard to the depth distribution of molybdenum in the sample. Distribution studies are possible because the distribution of the indicator radionuclide activity with depth differs for the individual principal reactions.

173

SUMMARY

The application of proton activation analysis to the determination of molybdenum is described. Thick molybdenum targets were bombarded with 12-MeV and 15-MeV protons. The reactions studied were g2Mo(p,n)g2Tc, g4Mo(p,n)g4gTc + g5Mo(p,2n)g4gTc, g5Mo(p,n)g5mTc + gGMo(p,2n)gsmTc, gSMo(p,n)gsgTc + gGMo(p,2n)g5gTc, gGMo(p,n)gGgTc + g7Mo(p,2n)gGgTc and *O”Mo(p,2n) ““Tc. Except for the MO -i-p + gsmTc reaction, all these reactions give high analytical sensitivities. For 12-MeV protons and an irradiation time of one half-life or a maximum of 2 h, the sensitivities range from 5 lo2 to 6 lo3 d.p.m./p.p.m. PA, and for 15-MeV protons and the same irradiation conditions from lo3 to lo* d.p.m./p.p.m. MA. In addition to the high sensitivity, the great advantage of proton activation is that different principal reactions yield indicator radionuclides with half-lives between 4.4 min and 61 d. Simultaneous determinations by these reactions are of value for checking the accuracy. For each reaction, detailed data are given on nuclear and instrumental interferences. Analytical application of this multi-reaction proton activation analysis is illustrated by the instrumental determination of molybdenum in cobalt. l

l

REFERENCES 1 W.T. Elwell and D.F. Wood, Analytical Chemistry of Molybdenum and Tungsten, Pergamon, Oxford, 1971. 2 J. Stary, A. Zeman and J. RBfitika, Anal. Chim. Acta. 29 (1963) 103. 3 H. Grosse-Ruyken and H.G. Dogc, Talanta, 12 (1965) 73. 4 R.A. Nadkarni and B.C. Haldar, Telanta, 16 (1969) 116. 5 N.K. Baisha and R.B. Heslop, Anal. Chim. Acta. 50 (1970) 209. 6 V. Krivan. D.L. Swindle and E.A. Schweikort, Anal. Chem., 46 (1974) 1626. 7 V. Krivan, Anal. Chem., 47 (1975) 469. 8 K.A. Keller, H. Miinzel and J. Lange, in K.-H. Hellwege (Ed.), Q-values for Nuclear Reactions, Landolt-Bornstein, New Series, Group I, Vol. 5, Part a, Springer, Berlin, 1973. 9 K.A. Keller, J. Lange and H. Miinzel, in K.-H. Hellwege (Ed.), Estimation of Unknown Excitation Functions and Thick Target Yields for p, d, ‘He and (Y Reactions, LandoltBornstein, New Series, Group I, Vol. 5. Part c, Springer, Berlin, 1974. 10 K.A. Keller, J. Lange, H. Miinzel and G. Pfennig, in K.-H. Hellwege (Ed.), Excitation Functions for Charged-Particle Induced Nuclear Reactions, Landolt-Biirnstein, New Series, Group I, Vol. 5, Part b, Springer, Berlin, 1973. 11 W. Seelmann-Eggebcrt, 0. Pfennig and H. Miinzel, Chart of the Nuclides, Bundesministerium fiir Bildung und Forschung, Bonn, 3rd edn., 1968. 12 C.M. Lederer, J.M. Hollander and I. Perlman, Table of Isotopes, Wiley, New York, 6th edn., 1967. 13 G. Erdtmann and W. Soyka, Die Gamma-Linien der Radionuklide, Band 1-3, Jul1003.AC, Jiilich, West Germany, September 1973. 14 S.M. Kormali and E.A. Schweikcrt, J. Radioanal. Chem., 22 (1974) 139. 15 J.N. Barrandon, J.L. Debrun and A. Kohn, J. Radioanal. Chem., 16 (1973) 617. 16 N.H. Krasnov, Yu.G. Sevastyanov, 1.0. Konstantinov, V.G. Vinogradova and V.V. Malukhin, J. Radioanal. Chem., 16 (1973) 395. 17 V. Krivan, J. Radioanal. Chem., 26 (1975) 151. 18 To be published. 19 L.A. Currie. Anal. Chem., 40 (1968) 586.