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Use of a faraday cup in target gas for low energy or heavy charged particle recording
NUCLEAR
INSTRUMENTS
AND
METHODS
75
(I969) I 7 I - [ 7 2 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
USE OF A FARADAY C U P IN T A R G E T GAS FOR LOW ENERGY OR HEAVY C H A R G E D PARTICLE RECORDING* L. S. C H U A N G
Chung Chi College, The Chinese University of Hong Kong, Hong Kong Received 28 M a y 1969 A F a r a d a y cup for charged particles was f o u n d to give consistent results w h e n used in 4He target gas, a n d in v a c u u m isolated by a Ni-foil o f 1 /~m thickness.
In nuclear scattering using a gas target, the number of incident particles are counted by a Faraday cup equipped with a current integrator. When using lowenergy or heavy charged particles as incident particles, a thin foil 1) or a differential pumping system 2) has been applied to keep the Faraday cup in vacuum. These methods are laborious, and unless rigorous care is exercised, numerous errors may be introduced into the experimental result. To test the validity of using a Faraday cup in the target gas, 4He, of pressure around 10 m m of Hg, the following procedures were followed: Two silicon surface barrier type solid state detectors, with the sensitive area well defined, were used as the monitors. They were fixed at laboratory angles of 20 ° (for
T-4He scattering) and 50 ° (for 3He-4He scattering) in the scattering chamber filled with 4He-gas. Tritonand aHe-beams were accelerated to 2.132 and 2.975 MeV respectively, by means of the 4 MeV Van de Graaff accelerator3), and the elastic scattering of T-4He and 3He-4He were studied at various target gas pressures ranging from 0.6 to 13.2 m m of Hg. Exposures of the target to the accelerated particles were continued till the monitor counting statistics of about 3% was attained. It seems worth noting here that under present experimental conditions, 54 min were spent in obtaining the monitor counting statistics of 6% when the target gas pressure was 0.6 m m of Hg and the incident beam current was about 0.04 pA. The result of each test measurement was expressed in terms of a value A, expressing the number of monitor
* Research at T o k y o University o f Education, T o k y o , Japan.
35
I 1.5
- - 4 33 ~
I ~r i
-c
!31
M
h
i
LL
~29 v
{
1.0
%
÷
{
{
% -[27
a_
'7. [2-
II
< 0.5 0
~'o
16o
1;o
23 2O0
P r e s s u r e ( m m - - o i l ) (147.8 m m - o i l = 1 0 m m - H g l Fig. 1. T h e values o f A [i.e. the n u m b e r o f m o n i t o r c o u n t s (M.C.) per unit pressure (P) a n d unit current integrator reading (C.I.)] vs F a r a d a y cup pressures. • ET = 2.132 MeV, for (T-4He) scattering; • E3He = 2.975 MeV, for (aHe-4He) scattering.
171
172
L. S. C H U A N G
counts per unit pressure and unit current integrator reading. The values of A were plotted on the ordinate and the gas pressures on the abscissa of fig. 1. Lines with practically zero slopes were plotted to fit the exp6rimental points by a least squares procedure. The estimated fractional standard deviation for the value A was about ___4%. Constancy of the values A for different pressures extending down to the pressure 0.6 mm of Hg, and possibly to zero pressure, is a satisfactory proof of the validity of using the Faraday cup in the 4He-gas for accelerated particles recording. This should be true at least under present experimental conditions. To test further the validity of the method used, the Faraday cup was isolated in vacuum by a Ni-foil of 1/am thickness. The circular cross-sectional diameter of the foil was 23 mm which is large enough to allow the beam, circular in cross-section with diameter of about 10 mm, to pass through the entrance to the Faraday cup without causing any edge scattering. The separation of the Ni-foil from the fringe of the Faraday cup was 73 mm, in which a magnetic field of approximately 500 G was applied. The secondary electrons, both from the foil and the cup-wall will then be repelled back. Differential cross-sectiona) at 50 ° laboratory angle was measured for 3He-4He elastic scattering with the energy of incidence at 2.975 MeV. Comparison with the differential cross section obtained under the same experimental condition but with the Faraday cup fixed in the 4He-gas showed a factor of 1.6 of difference; fewer beam particles were collected when the Faraday cup was used in vacuum through the Ni-foil. Calculation of the fraction of beam particles scattered out of the Faraday cup in passing through the Ni-foil of 1/am thickness was attemptedS). The calculation was made on the assumption that the beam of charged particles was a parallel one, with the radius of the cross-section of
4.5 mm. For Moli6re's theory of multiple scattering 5) to be valid in the calculation, a Ni-foil thickness greater than 3/am has to be used. The thickness of the Ni-foil used was only 1/am, therefore, the calculated value of 32.3% particle loss is only a rough indication of the order of particle loss for the Ni-foil and the geometry of measurement used. Bat, it should not be too far from the true value. If the assumption that the non-parallel beam was a parallel one is taken into consideration, the calculated particle loss will come closer to the experimental value of 37%. This result, therefore could be thought of as a further support of the conclusion made regarding the use of a Faraday cup in the 4He-gas. The author wishes to express his sincerest thanks for the instructive discussions and suggestions given by Prof. J. Sanada. References 1) H. R. Worthington, J. N. McGruer and D. E. Findley, Phys Rev. 90 (1953) 899; J. L. Russell, Jr., G. C. Phillips and~ C. W. Reich, Phys. Rev. 104 (1956) 135; L. H. Johnston and D. A.rSwenson, Phys. Rev. l U (1958) 212; D. J. Knechts, S. Messelt,!E. D. Berners and L. C. Northcliffe, Phys. Rev. 114 (1959) 550; C. Miller Jones, G. C. Phillips, R. W. Harris and E. H. Becker, Nucl. Phys. 37 (1962) 1; T. A. Tombrello and L. S. Senhouse, Phys. Rev. 129 (1963) 2252; T. A. Tombrello and P. D. Parker, Phys. Rev. 130 (1963) 1112. 2) p. D. Miller and G. C. Phillips, Phys. Rev. 112 (1958) 2048; A. C. L. Barnard, C. M. Jones and J. L. Well, Nucl. Phys. 50 (1964) 604. a) Installed at Nippon Atomic Industry Group, Tokyo, Japan. 4) j. C. Allred, L. Rosen, F. K. Tallmadge and J.rH. Williams, Rev. Sci. Instr. 22 (1951) 191. 5) W. C. Dickinson and D. C. Dodder, Rev. Sci. Instr. 24 (1953) 428 ; G. Moli~re, Z. Naturforsch. 3a (1948) 78.
INSTRUMENTS
AND
METHODS
75
(I969) I 7 I - [ 7 2 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
USE OF A FARADAY C U P IN T A R G E T GAS FOR LOW ENERGY OR HEAVY C H A R G E D PARTICLE RECORDING* L. S. C H U A N G
Chung Chi College, The Chinese University of Hong Kong, Hong Kong Received 28 M a y 1969 A F a r a d a y cup for charged particles was f o u n d to give consistent results w h e n used in 4He target gas, a n d in v a c u u m isolated by a Ni-foil o f 1 /~m thickness.
In nuclear scattering using a gas target, the number of incident particles are counted by a Faraday cup equipped with a current integrator. When using lowenergy or heavy charged particles as incident particles, a thin foil 1) or a differential pumping system 2) has been applied to keep the Faraday cup in vacuum. These methods are laborious, and unless rigorous care is exercised, numerous errors may be introduced into the experimental result. To test the validity of using a Faraday cup in the target gas, 4He, of pressure around 10 m m of Hg, the following procedures were followed: Two silicon surface barrier type solid state detectors, with the sensitive area well defined, were used as the monitors. They were fixed at laboratory angles of 20 ° (for
T-4He scattering) and 50 ° (for 3He-4He scattering) in the scattering chamber filled with 4He-gas. Tritonand aHe-beams were accelerated to 2.132 and 2.975 MeV respectively, by means of the 4 MeV Van de Graaff accelerator3), and the elastic scattering of T-4He and 3He-4He were studied at various target gas pressures ranging from 0.6 to 13.2 m m of Hg. Exposures of the target to the accelerated particles were continued till the monitor counting statistics of about 3% was attained. It seems worth noting here that under present experimental conditions, 54 min were spent in obtaining the monitor counting statistics of 6% when the target gas pressure was 0.6 m m of Hg and the incident beam current was about 0.04 pA. The result of each test measurement was expressed in terms of a value A, expressing the number of monitor
* Research at T o k y o University o f Education, T o k y o , Japan.
35
I 1.5
- - 4 33 ~
I ~r i
-c
!31
M
h
i
LL
~29 v
{
1.0
%
÷
{
{
% -[27
a_
'7. [2-
II
< 0.5 0
~'o
16o
1;o
23 2O0
P r e s s u r e ( m m - - o i l ) (147.8 m m - o i l = 1 0 m m - H g l Fig. 1. T h e values o f A [i.e. the n u m b e r o f m o n i t o r c o u n t s (M.C.) per unit pressure (P) a n d unit current integrator reading (C.I.)] vs F a r a d a y cup pressures. • ET = 2.132 MeV, for (T-4He) scattering; • E3He = 2.975 MeV, for (aHe-4He) scattering.
171
172
L. S. C H U A N G
counts per unit pressure and unit current integrator reading. The values of A were plotted on the ordinate and the gas pressures on the abscissa of fig. 1. Lines with practically zero slopes were plotted to fit the exp6rimental points by a least squares procedure. The estimated fractional standard deviation for the value A was about ___4%. Constancy of the values A for different pressures extending down to the pressure 0.6 mm of Hg, and possibly to zero pressure, is a satisfactory proof of the validity of using the Faraday cup in the 4He-gas for accelerated particles recording. This should be true at least under present experimental conditions. To test further the validity of the method used, the Faraday cup was isolated in vacuum by a Ni-foil of 1/am thickness. The circular cross-sectional diameter of the foil was 23 mm which is large enough to allow the beam, circular in cross-section with diameter of about 10 mm, to pass through the entrance to the Faraday cup without causing any edge scattering. The separation of the Ni-foil from the fringe of the Faraday cup was 73 mm, in which a magnetic field of approximately 500 G was applied. The secondary electrons, both from the foil and the cup-wall will then be repelled back. Differential cross-sectiona) at 50 ° laboratory angle was measured for 3He-4He elastic scattering with the energy of incidence at 2.975 MeV. Comparison with the differential cross section obtained under the same experimental condition but with the Faraday cup fixed in the 4He-gas showed a factor of 1.6 of difference; fewer beam particles were collected when the Faraday cup was used in vacuum through the Ni-foil. Calculation of the fraction of beam particles scattered out of the Faraday cup in passing through the Ni-foil of 1/am thickness was attemptedS). The calculation was made on the assumption that the beam of charged particles was a parallel one, with the radius of the cross-section of
4.5 mm. For Moli6re's theory of multiple scattering 5) to be valid in the calculation, a Ni-foil thickness greater than 3/am has to be used. The thickness of the Ni-foil used was only 1/am, therefore, the calculated value of 32.3% particle loss is only a rough indication of the order of particle loss for the Ni-foil and the geometry of measurement used. Bat, it should not be too far from the true value. If the assumption that the non-parallel beam was a parallel one is taken into consideration, the calculated particle loss will come closer to the experimental value of 37%. This result, therefore could be thought of as a further support of the conclusion made regarding the use of a Faraday cup in the 4He-gas. The author wishes to express his sincerest thanks for the instructive discussions and suggestions given by Prof. J. Sanada. References 1) H. R. Worthington, J. N. McGruer and D. E. Findley, Phys Rev. 90 (1953) 899; J. L. Russell, Jr., G. C. Phillips and~ C. W. Reich, Phys. Rev. 104 (1956) 135; L. H. Johnston and D. A.rSwenson, Phys. Rev. l U (1958) 212; D. J. Knechts, S. Messelt,!E. D. Berners and L. C. Northcliffe, Phys. Rev. 114 (1959) 550; C. Miller Jones, G. C. Phillips, R. W. Harris and E. H. Becker, Nucl. Phys. 37 (1962) 1; T. A. Tombrello and L. S. Senhouse, Phys. Rev. 129 (1963) 2252; T. A. Tombrello and P. D. Parker, Phys. Rev. 130 (1963) 1112. 2) p. D. Miller and G. C. Phillips, Phys. Rev. 112 (1958) 2048; A. C. L. Barnard, C. M. Jones and J. L. Well, Nucl. Phys. 50 (1964) 604. a) Installed at Nippon Atomic Industry Group, Tokyo, Japan. 4) j. C. Allred, L. Rosen, F. K. Tallmadge and J.rH. Williams, Rev. Sci. Instr. 22 (1951) 191. 5) W. C. Dickinson and D. C. Dodder, Rev. Sci. Instr. 24 (1953) 428 ; G. Moli~re, Z. Naturforsch. 3a (1948) 78.