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An electronic ion beam shutter
NUCLEAR
INSTRUMENTS
AND
METHODS
84 (i97o)
i49-i5o;
©
NORTH-HOLLAND
PUBLISHING
CO.
AN E L E C T R O M A G N E T I C I O N BEAM S H U T T E R A. S Z A L A Y and I. D O M B I
A TOMKI*, Debrecen, Hungary Received 23 March 1970 A mechanical ion b e a m shutter is described, which is operated by e l e c t r o m a g n e t s from outside o f v a c u u m . Its quick operation (0.07 s) and reliability d e m o n s t r a t e d its usefulness in n e u t r o n activation e x p e r i m e n t s connected with a 300 kV a n d 300/~A D b e a m o f a n e u t r o n generator.
In connection with experiments for the production of short period isotopes, by means of a n-generator of a 300 kV and 300-500/~A D beam, an electromechanical shutter was developed for the quick periodical interruption of the deuteron beam. The following technical difficulties arise in solving this problem:
c. The shutter must be quick and so the moving parts should have a small mass. d. The moving part of the shutter has to be activated from the outside by an electronic-automatic control, connected with the activation program. We solved the mentioned controversial problems in the following way: We have chosen 0.3 m m thick tantalum for the ion beam capturing sheet. The D beam is usually well focused and a small focal spot destroys even tantalum, when impinging perpendicularly. It is advantageous to divide the beam energy between two tantal sheets which shut the beam from two opposite sides. Further it is advisable to let the ion beam impinge under an oblique angle and so to distribute the energy over an extended elliptical spot. The Ta sheet is still heated to a
a. The beam capturing shield of the shutter is exposed to the bombardment of the ion beam and must dissipate an energy flux of about 100-150 W. b. The ion beam capturing metal is exposed to the sputtering effect of the beam.
* Institute o f Nuclear Research o f the H u n g a r i a n A c a d e m y o f Science, Debrecen l, P.O.B. 51, Hungary.
I
t
-
.
I~ ---
~-~,_-*-
/
~ /
~ ~L.~
/ Fig. 1. Principle o f the ion b e a m shutter. Above: " b e a m o n " ; below: " b e a m s h u t " position; right: iron yoke.
149
150
A. S Z A L A Y A N D I. D O M B I
.
1......L.....L
............
k ............
.
.
.
.
.
.
L......L ......
i l Fig. 2. The actual design o f the ion beam shutter (simplified, without electromagnets).
high temperature, which is sufficient for degassing and for to avoid D + D selfactivation. Fig. 1 demonstrates the principle of the ion beam shutter. An iron frame (yoke) is supporting the two Ta sheets. The motion of this yoke is rotating and it is operated by two U shaped electromagnets from the outside of the vacuum from the axial ends of the yoke (not shown on the figures). One of the two magnets keeps it in " b e a m o n " position, the other in " b e a m shut" position. The heat developed by the ion beam energy is dissipated largely by radiation. Fig. 2 demonstrates the actual design of the shutter, which is to be inserted into the ion beam channel. The tube and flanges are made of copper and are water cooled. Some additional cooling by heat conduction is given to the iron yoke which touches two copper blocks hardsoldered to the inside of the copper tube. This is advisable in order to avoid an overheating of the iron frame above the Curie-temperature.
It should be mentioned here that at first we tried to keep the iron frame in one stable (e.g. shut) position by means of a steel spring. Unfortunately these attempts failed, because the strong radiant heat annealed the springs. The shutter operates very quickly because the moving mass is small. The " b e a m o n " ~ " b e a m shut" operation takes about 0.07 s. We have used this shutter for six months, altogether for about 250 operating hours with a beam of 300 kV and a current of 300/tA. No breakdown or trouble was encountered during this operating period. It is operated in connection with a pneumatic rabbit, in carrying out short activations and measurements, with automatic collection of the data from many repeated operations. The ion source and accelerator tube are kept in constant continuous operation and so free of relaxation effects. The interruption of the ion beam during the activity measurements reduces the background of the detectors to a very large extent.
INSTRUMENTS
AND
METHODS
84 (i97o)
i49-i5o;
©
NORTH-HOLLAND
PUBLISHING
CO.
AN E L E C T R O M A G N E T I C I O N BEAM S H U T T E R A. S Z A L A Y and I. D O M B I
A TOMKI*, Debrecen, Hungary Received 23 March 1970 A mechanical ion b e a m shutter is described, which is operated by e l e c t r o m a g n e t s from outside o f v a c u u m . Its quick operation (0.07 s) and reliability d e m o n s t r a t e d its usefulness in n e u t r o n activation e x p e r i m e n t s connected with a 300 kV a n d 300/~A D b e a m o f a n e u t r o n generator.
In connection with experiments for the production of short period isotopes, by means of a n-generator of a 300 kV and 300-500/~A D beam, an electromechanical shutter was developed for the quick periodical interruption of the deuteron beam. The following technical difficulties arise in solving this problem:
c. The shutter must be quick and so the moving parts should have a small mass. d. The moving part of the shutter has to be activated from the outside by an electronic-automatic control, connected with the activation program. We solved the mentioned controversial problems in the following way: We have chosen 0.3 m m thick tantalum for the ion beam capturing sheet. The D beam is usually well focused and a small focal spot destroys even tantalum, when impinging perpendicularly. It is advantageous to divide the beam energy between two tantal sheets which shut the beam from two opposite sides. Further it is advisable to let the ion beam impinge under an oblique angle and so to distribute the energy over an extended elliptical spot. The Ta sheet is still heated to a
a. The beam capturing shield of the shutter is exposed to the bombardment of the ion beam and must dissipate an energy flux of about 100-150 W. b. The ion beam capturing metal is exposed to the sputtering effect of the beam.
* Institute o f Nuclear Research o f the H u n g a r i a n A c a d e m y o f Science, Debrecen l, P.O.B. 51, Hungary.
I
t
-
.
I~ ---
~-~,_-*-
/
~ /
~ ~L.~
/ Fig. 1. Principle o f the ion b e a m shutter. Above: " b e a m o n " ; below: " b e a m s h u t " position; right: iron yoke.
149
150
A. S Z A L A Y A N D I. D O M B I
.
1......L.....L
............
k ............
.
.
.
.
.
.
L......L ......
i l Fig. 2. The actual design o f the ion beam shutter (simplified, without electromagnets).
high temperature, which is sufficient for degassing and for to avoid D + D selfactivation. Fig. 1 demonstrates the principle of the ion beam shutter. An iron frame (yoke) is supporting the two Ta sheets. The motion of this yoke is rotating and it is operated by two U shaped electromagnets from the outside of the vacuum from the axial ends of the yoke (not shown on the figures). One of the two magnets keeps it in " b e a m o n " position, the other in " b e a m shut" position. The heat developed by the ion beam energy is dissipated largely by radiation. Fig. 2 demonstrates the actual design of the shutter, which is to be inserted into the ion beam channel. The tube and flanges are made of copper and are water cooled. Some additional cooling by heat conduction is given to the iron yoke which touches two copper blocks hardsoldered to the inside of the copper tube. This is advisable in order to avoid an overheating of the iron frame above the Curie-temperature.
It should be mentioned here that at first we tried to keep the iron frame in one stable (e.g. shut) position by means of a steel spring. Unfortunately these attempts failed, because the strong radiant heat annealed the springs. The shutter operates very quickly because the moving mass is small. The " b e a m o n " ~ " b e a m shut" operation takes about 0.07 s. We have used this shutter for six months, altogether for about 250 operating hours with a beam of 300 kV and a current of 300/tA. No breakdown or trouble was encountered during this operating period. It is operated in connection with a pneumatic rabbit, in carrying out short activations and measurements, with automatic collection of the data from many repeated operations. The ion source and accelerator tube are kept in constant continuous operation and so free of relaxation effects. The interruption of the ion beam during the activity measurements reduces the background of the detectors to a very large extent.