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Production of thin tritium sources by glow discharge
NUCLEAR
]NSIRUMENTS
AND
PRODUCTION
METHODS
71
(1969) I87-I88; ©
NORTH-HOLLAND
PUBLISHING
CO.
O F T H I N T R I T I U M S O U R C E S BY G L O W D I S C H A R G E R.
DARIS and C. ST-PIERRE
Department of Physics, Laval University, Quebec, Que., Canada
Received 8 January 1969 Very thin tritium sources have been prepared by glow discharge in tritium with aluminum electrodes. Although about 60% of the total activity originates from tritium absorbed on the surface, these sources are very stable under vacuum. When most of this
absorbed gas was removed by immersion in water, still 90% of the remaining activity was found to be in a thickness of less than 3 #g/cm2.
L o w voltage glow discharge in pure tritium gas was used to prepare ultra thin tritium sources for the investigation o f the shape o f the beta-ray spectrum near the end point. The most important characteristics for such sources are, as discussed in a previous paper1), high intensity for a thickness less than 5 #g/cm 2 and stability o f the active deposit under v a c u u m in order to prevent fluctuations of intensity and contamination o f the spectrometer chamber. A l u m i n u m was chosen as the backing material because it is relatively easy to study the distributions o f ions as a function o f their penetration by the thin layer electrolytic peeling process described by Davies and al.2), and also because the ever present aluminum oxyde film is reputedly difficult to leak through3). The metal was first electropolished following the method
o f T e g a r P ) and oxydized at 1 0 0 V in an orthop h o s p h o r i c - c h r o m i u m trioxyde solution at 90°C. The backings are then cut to size, deoxydised again, and placed in the apparatus shown in fig. I which is p r o m p t l y evacuated. W h e n a v a c u u m o f less than 10 -3 Torr is obtained with the mechanical p u m p and the liquid nitrogen cooled trap, the outlet to the p u m p is closed and the tritium is allowed to leak into the discharge c h a m b e r which is kept connected to the trap. We assume that the system is reasonably free o f condensable impurities. A steady discharge is maintained at 600-900 V with a stabilizing resistor o f 100 MI2, while the gas pressure is kept between 1.2 to 0.2 Torr. The voltage is measured with a high impedance electrostatic voltmeter between the electrodes and f r o m the potential
TO
MECHANICALPUMP TUNGSTENE CENTER ELECTRODE
INLET
#'~AIR
DETAIL OF ELECTRODEI
..k"
LIQUID AIR "RAP MANOMETER
SLITS EOR B SOURCE
J~O-O~ ~'~t::~0
BACKINGS I
STA}NLESS STEEL SL ITTED CYLINDER
ELECTRODE
]Fig. 1. Arrangement of the glass apparatus used for the production of thin tritium sources by electrical discharge. The detail of the cylindrical electrode system inserted in the electrical discharge vessel is also shown. 187
188
R. DARIS AND C. ST-PIERRE
d r o p across the resistor, the charge and hence the a m o u n t o f ion d e p o s i t i o n is readily d e t e r m i n e d . Off the six sources p r e p a r e d in a single process, some samples were used for stability tests and ion range d i s t r i b u t i o n studies, the other being kept under v a c u u m until used in the spectrometer. The activity o f sources p r e p a r e d by gas discharge originates f r o m the gas a b s o r b e d on the surface o f the metal and from ions driven into the a l u m i n u m or its oxyde layer. A g o o d p a r t of the a b s o r b e d activity can be easily removed by immersion of the sources in water. Since the sources seemed very stable under v a c u u m even when the a b s o r b e d gas was not removed, we decided later on to t a k e a d v a n t a g e o f this fact in o r d e r to i m p r o v e on the low effective thickness. Finally, the heating of the sources at 100°C for several
40
30
O
20
,-t "O .r4 m 0)
~z 10
\ 0•0•0 i 2
References
~--- 0 _ . _ _ 0
I
I 4
I
hours in v a c u u m did not p r o d u c e a p p r e c i a b l e outgassing. The activity of the sample sources was m e a s u r e d with an ion c h a m b e r - e l e c t r o m e t e r system after successive electrolytic peelings o f 1.0 pg/cm 2. Fig. 2 gives the residual activity as a function o f the thickness r e m o v e d for a source p r e p a r e d by glow discharge at 700 V a n d washed in water p r i o r to the first a n o d i sation. The result o f the washing was a r e d u c t i o n o f the intensity by 64%. The thickness c o r r e s p o n d i n g to 90% o f the source activity is less t h a n 3/~g/cm 2. The remaining activity is distributed in an exponential tail as expectedS). Using the unwashed sources, the m a x i m u m counting rates were up to 700 c p m with our s p e c t r o m e t e r which has a 0.25% m o m e n t u m resolution a n d a 0.07% transmission. F o u r spectrometer runs were d o n e with these sources w i t h o u t any difficulty concerning fluctuation o f intensity or b a c k g r o u n d counting rate. A p r e l i m i n a r y analysis o f the results indicates t h a t sources p r e p a r e d by this m e t h o d meet the requirements for the determ i n a t i o n o f the shape o f the tritium b e t a spectrum near the end p o i n t at 0.25% m o m e n t u m resolution. The e x t r a p o l a t e d end p o i n t has been extracted f r o m the m e a s u r e m e n t s done so far a n d its value is 18.565___0.075 keV. F o r the calibration, we used the 238.6 keV K-line a n d the 39.85 keV Ll-Conversion lines in 212Bi and 2°8T1 corrected for the D o p p l e r effects as m e a s u r e d by Ewan 6) a n d also corrected for our i n s t r u m e n t a l distortion. A m o r e c o m p l e t e analysis o f the shape o f these spectra near the end p o i n t will be given in a f o r t h c o m i n g publication.
I
I
6
Thickness remqved (~g/cm 2) Fig. 2. Residual activity of a source prepared in a gaseous electrical discharge at 700 V vs the thickness removed by electrolytic peeling at 1.53 V. The initial activity of 104 arbitrary units was reduced to 37 by simple immersion in distilled water.
1) R. Daris and C. St-Pierre, Nucl. Instr. and Meth. 64 (1968) 346. 2) j . A . Davies, J. Friesen and J. D. Mclntyre, Can. J. Chem. 38 (1960) 1526. a) I. BergstrOm et al., Nucl. Instr. and Meth. 21 (1963) 249. 4) W.J. McG. Tegart, Electrolytic and chemical polishing of metals (Pergamon Press, London, 1956) p. 52. 5) j. A. Davies, J. Friesen and J. D. McIntyre, Can. J. Chem. 38 (1960) 1535. 6) G.T. Ewan, Can. J. Phys. 41 (1963) 2202.
]NSIRUMENTS
AND
PRODUCTION
METHODS
71
(1969) I87-I88; ©
NORTH-HOLLAND
PUBLISHING
CO.
O F T H I N T R I T I U M S O U R C E S BY G L O W D I S C H A R G E R.
DARIS and C. ST-PIERRE
Department of Physics, Laval University, Quebec, Que., Canada
Received 8 January 1969 Very thin tritium sources have been prepared by glow discharge in tritium with aluminum electrodes. Although about 60% of the total activity originates from tritium absorbed on the surface, these sources are very stable under vacuum. When most of this
absorbed gas was removed by immersion in water, still 90% of the remaining activity was found to be in a thickness of less than 3 #g/cm2.
L o w voltage glow discharge in pure tritium gas was used to prepare ultra thin tritium sources for the investigation o f the shape o f the beta-ray spectrum near the end point. The most important characteristics for such sources are, as discussed in a previous paper1), high intensity for a thickness less than 5 #g/cm 2 and stability o f the active deposit under v a c u u m in order to prevent fluctuations of intensity and contamination o f the spectrometer chamber. A l u m i n u m was chosen as the backing material because it is relatively easy to study the distributions o f ions as a function o f their penetration by the thin layer electrolytic peeling process described by Davies and al.2), and also because the ever present aluminum oxyde film is reputedly difficult to leak through3). The metal was first electropolished following the method
o f T e g a r P ) and oxydized at 1 0 0 V in an orthop h o s p h o r i c - c h r o m i u m trioxyde solution at 90°C. The backings are then cut to size, deoxydised again, and placed in the apparatus shown in fig. I which is p r o m p t l y evacuated. W h e n a v a c u u m o f less than 10 -3 Torr is obtained with the mechanical p u m p and the liquid nitrogen cooled trap, the outlet to the p u m p is closed and the tritium is allowed to leak into the discharge c h a m b e r which is kept connected to the trap. We assume that the system is reasonably free o f condensable impurities. A steady discharge is maintained at 600-900 V with a stabilizing resistor o f 100 MI2, while the gas pressure is kept between 1.2 to 0.2 Torr. The voltage is measured with a high impedance electrostatic voltmeter between the electrodes and f r o m the potential
TO
MECHANICALPUMP TUNGSTENE CENTER ELECTRODE
INLET
#'~AIR
DETAIL OF ELECTRODEI
..k"
LIQUID AIR "RAP MANOMETER
SLITS EOR B SOURCE
J~O-O~ ~'~t::~0
BACKINGS I
STA}NLESS STEEL SL ITTED CYLINDER
ELECTRODE
]Fig. 1. Arrangement of the glass apparatus used for the production of thin tritium sources by electrical discharge. The detail of the cylindrical electrode system inserted in the electrical discharge vessel is also shown. 187
188
R. DARIS AND C. ST-PIERRE
d r o p across the resistor, the charge and hence the a m o u n t o f ion d e p o s i t i o n is readily d e t e r m i n e d . Off the six sources p r e p a r e d in a single process, some samples were used for stability tests and ion range d i s t r i b u t i o n studies, the other being kept under v a c u u m until used in the spectrometer. The activity o f sources p r e p a r e d by gas discharge originates f r o m the gas a b s o r b e d on the surface o f the metal and from ions driven into the a l u m i n u m or its oxyde layer. A g o o d p a r t of the a b s o r b e d activity can be easily removed by immersion of the sources in water. Since the sources seemed very stable under v a c u u m even when the a b s o r b e d gas was not removed, we decided later on to t a k e a d v a n t a g e o f this fact in o r d e r to i m p r o v e on the low effective thickness. Finally, the heating of the sources at 100°C for several
40
30
O
20
,-t "O .r4 m 0)
~z 10
\ 0•0•0 i 2
References
~--- 0 _ . _ _ 0
I
I 4
I
hours in v a c u u m did not p r o d u c e a p p r e c i a b l e outgassing. The activity of the sample sources was m e a s u r e d with an ion c h a m b e r - e l e c t r o m e t e r system after successive electrolytic peelings o f 1.0 pg/cm 2. Fig. 2 gives the residual activity as a function o f the thickness r e m o v e d for a source p r e p a r e d by glow discharge at 700 V a n d washed in water p r i o r to the first a n o d i sation. The result o f the washing was a r e d u c t i o n o f the intensity by 64%. The thickness c o r r e s p o n d i n g to 90% o f the source activity is less t h a n 3/~g/cm 2. The remaining activity is distributed in an exponential tail as expectedS). Using the unwashed sources, the m a x i m u m counting rates were up to 700 c p m with our s p e c t r o m e t e r which has a 0.25% m o m e n t u m resolution a n d a 0.07% transmission. F o u r spectrometer runs were d o n e with these sources w i t h o u t any difficulty concerning fluctuation o f intensity or b a c k g r o u n d counting rate. A p r e l i m i n a r y analysis o f the results indicates t h a t sources p r e p a r e d by this m e t h o d meet the requirements for the determ i n a t i o n o f the shape o f the tritium b e t a spectrum near the end p o i n t at 0.25% m o m e n t u m resolution. The e x t r a p o l a t e d end p o i n t has been extracted f r o m the m e a s u r e m e n t s done so far a n d its value is 18.565___0.075 keV. F o r the calibration, we used the 238.6 keV K-line a n d the 39.85 keV Ll-Conversion lines in 212Bi and 2°8T1 corrected for the D o p p l e r effects as m e a s u r e d by Ewan 6) a n d also corrected for our i n s t r u m e n t a l distortion. A m o r e c o m p l e t e analysis o f the shape o f these spectra near the end p o i n t will be given in a f o r t h c o m i n g publication.
I
I
6
Thickness remqved (~g/cm 2) Fig. 2. Residual activity of a source prepared in a gaseous electrical discharge at 700 V vs the thickness removed by electrolytic peeling at 1.53 V. The initial activity of 104 arbitrary units was reduced to 37 by simple immersion in distilled water.
1) R. Daris and C. St-Pierre, Nucl. Instr. and Meth. 64 (1968) 346. 2) j . A . Davies, J. Friesen and J. D. Mclntyre, Can. J. Chem. 38 (1960) 1526. a) I. BergstrOm et al., Nucl. Instr. and Meth. 21 (1963) 249. 4) W.J. McG. Tegart, Electrolytic and chemical polishing of metals (Pergamon Press, London, 1956) p. 52. 5) j. A. Davies, J. Friesen and J. D. McIntyre, Can. J. Chem. 38 (1960) 1535. 6) G.T. Ewan, Can. J. Phys. 41 (1963) 2202.