Activation yield curves of photonuclear reactions for multielement photon activation analysis

NUCLEAR

INSTRUMENTS

AND METHODS

157 ( 1 9 7 8 )

567-577 ; ©

NORTH-HOLLAND

P U B L I S H I N G CO.

ACTIVATION YIELD CURVES OF PHOTONUCLEAR REACTIONS FOR MULTIELEMENT PHOTON ACTIVATION ANALYSIS KAZUYOSHI MASUMOTO, TOYOAKI KATO* and NOBUO SUZUKI

Department of Chemistry, Faculty of Science, Tohoku University, Sendal, Japan Received 9 May 1978. The activation yield curves have been presented for a number of photonuclear reactions in the energy range from 30 to 68 MeV, in order to evaluate quantitatively the interferences due to competing reactions in multielement photon activation analysis. The general features of the yields as functions of both target mass number and excitation energy were elucidated from the data obtained, discussion being given on the results in terms of the reaction mechanism. Simultaneous neutron activation due to appreciable neutron production from the converter and surrounding materials has also been studied, and, finally, the magnitudes of interferences in real multielement analysis were given in the form of their energy dependences.

1. Introduction Activation analysis with high-energy gammaphotons has been exploited so far as one of the most useful analytical techniques for the determination of a number of elements in the periodic chart. The important developments are nondestructive multielement determinations in complex matrices of current interest such as geological, biological and environmental materials. Of the sources of photons, linear electron accelerators have preferably been selected for photon activation analysis. They are operated at relatively high currents, the energies of the beam being easily varied, and the sensitivities obtained for elements like oxygen, carbon, fluorine, nickel, and lead are in the range of micrograms or even nanogramsl-3). The reactions of the types (7, n) and (7, P) appear promising in the determination of most of the eleme,nts, and other possible reactions, such as (7, pn), (7, ~z) and (7, on), are less important from the point of view of their use in actual determinations. Nevertheless they have to be considered because their occurrence, especially when working at higher photon energies, often causes interference problems. The production of the radionuclide identical to that used for determination from two or more different target elements through different reaction paths should be taken into account. To evaluate the magnitudes of interference and to correct the abundance data for them, the quantitative estimations of the yields of the interference P~,esent address: Department of Chemistry, College of General Education, Tohoku University, Kawauchi, Sendai, Japan.

reactions are thus important in designing multielement work by photon activation analysis. In this work, these problems have been studied in the form of the activation yield curves which were obtained by absolute measurements of the induced activity in a given element as a function of electron energy ranging from 30 to 68 MeV. Considering our previous results for the analytical purposes4), selection of the elements was ruled in this work as follows. They were the elements leading to photon activation products which exhibit preferable nuclear properties for use of analysis, thereby making the method advantageous over competing methods with respect to sensitivity, accuracy and versatility. They were Na, Mg, Ti, V, Cr, Mn, Fe, Co, Zr, Y and Pb. The adjacent elements to the above were then given consideration for interference study. They were Mg, Na, AI, Ca, Mn, Fe, Ni, Zr, Nb, Mo and Hg. Additionally, five elements (C, Cu, Au, I and TI) were selected in order to elucidate the general features of the photonuclear reactions of various types. These totalled 22 elements covering the periodic chart. Attention has also been paid to simultaneous neutron activation, due to appreciable neutron production from the converter target and surrounding materials. Finally, the magnitudes of interferences in real multielement analysis were evaluated from the experimental data and were given in the form of the energy dependences of interferences. 2. Theoretical The photonuclear reaction yield, Y, can be derived by the following integral by using the brems-

568

K. MASUMOTO et al,

strahlung spectrum and the excitation function for the relevant reaction. !E ~rnax g = No a(k) ~'(k,~max) 1'-~ dk, (1) th

end of irradiation, N is the number of target atoms, I is the bremsstrahlung intensity in equiv. quanta/cm 2 s at the sample position, it is the decay constant, and t is the length of irradiation.

where NO is the Avogadro's number, Eth is the threshold energy for the reaction, Em~x is the bremsstrahlung maximum energy, ~(k) is the cross section in cm 2 per nucleus, and ~(k, Ema×)k I iS the number of photons at a given energy k. If the bremsstrahlung spectrum is normalized to 1 R as reported by Johns et al.5), a yield can be obtained as that per mol of target nucleus per roentgen. The numerical calculations can be made with the aid of the tables of the thick target bremsstrahlung spectrum, q~(k,Ema0, prepared by Penfold and Leiss6), to be proportional to the integrated angular cross section as given by SchiffT). When equivalent quanta, Q, are introduced, the yield per equivalent quanta for the given reaction, aQ, is given by Emax

3. Experimental Bremsstrahlung irradiations were conducted with the 300 MeV linear electron accelerator of Tohoku University. The machine was operated with the high-current accelerating section, the peak current being at least 100 mA with energies up to 68 MeV. The spread of electron energies was 3% from the selected value. The pulse repetition rate was 300 pps with a pulse width of 3/is. The electron beam produced bremsstrahlung in a platinum converter with a thickness of 2 ram. The dose rates were measured by the 64Cu activity from the 65Cu(?~,n)64Cu reaction in copper monitors, and found, in typical irradiation conditions, to be 7 . 0 x l 0 6 R / m i n ( O = 2 . 0 × 1 0 1 3 c m 2s ~) and 5.2×10VR/min ( Q = l . 0 × 1 0 1 4 c m - 2 s -1) at 30 .and 60 MeV, respectively. Samples of 22 elements, either in the form of metals or their oxides, with chemical purity of 99.9% or better, were used. Metallic samples were small discs 6 mm in diameter with a thickness of 0.1 ram. The powdered samples were individually wrapped in small pieces of aluminum foil and made into small discs 6 mm in diameter with a thickness of 2 ram. Accurately weighed discs of copper, 0.1 mm thick, were placed at the front and back of each sample. They were stacked in a silica tube and this unit was placed in a water-cooled sample holder on the bremsstrahlung beam axis directly behind the converter, for simultaneous irradiation. Gamma-ray spectra were measured with either a 33 cm 3 or 68 cm 3 Ge(Li) detector (Ortec Model 8101-0525 or Canberra Model 7200-7600-1423) and its associated electronics, coupled to a 4096channel pulse-height analyzer (Canberra Model 8100 or Toshiba USC-1 Model MS 201-000). The sample and copper monitor to be measured were separately sandwiched between 10 mm thick lucite plates to absorb positrons from any positron emitters, and they were counted at a fixed position from the active surface of the detector. Decay curve analyses were applied to the relevant photopeak areas to separate undesired activities. Nuclides were identified from a knowledge of the target nuclide, the gamma-ray spectra, decay data, data listed in the Table of Isotopes~°), and current literature data~l-~3).

o'(2 = Q - '

f

if(k) ~(k, Emax) k -1 dk,

(2)

'~(k, Eraax) dk.

(3)

Eth

where

Q = E~aJx

i

Emax

d Eth

In determining the yield values experimentally, a sample for the yield determination and a suitable monitor are simultaneously irradiated by the bremsstrahlung beam of a selected maximum energy. Each datum of a measurement reflects the relative yield of the monitor reaction and each of the photonuclear reactions induced in the sample to the bremsstrahlung beam. Hence, if the yield on the monitor is known, the absolute yield of each reaction can be deduced. The monitor reactions selected in this work were the (~,, n) reactions of 65Cu, 63Cu and ~2C. To obtain the yield curves for these reactions, the excitation functions for the 65Cu(~', n)64Cu and 63Cu()), n)62Cu reactions reported by Katz and Cameron8), and that for the 12C(y, n)~lC reaction by Barber et al. 9) were used. For a given photonuclear reaction, the yield was determined from the experimental data obtained by absolute measurement of the induced activity by the following equation, D y _ (4) NI ( 1 - e - ~ t ) ' where D is the disintegration rate in dps at the

569

A C T I V A T I O N YIELD CURVES TABLE 1

4. Results and discussion

Relative yields for the monitor reactions.

4.1. YIELD CURVES FOR THE MONITOR REACTIONS

The yields for the reactions 12C(},,n)I~C, 65Cu(7, n)64Cu and 63Cu(7, n)62Cu, calculated by eqs. (1) and (2), have been plotted as functions of bremsstrahlung maximum energy (Ema0 as shown in fig. 1. Numerical integrations have been applied over energy intervals of 20-70 MeV with a bin width of 1 MeV. The curves for the yields per tool per roentgen exhibit maxima around 20-30 MeV, and then drop gradually with increasing energy. This behavior corresponds to the drop in the energy flux of the incident beam required to produce 1 R as a function of the photon energyS). The yield per equivalent quanta for the 63Cu(),', n)62Cu reaction can be well extended to the yield curve determined by Masaike ~4) in the energy region above 200 MeV. Some numerical values from both the calculated yield curves and the experimental data are set out in table 1 in terms of the yield ratios at 30, 40 and 60 MeV. Fairly good agreement can be seen between calculated and experimental values for the 63Cu('y,n)62Cu to the 12C(7,n)~lC yMd ratios. The experimental ratios are also in good agreement with those obtained by Perlman and Friedlander~5), 15.2 at 50MeV and 14.4 at 100MeV, whereas the experimental values are somewhat lower than those calculated for the 65Cu(7, n)64Cu

to 63Cu(~, n)62Cu

Emax (MeV)

Yield ratio 65Cu(7 , n)64Cu/12C(y, n)11C

Calculated

Experimental

Average

42.2 32.8 29.6

49.8 4 1 . 1 43.0 25.5 23.5 30.2 19.1 22.0 19.7

44.6 26.4 20.3

30 40 60

63Cu (7, n) 62Cu/12 C(7, n) 1l C

Calculated

Experimental

Average

25.4 19.5 17.5

33.6 34.4 34.7 19.2 1 9 . 9 21.0 15.7 1 6 . 4 14.2

34.2 20.0 15.4

30 40 60

65Cu(y,n)64Cu/63Cu(7, n)O2Cu

Calculated 30 40 60

Experimental

1.7 1.7 1.7

1.5 1.3 1.2

1.2 1.4 1.4

1.3 1.3 1.3

yield ratios. T h e

experimental value of 1.3 is, however, in excellent agreement with the integrated cross section ratio obtained by Strauchl6), 1.30 up to 300 MeV. It was concluded that the excitation functions used for

//

6

O'Q=N T )0 2

'101

1.2 1.2 1.3

Average

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'"6

/

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I

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E max(MeV )

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(3)

101

~ ..... (3)

°

t Itll[I

,

102

:

10 °

It,,,

i

Jlil[

i

i

101

103

E max ( M e V )

Fig 1. Activation yield curves for the monitor reactions. (1) 65C'u(y,n)64Cu; (2) 63Cu(y,n)62Cu; (3) 12C(y,n)IIC.

i

: Z:Ta i

i

~lll

I

I

102

NT Fig. 2. Yield per equivalent quanta versus target neutron n u m

ber.

570

K. M A S U M O T O

the ]2C(7, n)llC and 6 3 C u ( 7 , n)62Cu reactions were more reliable than that for the 65Cu(7, n)64Cu reaction. In this work, therefore, the activation yield curve for the 6 3 C u ( 7 , n)62Cu reaction in fig. 1 was selected as a monitor yield curve, and the value of 1.3 for the 65Cu(7, n)64Cu t o 6 3 C u ( 7 , n)62Cu yield ratio was used for comparisons in the energy range covered in these experiments. In order to assess this procedure, the yields per equivalent quanta in mb obtained in this sequence at 60MeV for several (7, n) reactions were plotted against the number of neutrons in target nuclei as shown in fig. 2, together with those obtained by Napoli et al. 17) at 300 MeV. Both the present data and the literature data lie on a straight line. The empirical formula Q(mb)= NrL6, was derived, where NT denotes the number of neutrons in a target nucleus, when least-squares analysis was applied to the present data. It was found that this yield measurement was easier than the absolute method 14,~7) and gives a fairly good result. 4.2. ACTIVATIONYIELDCURVES All of the activation yield curves obtained in this work are given in figs. 3-8. In many cases, a radioactive end-product could be formed through

lo;

et al.

several different reaction paths in an irradiated element. The main reaction path was assumed by considering natural isotopic abundances, massthresholds, potential barrier heights and other nuclear effects. Some comments and discussion on individual yield curves are given separately with respect to mass region below. a) C, Na, Mg and AI (fig. 3) For low-Z nuclides, the (7, n) and (7, P) yields are of the same order of magnitude, and show similar energy dependences over the energy region investigated. The yields for the (7, pn) reactions are almost one order of magnitude lower than those for the (7, n) and'(7, p) reactions, and the yields for the (7, an) reactions appear to be lower by another order of magnitude in this mass region. Regarding the reactions leading to 24Na from 27A1, the (n, ~z) process gives a considerable contribution to the total yields of 24Na. Below a threshold energy for the reaction 27A1(7, 2pn)24Na ( - Q - = 31,45 MeV), the yield can be deduced by the 27Al(n, ~z)24Na reaction.

=

10z

=

1 01

o

,00

;0 °

E

E

A

16;

62

1~

| 30

I 40

I [ 50 60 E max (M eV)

q 70

Fig. 3. Activation yield curves for the reactions on C, Na, Mg and AI. [] 12C(7, n)1|C, • 12C(7,~'n)7Be, • 23Na(y,n)22Na, O 25Mg(7,p)24Na, • 24Mg(7,pn)22Na, A 27A1(7,~n)22Na, V 27A1 ~ 24Na.

11]3 30

40

I 5O Emax ( M e V )

I 60

7O

Fig. 4. Activation yield curves for the reactions on Ca, Ti and V. • 48Ca(7,n)47Ca, [] 44Ca(7,p)43K, • 46Ti(7,n)45Ti, 0 48Ti(7,p)47S¢, tD 49Ti(7,p)48Sc, A 51V(7 , ~)478c, • 5Iv(y, ~n)46Sc.

ACTIVATION YIELD CURVES

571

I0 2

Iol ~

~

~

,o--

I0 2

lo'

10°

E

A,

I(3¢

I()2

30

40

50 60 70 E max(Me'C) Fig. 5. Activation yield curves for the reactions on Cr, Mn and Fe. (3 52Cr(y,n) 51Cr, @ 5°Cr(7, pn)48V, ¢ 5°Cr(y, 2n)48Cr, 55Mn(y, n)SaMn, • 57Fe(~, p)56Mn, [] 54Fe(y, pn)-~2gMn, ~1 56Fe(7, pn)54Mn,~ 56Fe(y, om)SICr, ~ 54Fe(y,2n)52Fe. lo 2

10 ~

l°°~ o

b 16~

1():

I

I

I

I

30

40

50

60

E max(MeV)

Fig. 6. Activation yield Cu. 0 59C0(y, n)58C0, • V 58Ni(7, pn)56Co, • [] 65Cu(7, n)64Cu, II

curves for the reactions on Co, Ni and 59Co(y, 2n)57C0, A 58Ni(y, n)57Ni, 6ONi(7' pn)SSCo, • 58Ni(7' 2n)56Ni' 63Cu(7, 2n)61Cu, O 63Cu(7, ~'1)58C0.

30

40

50 E max ( M e V )

60

70

Fig. 7. Activation yield curves for the reactions on Y, Zr, Nb and Mo. , O 89y(y, n)SSy, • 90Zr(7 ' n)S9Zr, © 90Zr(7 ' pn)SSy, A 93Nb(7, n)92mNb, A 93Nb(7,~zn)88Y, • lOOMo(y' nj99Mo, <1~ 97Mo(y, p)96Nb, I~ 96Mo(y' p)95mNb,~. 94Mo(y' pn)92mNb' [] 92Mo(y, 2n)90Mo,v 94Mo(y, ~zn)89Zr.

b) Ca, 17 and V (fig. 4) The yield values for the reactions leading to 47Ca from 48Ca are high in comparison with those for the 46Ti(y, n)4STi reaction. They are the sum of the yields for the 48Ca(y,n)47Ca and 48Ca(7, p)47K~ 47Ca processes. The contribution of the latter to the total 47Ca yields could be estimated to be up to 30%, when considering a relative probability of forming (7, n) and (7, p) reactions in this mass region and nuclear data of the products. Although the parent 4SCa is magic with respect to both neutron and proton numbers, these high yields can be considered as a result of very low neutron binding energy due to a large excess of neutrons. The cross section for photoproton emission from a neutron-excess isotope is expected to be small. An example of this sort can be seen in the (y, p) reactions on titanium isotopes. The yield is higher for the (7, p) reaction on 48Ti than for that on 49Ti. The 46Sc activity was detected, but was not subjected to a yield calculation, because it was the sum of the 47Ti(y, p)46Sc and

K. MASUMOTO et al.

572 103~-

of excitation energy on the (~,, 2n) reactions for Co and Ni is not so remarkable as that for Cr and Fe in this energy region.

I0~

d) Zr, Nb and Mo (fig. 7) Over the energy range investigated, the yield increases gradually with increasing energy. The contribution of the direct photoproton emission mechanism will become important at high energies, thereby resulting in the enhancement of the yield values. Several of the other complex reactions with the emission of more particles, up to 5, were observed. To the yield of the-94Mo(~,, ~n)ggzr reaction, the (n, ~z) and (7, 2pn) processes on 92Mo can contribute. In addition, fed-in yields from the decay of 89Mo which would be produced by the (~,p2n) and (7,3n) reactions are the possible source of the yields, thereby causing the apparently high yield values.

17/

L

I

30

40

~_ 50

I

,

60

70

E max(M~V) Fig. 8. Activation yield curves for the reactions on Pb, TI and Hg.

[3 2°4Hg(?, n)2°3Hg, ~1 |98Hg(~, n)lgvmHg, V 2°4pb(7, n)2°3pb, A 2°4pb(7, 3n) z°l Pb, 2°3T1(7, 2n)201TI,

O 198Hg(7, n)197gHg, <1~ 199Hg(7, p)I98Au; • 2°4pb(?, 2n)2°2mpb, <3 2°3T1(7, n)2°2Tl, • 2°3T1(7, 3n)200Tl.

48Ti(~,, pn)46Sc reactions. In the quantitative determination of titanium, these (y, p) reaction products can be used, but simultaneous productions of these nuclides from vanadium should also be taken into account. c) Cr, Mn, Fe, Co, Ni and Cu (figs. 5 and 6) Dependences of excitation energy and mass of target nucleus on the (?, n) yields are not remarkable, because the energy range of 30 MeV bremsstrahlung covers a substantial part of the excitation function of the (},, n) reaction for most nuclei in this mass region. When quantitative determinations of Mn and Cr are undertaken by utilizing (?,n) reactions, iron is a source of interference, since the (~, pn) and (~,, on) reactions compete in the production of 54Mn and 5~Cr, respectively. These interferences become significant at high energies. Almost exponential decrease in the yield values can be seen for the (},, n), (~', pn) and (?, an) reactions in this order. Because the threshold energies of the (y, 2n) reactions for Cr and Fe are higher than those for Co and Ni, the dependence

e) Hg, TI and Pb (fig. 8) In this heavier mass region, a general feature is the enhancement of the relative probability of forming more neutron deficient nuclides by the (7,xn) reactions. The reactions with the emission of charged particles are considerably suppressed as a result of the potential barrier restrictions, and, hence, these become insignificant. The (7,2n) yield values reach about 10% of those of the (7, n) reactions. The (7,3n) yields amount to about 0.5 and 2% of those of the (y,n) at 30 and 60MeV, respectively. The contribution of the 2°6pb(?, 3n)2°3pb reaction to the total 2°3pb production rate is thus significant, and amounts to about 43% at 60MeV. By the same reason, 2°4pb(7, 2n)2°2mPb and 2°4pb(7, 3n)2°lpb reaction yields were increased by the (?, 4n) and (y, 5n) reactions from 2°6pb, respectively. 4.3. GENERAL FEATURE OF PHOTONUCLEAR YIELDS In an attempt to elucidate the general feature of the yield values, the yields for a series of each of the (?, n), (},, p) and (y, pn) reactions have been given in the form of two-dimensional graphs in figs. 9-11. In these figures, smooth curves were drawn through the points giving the same magnitudes of the yield values as functions of bremsstrahlung maximum energy and target mass number. The yield "zero" means the calculated massthreshold. The yield values at 20 MeV were taken from the data reported by Oka et al.18,Jg), being converted into corresponding yield values per

573

ACTIVATION YIELD CURVES BO

80 5 10

50

100

t50

300

2 3 4

4

3

2

1

A

'•.0 E

W

20

20 -

--0 I

I

I

I

0

I 1 O0

Target

l

I

mass

l

I

I 200

I

n

,~

F

!

~

,

o

f

/~

/

.,II

2~ ~

1

]

'°f W -J7 ol 0

100 T a r g e t mass n u m b e r

I

I

Target

s J

I

number

~

6ol-tB= ~ l ~ / ' / J ~/

I 0

Fig. 9. Yields of the (7, n) reactions as a function of bremsstrahlung maximum energy and target mass number. The numerical values in the figure are yields per equivalent quanta in rob.

e°

~ 0 I 100

I

mass

I

I

I

I 200

number

Fig. 11. Yields of the (7, pn} reactions as a function of bremsstrahlung maximum energy and target mass number. The numericalvalues in the figure are yields per equivalent quanta in rob.

exhibit behavior similar to the (7, P) reactions, though the former have higher thresholds and lower yield values than the latter. They appear to be more energy dependent up to 20-40 MeV. The relative probability of forming a nuclide from adjacent elements in the periodic table is then considered. The yields of forming such a 10 ~

200

Fig. 10. Yields of the (7, P) reactions as a function of bremsstrahlung maximum energy and target mass number. The numerical values in the figure are yields per equivalent quanta in rob.

100

equivalent quanta. The (7, n) reactions take place in the 10MeV photon energy region, and their ~ lo' f ~ '~ yield values increase markedly with energies up to 30MeV'Theyieldsarestr°nglydependent°nthe ~ f max(MeV)~ mass number rather than on the excitation energy. E This tendency is especially remarkable for the -~f ~ 60.0 \ reactions on the targets with mass number up to lo ~- ~ 40.5 \ 50. The reactions of the type (7, p ) a n d (y, p n ) 3 I show quite different behavior. The curves for the (7, P) reactions show minima for target mass numbers lying between 50 and 60. In this mass region, ld therefore, the yields reach a m a x i m u m at a given ma:~imum energy, and then decrease with increas-

- ~ 3°'°1

o

1

~

] / /

[ /

2

d3Na) (251vlg)(27A[)

ing mass number, due to the difficulty of expel- Fig. 12. The ling', a proton from a high-Z nucleus because of differencein potential barrier restrictions. The (7, pn) reactions clides.

AZ

reaction yields leading to 22Na as a function of atomic number between target and product nu-

574

K. MASUMOTO et al.

fixed product are shown in fig. 12 in which the yields of the reactions leading to 22Na were plotted against differences in atomic number of the target and product nuclides. The exponential decrease in the yields versus the difference in atomic number is shown at each bremsstrahlung maximum energy. The gradient of the logarithm of the yield against the difference in atomic number increases as the excitation energy decreases. Similar results and their discussion have been reported earlier4). Despite the empirical relationship, these results may prove useful in a general consideration of the selection of an electron energy in multielement photon activation analysis. 4.4. NEUTRON-INDUCED REACTIONS The spatial distribution of bremsstrahlung photons behind the conversion target has been studied by Engelmann2°). The bremsstrahlung photons are more closely concentrated in the forward direction along the beam axis. The distribution of neutrons produced in the target assembly may be described in terms of transverse and longitudinal activation gradients. Such problems have been demonstrated Distance 1.0

g g

~.

1 i l ~

2

the converter

0

3

I |0l , ~ i

0.5

0.5!

03 --

O.I

0.05

from

( cm

5

~

I

)

15

'

''

'(('

I

i

i I

0.05

0.

Verticals.,

%, 1

1.0

2

beam axis .,.

Lateral

3

O

,.o_t~,~.

5

, i i i , 1(~,

15

i , , t

0.5

g 03 "1o o 0.05 ix,

--

0.1

0.05

Fig. 13. Production rates of the neutron reactions and the photonuclear reactions as a function of distance from the converter in vertical and lateral directions. © 55Mn(n, ~,)56Mn, • 23Na(n, y)24Na, A 27Al(n, ~z)24Na, • 55Mn(y, n)54Mn, [] 23Na(7, n)22Na, t 65Cu(?, n)64Cu.

TABLE 2 The reactions used for determination and interference reactions. Element

Analytical reaction

Na

23Na(y, n)22Na

Mg

25Mg(y,p)24Na 48Ti(y, p)47Sc

Ti

Major competing reaction Photonuclear Neutron reaction reaction 24Mg(y, pn)22Na 27AI(y, ~zn)22 Na ?7Al(y, 2pn)24Na 23Na(n, 7)24Na 51V(y, ~z)47Sc 48t. [(~, n)

Cr Mn Fe Co y Zr Nb Hg Pb

-1 47Ca3 47Sc

52Cr(y, n)Sl Cr 55Mn(),,n)54Mn 57Fe(7, p)56Mn 59C0(7 ' n)58C0

56FeC<~zn)51Cr 54Fe(n, ~)5]Cr 56Fe(~, pn)54Mn 54Fe(n, p)54Mn 59C0(7,2pn)56Mn 55Mn(n, ;~)56Mn 60Ni(? ' pn)58Co 58Ni(n, p)58C0 63Cu(y, ~.n)58Co 89y(y, n)88y 90Zr(y ' pn)88y 93Nb(~, :zn)88Y 9°Zr(y,n)89Zr 94Mo()~,~n)89Zr 92Mo(n,~z)89Zr 93Nb(2, n)92mNb 94Mo(y, pn)92mNb 92Mo(n, p)92mNb 204Hg(7,n)2°3Hg 207pb(7,~z)2°3Hg 2°4pb(7. n)2°3Pb 2°9 Bi(7, p5n)2°3Bb

here in terms of relative amounts of (n, y) and (n, ~z) activation to those of (7, n) activation. Sample materials investigated consisted of foils of aluminum and copper, manganese dioxide and sodium carbonate. The preparation of each sample was described earlier. These samples were separately stacked behind the converter either along the beam axis or along the vertical direction, and irradiated simultaneously with a 30 MeV bremsstrahlung beam for 1 h. The initial decay rates of S6Mn, 24Na, =Na and 6 4 C u were determined from decay curve analyses of the specified gamma-ray photopeak areas and normalizations were applied to them for the sample weight. They were plotted against the distance from the converter as shown in fig. 13, in which the initial decay rate of a sample placed immediately behind the converter was taken as unity. The production rates of both SSMn(n, y)56Mn and 23Na(n, 7)24Na reactions show a similar tendency in transverse and longitudinal gradients. No (Y, n) activation products could be detected in the samples at vertical positions. It can be said, therefore, that the photon-producing converter is also a main source of neutrons and that the neutron flux is less directional than the bremsstrahlung photons. The transverse decrease in neutron fluxes is less serious than that in bremsstrahlung photons. A significant variation in

A C T I V A T I O N YIELD CURVES

,¢

575

104

0

103

.o

1o~

~z~ 102 m

,° E i

101

IC

101 I

--

I 30

AO

I 50

I 60

~oOI

I 70

--o

~

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I

I

I

I

30

40

50

60

70

E max ( M eV ) Fig,,. 14. Energy dependence of interference (1). See text for definition. © AI/Mg(24Na), @ AI/Na(22Na), ~ Mg/Na(22Na), & Ca/Ti(47Sc), • V/Ti(47Sc), [] Fe/Mn(54Mn), • Fe/Cr(51Cr).

E max (MeV) Fig. 15. Energy dependence of interference (2). See text for definition. • Cu/Co(58Co), O Ni/Co(58Co), ~ Mo/Zr(89Zr), [] Mo/Nb(92rnNb), A Zr/y(88y), • Nb/y(88y), V Hg/Pb(2°3 Pb).

the relative rates of neutron-induced reactions to photonuclear reactions would result in a difference in interference which would cause a complex problem in real analysis, unless the neutron flux is monitored at a position very close to the sample. Similar experiments and analyses were made with 40, 50 and 60 MeV electrons, but the results were substantially the same as described with 30 MeV electrons.

4.5. INTERFERENCEIN MULTIELEMENTANALYSIS Pertinent nuclear data for the nuclides to be used for determinations are listed in table 2, along with interfering reactions yielding identical nuclides. Degrees and the relative significance of these interfering reactions are illustrated by the curves shown in figs. 14 and 15 as a function of bremsstrahlung maximum energy. The effects of interferences were expressed as a ratio of the

TABLE 3 Sensitivity and interference in multielement analysis of standard Orchard Leaf sample. Element

Sensitivity

(~g) a

Abundance ratio b

Magnitude of interference (%) 30 MeV 40 MeV 60 MeV

k

Na

6.9

Mg Mn Cr Zr Pb Fe Mg

41 2.5 0.1 7.5 170 41

Mg/Na AI/Na AI/Mg Fe/Mn Fe/Cr Mo/Zr Hg/Pb Mn/Fe Na/Mg

= = = = = = = = =

75.6 4.99 0.0660 3.30 130 15.4 0.0034 0.303 0.0132

a Obtained with 30 MeV bremsstrahlung. See text for definition. b Abundance data were taken from refs. 21 and 22. c Due to neutron reaction.

51.9 0.38 0.031 2.0 0.76 0.073 0.069 9.77 c 5.9x 10 -5c

88.7 6.2 0.090 13.0 37.3 0.95 0.067

92.5 13.9 0.56 20.2 54.2 7.1 0.047

576

K. M A S U M O T O et al.

TABLE 4 Sensitivity and interference in multielement analysis of standard JB-1 rock sample. Element

Sensitivity f~g) a

Na

82

Mg Ti

37 3.8

Mn Cr Zr Nb Pb Y

5.2 13 1.1 0.5 1.0

Fe Mg

400 37

Abundance ratio b

Magnitude of interference (%) 30 MeV 40 MeV 60 MeV

Mg/Na = AI/Na AI/MgCa/Ti = V/Ti = Fe/MnFe/Cr = Mo/Zr = Mo/NbHg/Pb Zr/Y Nb/Y Mn/Fe Na/Mg =

2.83 0.28 0.78 12.2 0.12 31.1 0.96 0.00071 0.096 0.026 0.83 0.66 c 0.20 c

2.25 3.70 1.65 8.32 0.0266 54.1 155 0.15 1.5 0.0013 6.00 0,59 0.0185 0.445

17.7 4.69 2.2 10.8 0.13 71.1 41.3 0.0094 0.53 0.026 -

26.7 10.7 12.1 8.1 0.13 80.6 58.5 0.075 0.95 0.017 4.1 0.067

a Obtained with 30 MeV bremsstrahlung. See text for definition. b Abundance data were taken from ref. 23. c Due to neutron reaction.

weights of the elements to produce the same counted at the optimal time intervals after irraamounts of nuclide under consideration. For ex- diation. For most of the elements, these sensitiviample., AI/Mg (24Na) indicates the weight ratio of ties are reduced by one-third with 60 MeV bremsAI and Mg to produce the same activity of 24Na. strahlung with our accelerator. Severe interference To evaluate the effects, the data in figs. 3-8 were problems are found in the determinations of Na, used. Once these relationships are determined un- Mn and Cr in Orchard Leaves and Na, Ti, Mn and der the given experimental conditions, corrections Cr in JB-1. In such cases where the interfering required for a sample in real analysis can easily be contributions turn out to be large, careful corrections should be made by using monitor materials evaluated. Interfering contributions thus evaluated are irradiated under identical conditions to those used presented in tables 3 and 4 at three different for samples. In conclusion, the foregoing presentation may bremsstrahlung maximum energies, 30, 40 and 60 MeV, for two different matrices, a standard ref- prove useful in designing multielement activation erence biological material Orchard Leaves analysis experiments in which high-energy brems(NBS SRM-1571) and a standard basalt rock JB-1 strahlung photons are to be used. issued from the Geological Survey of Japan. The The authors would like to express their appreabundance data reported by LaFleur 21) and by Chattopadhyay and Jervis 22) for the Orchard • ciation to members of the linac machine and raLeaves and-by Ando et al. 23) for the JB-1 were dioisotope groups at the Institute of Nuclear used. The sensitivity data in tables 3 and 4 were Science, Tohoku University, for their kind cooperbased on the minimum detectable photopeak areas ation with the irradiations. found in the spectra of the standard samples. They were assumed to be the amounts of elements to give a full-energy peak area which corre- R e f e r e n c e s l) G. J. Lutz, Anal. Chem• 41 (1969) 424. sponded to 3a of the area under the peak of in2) Ch. Engelmann, J. Radioanal. Chem. 6 (1970) 399. terest, with 30 MeV bremsstrahlung activation for 3) T. Kato, J. Radioanal. Chem. 16 (1973) 307. periods of 2 h for Orchard Leaves and 5 h for the 4) T. Kato, K. Masumoto, N. Sato and N. Suzuki, J. RadioJB-1, 1 g of Orchard Leaves and 500 mg of JB-1, anal. Chem. 32 (1976) 51. 5) H. E. Johns, L. Katz, R. A. Douglas and R. N. H. Haslam, detection with a 68cm 3 Ge(Li) detector, and

ACTIVATION Phys. Rev. 80 (1950) 1062.

611 A. S. Penfold and J. E. Leiss, Analysis of photo cross-sections (Rept. Physics Res. Lab., Univ. Illinois, 1958).

7) L. I. Schiff, Phys. Rev. 83 (1951) 252. 8~ L. Katz and A. G. W. Cameron, Can. J. Phys. 29 (1955) 518. 9) W. C. Barber, W. D. George and D. D. Reagan, Phys. Rev. 98 (1955) 73. 10,; C. M. Lederer, J. M. Hollander and 1. Perlman, Table t?fisotopes, 6th ed. (Wiley, New York, 1967). i1,, p. Anderson, J. S. Hislop and D. R. Williams, Rept. U. K. At. Energy Auth. AERE-R 7823 (1974). 12), M. E. Toms, J. Radioanal. Chem. 20 (1974) 177. 13) T. Kato, Res. Rept. Lab. Nucl. Sci., Tohoku Univ. 5 (1972) p. 137. 14) A. Masaike, J. Phys. Soc. Japan 19 (1964) 427.

YIELD CURVES

577

15) M. L. Perlman and G. Friedlander, Phys. Rev. 78 (1950) 86A. 16) K. Strauch, Phys. Rev. 81 (1951) 973. 17) V. di Napoli, F. Salvetti, M. L. Terranova, H. G. de Garvalho, J. B. Martins and O. A. P. Tavares, Gazz. Chim. Ital. 105 (1975) 1434. 18) y. Oka, T. Kato, K. Nomura and T. Saito, Bull. Chem. Soc. Japan 40 (1967) 575. 19) y. Oka, T. Kato, K. Nomura, T. Saito and H. T. Tsai, Bull. Chem. Soc. Japan 41 (1968) 380. 2o) Ch. Engelmann, Nucl. Instr. and Meth. 93 (1971) 197. 21) p. D. LaFleur, J. Radioanal. Chem. 19 (1974) 227. 22) A. Chattopadhyay and R. E. Jervis, Anal. Chem. 46 (1974) 1630. 23) A. Ando, H. Kurasawa, T. Ohmori and E. Takeda, Geochem. J. 8 (1974) 175.