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A study on proton induced X-ray analysis and its application to environmental samples
N U C L E A R INSTRUMENTS AND METHODS 142 ( 1 9 7 7 )
199-204 ; ©
N O R T t t - H O L L A N D P U B L I S t t l N G CO.
A STUDY ON PROTON INDUCED X-RAY ANALYSIS AND ITS APPLICATION TO ENVIRONMENTAl, SAMPLES T. SHIOKAWA, T. C. CHU, V R. NAVARRETE, H. KAJI, G. IZAWA
Department Of ('henTistry, Tohoku University, Sendal, Japan K. 1SHII, S. MORITA
Department (~/Physk's, Tohoku University, Sendai, Japan and H. TAWARA
Nuclear Engineering Department, Kyushu University, Fukuoka, Japan Environmental samples have been analysed by measuring the X-rays induced by proton bombardment, if necessary by addition of an internal standard. Lighter elements were analysed more suitably by protons of energy 200 keV, because the background in the lower region of emitted photon energies was considerably less than with 3.5 MeV protons. A wide range of elements in soil was analysable by using protons of both these energies. Analysis of acid-soluble constituents in soil and aerosol was carried out with 3,5 MeV protons after the addition of internal standards, and the results obtained were compared with those obtained by using atomic absorption spectrometry (AAS).
1. Introduction Generally the preparation of uniform, thin targets is difficult. To overcome these difficulties, the addition of an internal standard ~) has been employed in the present paper. In solid targets, the accurate data obtained by the other method were used as the reference, because it is difficult to add the internal standard solution on the sample uniformly. Backgrounds from the Mylar backing produced by 3.5MeV protons used in the experimental procedure which is presented in the other paper we present at this conference are rather higher in the low-energy region of induced X-rays. For analysis of lighter elements, the use of lower energy protons, e.g. 200 keV from a Cockcroft-Walton type accelerator, was preferable to that of 3.5 MeV protons. By employing 4/~m Mylar as the backing, targets from soil and acid extracts from soil and aerosol were bombarded with protons, and induced X-rays were measured at 90 ° with respect to the proton beam. 2. Experimental results Experimental arrangements are in general the same as in our other paper in this conference. Analytical data are mainly presented here. 2.1. hNTERNALSTANDARDMETHOD In trace element analyses of environmental samples, target preparation is considered to be a
major problem, because of difficulties in preparing uniform, thin targets and measuring their absolute thicknesses. To overcome these difficulties, the "internal standard" method has recently been developed~), in which the weight of an element of atomic number Z is determined by the following equation :
where W is the weight and Y is the counts for X-ray peaks corrected for absorption in the target, the window, the air-path and the AI absorber. The suffixes s and z refer to the standard and the element of the atomatic number Z respectively. Apparently, this equation is applicable with the provision that the energy loss of the proton beam and the absorption of the X-rays produced are negligible during the bombardment. In the case of 200 keV protons, it has been confirmed experimentally that the same equation is applicable, because the proton range is very small in the target. In the present analysis, uranium was chosen as an internal standard, because usually its concentration is very low in nature and the L X-ray production cross-section is fairly large2). When no reliable X-ray production cross-sections had been obtained experimentally, these were calculated from the scaled universal binary encounter approximation curve3). The errors of the experimental results are, in V. APPLICATION
TO BIOLOGICAL SAMPLES
200
T. S t t l O K A W A
general, estimated to be 10-14%, including the uncertainties associated with theoretical estimation of the X-ray production cross-section, counting statistics and correction factors such as detection efficiency and absorption. 2.2.
et al.
TABLE l Elemental constitution of soil as determined by non-destructive C P X E (ppm).
NON-DESTRUCTIVE ANALYSIS OF A WIDE RANGE OF ELEMENTS IN SOIL BY 2 0 0 keY AND 3.5 M e V PROTON BOMBARDMENT4)
A soil sample was collected from a site in Teizan Bori, Sendai City by the Municipal Institute of Public Health. The composition was determined by the AAS method in the same institute. A fraction of the same soil sample was made available to the present investigators. It was dried and powdered to micro-millimeter fineness. The target used for 3.5 MeV proton bombardment was prepared by scattering a few milligrams of soil onto 4 gm Mylar film and using a drop of 0.1 ml of polyvinyl acetate diluted with acetone as adhesive. It was ascertained that the soil film was homogeneous and sufficiently thin. The energy loss of the proton beam in the sample was so small as to
Element
200 keV
AI Si S CI K Ca Ti V Mn Fe Cu Zn Ga Pb Rb Sr Y Zr Nb Mo
1 978 123 700 5 217 4 834 17 480 11 100 520 88 218 (20 350)
3.5 MeV
605 97 264 (20 350) 18 62 15 61 49 286 18 106 15 8
,o~
I
'
5 AL K
Ep
=
200
keV
ABSORBER : 20 prn MYLAR
2 SK
10
A.
3
5
\i
'~ C L K
R K
z
8
~"~'
2
l
:
t ~M
102
CA K,f
J
r
FE K~ CA K8
TI K,~
V Kw
10 20
I
I
I
I00
2O0
3O0 CHANNEL
400 NUMBER
Fig. I. X-ray spectrum of soil bombarded by 200 keV protons.
CR K,~ MN K=(
5O0
FE KI]
600
700
APPLICATION
TO
ENVIRONMENTAL
201
SAMPLES
z
Ep = 3.5 MeV ABSORBER: 50 pm AL
N
.= 'I
ql
d O3 z
5
o(D 2
Lul
,I I
10
.~
1 30
11100
i
i
I00
200
i 300 CHANNEL
i
I
I
400
500
600
NUMBER
Fig. 2. X-ray s p e c t r u m o f soil s a m p l e b o m b a r d e d by 3.5 M e V p r o t o n s .
cause negligible error in determinations. An aluminium absorber of 50/~m thickness was used to minimize the low energy bremsstrahlung background. Another target was prepared for bombardment using 200keV protons. The soil sample was mixed with high-purity graphite in a 10:3 soil-graphite ratio and was compressed into a disk of 0.5 mm thickness and 10 mm diameter. A Mylar film of 20 #m thickness was placed in front of the Si(Li) detector as absorber to avoid the pile-up effect of energy X-rays due to the high concentration of AI and Si. In the present analysis, the concentration of Fe in the soil sample determined by AAS was chosen as the internal standard. This was necessary because of the difficulty i n a d d i n g an internal standard solution uniformly to the powdered solid sample. The X-ray spectra of soil sample obtained using 200 keV and 3.5 MeV protons are shown in figs. 1
and 2 respectively. The results obtained by nondestructive CPXE using 200keV and 3.5 MeV protons are listed in table 1. The maximum energy of secondary electrons ejected by 200 keV protons is only 435 eV 5). Thus the X-ray energy range of interest which is from 1.4 keV (A1-K) to 8.0 keV (Cu-K) in CPXE with 200keV protons is free from bremsstrahlung caused by secondary electrons. However, X-ray production cross-sections are very small compared to those for 3.5 MeV protons. Hence, the peaks of the lighter elements such as A1, Si, S, K, Ca, Ti, Mn, and Fe are clearly defined. In this case, the use of protons of low energy, such as 200 keV, is favorable for the determination of the above elements. Nevertheless, in the case of 3.5 MeV protons, a considerable bremsstrahlung background is observed up to 15 keV. The large X-ray production cross-section at this proton energy makes it possible to identify 14 elements from Ti to Mo. For V.
APPLICATION
TO
BIOLOGICAL
SAMPLES
T.
202
S t t l O K A W A et al.
these elements the concentrations of Ti, V, and Mn agreed with the values determined by using 200 keV protons.
the internal standard. Thus, Fe was used as the standard, its concentration being normalized to that given by AAS.
2.3. ANALYSIS OF ACID-SOLUBLE CONSTITUENTS FROM SOIL BY 3.5 M e V PROTON BOMBARDMENT6)
"TABLE 2 Analysis of acid-soluble constituents from soil bombarded by 3.5 MeV protons (ppm).
So far as environmental science is concerned, an acid-soluble constituent from soil is more significant than the total constituents of the soil. A soil sample of 10 g was treated with 10 ml of 50% HC1 and 10ml of 50% HNO3 and diluted to 100ml with pure water. An aliquot portion of 0.4 ml of the extract was carefully dried onto 4/.tm Mylar film under an IR lamp. The target was bombarded by 3.5 MeV protons. Fig. 3 shows an X-ray spectrum of the sample. The results are also compared with those determined by means of the AAS in table 2. Since the Rb K~ X-ray peak appeared at the position of the U L~X-ray peak, uranium could not be used as
Element
CPXE 3.5 MeV
Ti V Mn Fe Cu Zn Br Pb Rb Sr
417 57 235 (20 350) 27 206 74 185 17 86
AAS
267 20 350 32 149 16
1
f Ep = 3.5 MeV ABSORBER: 50 pm AL
03
,1' I
I' I
5
2
i
3
I
5 CO
102
I
I
l
m
uo z 0 u
2 l/100
10 5 2 1
30
I
i
100
200
I
300 CHANNEL NUMBER
J
/-,00
I
500
Fig. 3. X-ray spectrum of acid-soluble constituents from soil bombarded by 3.5 MeV protons.
600
APPLICATION
2.4.
TO
ENVIRONMENTAL TABLE 3
A N A L Y S I S OF ACID-SOLUBLE CONSTITUENTS FROM
AEROSOL
BY
3.5 MeV
PROTON
203
SAMPLES
Analysis of aerosol bombarded by 3.5 M e V proton.
BOMBARD-
MENT 6)
Element
Air dust was collected on a glass fiber filter (GelmanA, 8"×10"), through which air was passed for 24 h at a flow rate of about 1.5 m3/min at a definite point in Sendai. The sample solution was prepared from the aerosol sample by using the same technique as employed in the AAS method. It is as follows: Extraction of the aerosol sample was carried out with 20ml of 20% H C l a n d 5ml of 30%H202 in a modified Soxhlet extractor. The sample solution was prepared by dilution of combined extracts, to which a known amount of uranyl nitrate was added as an internal standard. Onto a 4/2m Mylar film, an aliquot of 0.4 ml of the solution containing 1.92/2g U was deposited and heated carefully to dryness under an IR lamp. An example of the X-ray spectrum of the aerosol sample is shown in fig. 4. The analytical re-
V Cr Mn Fe Ni Zn Ge Pb Br Sr Cd
Content
(,ug/m 3)
CPXE
AAS
0.097 -0.057 1.96 0.071 0.718 0.035 0.293 2.93 0.015 --
-0.005 0.062 2.51 -0.133 -0.128 --0.003
suits corrected for the amounts of impurities contained in the filter are listed in table 3, together with those obtained by the AAS. Quantitative analysis of Cu was not carried out, because during
2 Ep :
z
N
10 3
g
3.5 IvleV
ABSORBER : 50 pm
5
Y
2 10
2
,.J
ft. 5
I
5
2
I0
1
30
I
I
I00
200
I
30O CHANNEL NUMBER
I
__
400
I 500
600
Fig. 4. X-ray spectrum of aerosol bombarded by 3.5 M e V protons V
APPLICATION
TO
BIOLOGICAL
SAMPLES
204
T. SHIOKAWA et al.
s a m p l i n g c o n t a m i n a t i o n f r o m a n air s a m p l e r h a d been recognized. T h e c o n c e n t r a t i o n o f M n is in a g r e e m e n t w i t h that determined by using AAS, but the disagreem e n t for t h e c o n c e n t r a t i o n s o f Z n a n d P b in b o t h m e t h o d s is s e e n . T h e fact t h a t v a n a d i u m c a n e a sily be a n a l y s e d b y t h e C P X E m e t h o d s h o w s a c h a r a c t e r i s t i c o f this m e t h o d in c o m p a r i s o n w i t h the AAS method. T h e a u t h o r s w o u l d like to t h a n k Mr. M. K a t o a n d Mr. M. H i r a g a for t h e i r skilful o p e r a t i o n o f t h e a c c e l e r a t o r s in t h e c o u r s e o f t h i s e x p e r i m e n t . T h e y also w i s h to t h a n k t h e M i n i s t r y o f E d u c a t i o n for f i n a n c i a l s u p p o r t .
References 1) A. Pape, J. C. Sens, P. Fiutz, A. Gallman, H. E. Gove, G. Guillaume and D. M. Stupin, Nucl. Instr. and Meth. 105(1972) 161; K. lshii, S. Morita, H. Tawara, T. C. Chu, H. Kaji and T. Shiokawa, Nucl. Instr. and Meth. 126 (1975) 75. 2) H. Tawara, K. lshii, S. Morita, H. Kaji and T. Shiokawa, Phys. Rev. A l l (1975) 1560. 31 J. D. Oarcia, R. J. Fortner, T. M. Kavanagh, Rev. Mod. Phys. 45 (1973) I l l . 4) V. R. Navarrete, G. Izawa, T. Shiokawa, K. Ishii and S. Morita, Radiochem. Radionanal. Lett., in press. 5) E. Merzbacher and H. W. Lewis, Encycl. Phys. 34 (1958) 166. 6) T. C. Chu, V. R. Navarrete, H. Kaji. G. lzawa, T. Shiokawa, K. Ishii, S. Morita and H. Tawara, J. Radioanal. Chem., in press.
199-204 ; ©
N O R T t t - H O L L A N D P U B L I S t t l N G CO.
A STUDY ON PROTON INDUCED X-RAY ANALYSIS AND ITS APPLICATION TO ENVIRONMENTAl, SAMPLES T. SHIOKAWA, T. C. CHU, V R. NAVARRETE, H. KAJI, G. IZAWA
Department Of ('henTistry, Tohoku University, Sendal, Japan K. 1SHII, S. MORITA
Department (~/Physk's, Tohoku University, Sendai, Japan and H. TAWARA
Nuclear Engineering Department, Kyushu University, Fukuoka, Japan Environmental samples have been analysed by measuring the X-rays induced by proton bombardment, if necessary by addition of an internal standard. Lighter elements were analysed more suitably by protons of energy 200 keV, because the background in the lower region of emitted photon energies was considerably less than with 3.5 MeV protons. A wide range of elements in soil was analysable by using protons of both these energies. Analysis of acid-soluble constituents in soil and aerosol was carried out with 3,5 MeV protons after the addition of internal standards, and the results obtained were compared with those obtained by using atomic absorption spectrometry (AAS).
1. Introduction Generally the preparation of uniform, thin targets is difficult. To overcome these difficulties, the addition of an internal standard ~) has been employed in the present paper. In solid targets, the accurate data obtained by the other method were used as the reference, because it is difficult to add the internal standard solution on the sample uniformly. Backgrounds from the Mylar backing produced by 3.5MeV protons used in the experimental procedure which is presented in the other paper we present at this conference are rather higher in the low-energy region of induced X-rays. For analysis of lighter elements, the use of lower energy protons, e.g. 200 keV from a Cockcroft-Walton type accelerator, was preferable to that of 3.5 MeV protons. By employing 4/~m Mylar as the backing, targets from soil and acid extracts from soil and aerosol were bombarded with protons, and induced X-rays were measured at 90 ° with respect to the proton beam. 2. Experimental results Experimental arrangements are in general the same as in our other paper in this conference. Analytical data are mainly presented here. 2.1. hNTERNALSTANDARDMETHOD In trace element analyses of environmental samples, target preparation is considered to be a
major problem, because of difficulties in preparing uniform, thin targets and measuring their absolute thicknesses. To overcome these difficulties, the "internal standard" method has recently been developed~), in which the weight of an element of atomic number Z is determined by the following equation :
where W is the weight and Y is the counts for X-ray peaks corrected for absorption in the target, the window, the air-path and the AI absorber. The suffixes s and z refer to the standard and the element of the atomatic number Z respectively. Apparently, this equation is applicable with the provision that the energy loss of the proton beam and the absorption of the X-rays produced are negligible during the bombardment. In the case of 200 keV protons, it has been confirmed experimentally that the same equation is applicable, because the proton range is very small in the target. In the present analysis, uranium was chosen as an internal standard, because usually its concentration is very low in nature and the L X-ray production cross-section is fairly large2). When no reliable X-ray production cross-sections had been obtained experimentally, these were calculated from the scaled universal binary encounter approximation curve3). The errors of the experimental results are, in V. APPLICATION
TO BIOLOGICAL SAMPLES
200
T. S t t l O K A W A
general, estimated to be 10-14%, including the uncertainties associated with theoretical estimation of the X-ray production cross-section, counting statistics and correction factors such as detection efficiency and absorption. 2.2.
et al.
TABLE l Elemental constitution of soil as determined by non-destructive C P X E (ppm).
NON-DESTRUCTIVE ANALYSIS OF A WIDE RANGE OF ELEMENTS IN SOIL BY 2 0 0 keY AND 3.5 M e V PROTON BOMBARDMENT4)
A soil sample was collected from a site in Teizan Bori, Sendai City by the Municipal Institute of Public Health. The composition was determined by the AAS method in the same institute. A fraction of the same soil sample was made available to the present investigators. It was dried and powdered to micro-millimeter fineness. The target used for 3.5 MeV proton bombardment was prepared by scattering a few milligrams of soil onto 4 gm Mylar film and using a drop of 0.1 ml of polyvinyl acetate diluted with acetone as adhesive. It was ascertained that the soil film was homogeneous and sufficiently thin. The energy loss of the proton beam in the sample was so small as to
Element
200 keV
AI Si S CI K Ca Ti V Mn Fe Cu Zn Ga Pb Rb Sr Y Zr Nb Mo
1 978 123 700 5 217 4 834 17 480 11 100 520 88 218 (20 350)
3.5 MeV
605 97 264 (20 350) 18 62 15 61 49 286 18 106 15 8
,o~
I
'
5 AL K
Ep
=
200
keV
ABSORBER : 20 prn MYLAR
2 SK
10
A.
3
5
\i
'~ C L K
R K
z
8
~"~'
2
l
:
t ~M
102
CA K,f
J
r
FE K~ CA K8
TI K,~
V Kw
10 20
I
I
I
I00
2O0
3O0 CHANNEL
400 NUMBER
Fig. I. X-ray spectrum of soil bombarded by 200 keV protons.
CR K,~ MN K=(
5O0
FE KI]
600
700
APPLICATION
TO
ENVIRONMENTAL
201
SAMPLES
z
Ep = 3.5 MeV ABSORBER: 50 pm AL
N
.= 'I
ql
d O3 z
5
o(D 2
Lul
,I I
10
.~
1 30
11100
i
i
I00
200
i 300 CHANNEL
i
I
I
400
500
600
NUMBER
Fig. 2. X-ray s p e c t r u m o f soil s a m p l e b o m b a r d e d by 3.5 M e V p r o t o n s .
cause negligible error in determinations. An aluminium absorber of 50/~m thickness was used to minimize the low energy bremsstrahlung background. Another target was prepared for bombardment using 200keV protons. The soil sample was mixed with high-purity graphite in a 10:3 soil-graphite ratio and was compressed into a disk of 0.5 mm thickness and 10 mm diameter. A Mylar film of 20 #m thickness was placed in front of the Si(Li) detector as absorber to avoid the pile-up effect of energy X-rays due to the high concentration of AI and Si. In the present analysis, the concentration of Fe in the soil sample determined by AAS was chosen as the internal standard. This was necessary because of the difficulty i n a d d i n g an internal standard solution uniformly to the powdered solid sample. The X-ray spectra of soil sample obtained using 200 keV and 3.5 MeV protons are shown in figs. 1
and 2 respectively. The results obtained by nondestructive CPXE using 200keV and 3.5 MeV protons are listed in table 1. The maximum energy of secondary electrons ejected by 200 keV protons is only 435 eV 5). Thus the X-ray energy range of interest which is from 1.4 keV (A1-K) to 8.0 keV (Cu-K) in CPXE with 200keV protons is free from bremsstrahlung caused by secondary electrons. However, X-ray production cross-sections are very small compared to those for 3.5 MeV protons. Hence, the peaks of the lighter elements such as A1, Si, S, K, Ca, Ti, Mn, and Fe are clearly defined. In this case, the use of protons of low energy, such as 200 keV, is favorable for the determination of the above elements. Nevertheless, in the case of 3.5 MeV protons, a considerable bremsstrahlung background is observed up to 15 keV. The large X-ray production cross-section at this proton energy makes it possible to identify 14 elements from Ti to Mo. For V.
APPLICATION
TO
BIOLOGICAL
SAMPLES
T.
202
S t t l O K A W A et al.
these elements the concentrations of Ti, V, and Mn agreed with the values determined by using 200 keV protons.
the internal standard. Thus, Fe was used as the standard, its concentration being normalized to that given by AAS.
2.3. ANALYSIS OF ACID-SOLUBLE CONSTITUENTS FROM SOIL BY 3.5 M e V PROTON BOMBARDMENT6)
"TABLE 2 Analysis of acid-soluble constituents from soil bombarded by 3.5 MeV protons (ppm).
So far as environmental science is concerned, an acid-soluble constituent from soil is more significant than the total constituents of the soil. A soil sample of 10 g was treated with 10 ml of 50% HC1 and 10ml of 50% HNO3 and diluted to 100ml with pure water. An aliquot portion of 0.4 ml of the extract was carefully dried onto 4/.tm Mylar film under an IR lamp. The target was bombarded by 3.5 MeV protons. Fig. 3 shows an X-ray spectrum of the sample. The results are also compared with those determined by means of the AAS in table 2. Since the Rb K~ X-ray peak appeared at the position of the U L~X-ray peak, uranium could not be used as
Element
CPXE 3.5 MeV
Ti V Mn Fe Cu Zn Br Pb Rb Sr
417 57 235 (20 350) 27 206 74 185 17 86
AAS
267 20 350 32 149 16
1
f Ep = 3.5 MeV ABSORBER: 50 pm AL
03
,1' I
I' I
5
2
i
3
I
5 CO
102
I
I
l
m
uo z 0 u
2 l/100
10 5 2 1
30
I
i
100
200
I
300 CHANNEL NUMBER
J
/-,00
I
500
Fig. 3. X-ray spectrum of acid-soluble constituents from soil bombarded by 3.5 MeV protons.
600
APPLICATION
2.4.
TO
ENVIRONMENTAL TABLE 3
A N A L Y S I S OF ACID-SOLUBLE CONSTITUENTS FROM
AEROSOL
BY
3.5 MeV
PROTON
203
SAMPLES
Analysis of aerosol bombarded by 3.5 M e V proton.
BOMBARD-
MENT 6)
Element
Air dust was collected on a glass fiber filter (GelmanA, 8"×10"), through which air was passed for 24 h at a flow rate of about 1.5 m3/min at a definite point in Sendai. The sample solution was prepared from the aerosol sample by using the same technique as employed in the AAS method. It is as follows: Extraction of the aerosol sample was carried out with 20ml of 20% H C l a n d 5ml of 30%H202 in a modified Soxhlet extractor. The sample solution was prepared by dilution of combined extracts, to which a known amount of uranyl nitrate was added as an internal standard. Onto a 4/2m Mylar film, an aliquot of 0.4 ml of the solution containing 1.92/2g U was deposited and heated carefully to dryness under an IR lamp. An example of the X-ray spectrum of the aerosol sample is shown in fig. 4. The analytical re-
V Cr Mn Fe Ni Zn Ge Pb Br Sr Cd
Content
(,ug/m 3)
CPXE
AAS
0.097 -0.057 1.96 0.071 0.718 0.035 0.293 2.93 0.015 --
-0.005 0.062 2.51 -0.133 -0.128 --0.003
suits corrected for the amounts of impurities contained in the filter are listed in table 3, together with those obtained by the AAS. Quantitative analysis of Cu was not carried out, because during
2 Ep :
z
N
10 3
g
3.5 IvleV
ABSORBER : 50 pm
5
Y
2 10
2
,.J
ft. 5
I
5
2
I0
1
30
I
I
I00
200
I
30O CHANNEL NUMBER
I
__
400
I 500
600
Fig. 4. X-ray spectrum of aerosol bombarded by 3.5 M e V protons V
APPLICATION
TO
BIOLOGICAL
SAMPLES
204
T. SHIOKAWA et al.
s a m p l i n g c o n t a m i n a t i o n f r o m a n air s a m p l e r h a d been recognized. T h e c o n c e n t r a t i o n o f M n is in a g r e e m e n t w i t h that determined by using AAS, but the disagreem e n t for t h e c o n c e n t r a t i o n s o f Z n a n d P b in b o t h m e t h o d s is s e e n . T h e fact t h a t v a n a d i u m c a n e a sily be a n a l y s e d b y t h e C P X E m e t h o d s h o w s a c h a r a c t e r i s t i c o f this m e t h o d in c o m p a r i s o n w i t h the AAS method. T h e a u t h o r s w o u l d like to t h a n k Mr. M. K a t o a n d Mr. M. H i r a g a for t h e i r skilful o p e r a t i o n o f t h e a c c e l e r a t o r s in t h e c o u r s e o f t h i s e x p e r i m e n t . T h e y also w i s h to t h a n k t h e M i n i s t r y o f E d u c a t i o n for f i n a n c i a l s u p p o r t .
References 1) A. Pape, J. C. Sens, P. Fiutz, A. Gallman, H. E. Gove, G. Guillaume and D. M. Stupin, Nucl. Instr. and Meth. 105(1972) 161; K. lshii, S. Morita, H. Tawara, T. C. Chu, H. Kaji and T. Shiokawa, Nucl. Instr. and Meth. 126 (1975) 75. 2) H. Tawara, K. lshii, S. Morita, H. Kaji and T. Shiokawa, Phys. Rev. A l l (1975) 1560. 31 J. D. Oarcia, R. J. Fortner, T. M. Kavanagh, Rev. Mod. Phys. 45 (1973) I l l . 4) V. R. Navarrete, G. Izawa, T. Shiokawa, K. Ishii and S. Morita, Radiochem. Radionanal. Lett., in press. 5) E. Merzbacher and H. W. Lewis, Encycl. Phys. 34 (1958) 166. 6) T. C. Chu, V. R. Navarrete, H. Kaji. G. lzawa, T. Shiokawa, K. Ishii, S. Morita and H. Tawara, J. Radioanal. Chem., in press.