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A refrigerated gas target apparatus for 180° electron scattering
NUCLEAR
INSTRUMENTS
AND
M E T H O D S 77
(I97O) I36-14o;
©
NORTH-HOLLAND
PUBLISHING
CO.
A REFRIGERATED GAS TARGET A P P A R A T U S FOR 180 ° E L E C T R O N SCATTERING L. W. F A G G , E. C. J O N E S , Jr. and W. L. B E N D E L
Naval Research Laboratory, Washington, D.C. 20390, U.S.A. Received 16 September 1969 A gas target system for use with 180 ° electron scattering experiments is described. The target gas is cooled to 77 °K and maintained at about 4.4 atm with 6 l~m havar foils separating the gas from the v a c u u m of the main target chamber. A gas transfer
feature using liquid helium trapping is employed. The successful use of this apparatus in experiments with 2°Ne, ~He, and 4He is discussed. Salient features of an improved gas target design, now being tested, are also given.
1. Introduction
2. Gas transfer system
Although several gas target systems 1"5) have found use in experiments in electron scattering at angles less than 180 °, the first used in 180 ° electron scattering was an uncooled device developed by Barber et al.6). The characteristics of 180 ° scattering are sufficiently unique to necessitate a distinctly different design from that needed for other angles. An advantage of such a design is that, since the incident and 180°-scattered electron directions are along essentially the same line, a very simple cylindrical geometry with thin windows (foils) only at the cylinder ends can be used.
Among the primary considerations in the initial design of the presently operating system was the requirement that it be capable of transferring and storing valuable isotopically enriched gases with a minimum of loss. Since the gases are generally delivered to the laboratory at near atmospheric pressure and the desired cooled target pressure is usually about 4 or 5 arm, it is necessary to pressurize the gas. Also a favorable ratio of gas target volume to storage volume must be provided for. This can be accomplished by trapping out the gas from its original container into a low volume, high pressure container using liquid helium. In the present apparatus this container is a 2 m long, small bore (1 m m i.d.), piece of stainless steel tubing capped off at one end. The desired volume ratio is also enhanced by minimizing the i.d. of all the delivery tubing.
The length and diameter of the cylinder are governed by many considerations including 1)the acceptable gas target to foil thickness ratio, 2) the degree to which loss of accurate spectrometer focus can be tolerated as a result of the target elongation, 3) keeping double scatterings which add to 180 ° below a tolerable level, 4) whether the gas is to be cooled, and 5) the amount of gas available. A compromise generally has to be found between these criteria. The advantage of refrigeration is self-evident. With the liquid nitrogen cooling, a density gain of about a factor of three can be achieved at a given pressure. However, if cooling is undertaken the diameter of the cylinder cannot be too large; otherwise, the cooling will not be effective along the central axis where the beam passes. On the other hand, the diameter cannot be made too small because of double-scattering adding to 180 ° , primarily arising from generally forward scattering in the first foil and in the gas followed by generally backward scattering from the walls, particularly at the downstream end of the gas cylinder. In the succeeding sections we describe the apparatus that was originally designed and successfully tested v) four years ago at the Naval Research Laboratory and placed back in operation over a year ago. A discussion & i t s recent use with isotopes of helium and neon and of an improved design, now under construction, is given. 136
A semi-perspective diagram of the entire apparatus is presented in fig. 1. The procedure of transfer is quite straight forward. After the whole system except for the original gas container is evacuated, the junction manifold valves to the vacuum and to the target chamber are closed while the remaining two valves are opened. The end of the high pressure tubing is placed in a helium dewar and the gas from the original container is trapped into the tubing. After the original container and high pressure tubing valves are closed, the tubing is withdrawn from the dewar and the gas therein allowed to rise to room temperature and high pressure ( ~ 50 atm). Transfer to the target chamber is accomplished by first opening the manifold valve to the target chamber. Then the manifold valve to the high pressure tubing, which is a needle valve, is opened slightly, allowing gas to pass slowly to the target chamber. Both the tubing and target valves can be adjusted and the desired flow to fill the target chamber can be obtained. Clearly this must be done cautiously. When a data-
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138
L . W . FAGG et al.
taking run is completed, the gas in the target chamber can again be trapped out using the helium dewar and stored at r o o m temperature in the high pressure tubing. It is generally considered advisable to let a little gas into the target chamber before it is cooled in order that the foils assume a shape approximating their ultimate bowed out configuration. However, this procedure has been occasionally neglected with no ill effects. The liquid nitrogen is furnished to the target chamber from the reservoir by means of gravity (see fig. I). The supply in the reservoir is replenished automatically from a large dewar by means of air pressure. The temperature sensitivity of a resistive element placed near the bottom of the reservoir furnishes the signal that actuates the solenoid air valve connected to the dewar. The current through another circuit containing two resistors (fig. 1) serves as an indication of how full the reservoir is by means of a calibrated ammeter on the control panel.
3. Gas target A cross-sectional diagram of the gas target assembly proper is shown in fig. 2. The present device was designed to yield target thicknesses comparable to those used at this laboratory for solid targets ( ~ 7 0 m g / c m 2) with an effective thickness ratio of target to foils of roughly about seven. Basing our calculations on neon, this results in a choice of 5 cm length for the target gas chamber. Since some of the gases to be bombarded were expensive, the original design diameter was effectively diminished to 1.1 cm by a copper liner. As it has turned out, most experiments using the apparatus thus far have been conducted with the liner in. The 6/tin foils are held between two copper gaskets. The gasket surfaces facing the foil are indium coated. The foils are operated with a safety factor of about 1.25 on the average since they burst at about 5.5 atm for a 1.1 cm chamber diameter. Of course, it is vital that the linac be equipped with an automatic safety valve to provide for foil rupture or other vacuum defects. However, in our experience foil ruptures have been rare. Since, in most cases, the chamber diameter was so small, considerable care had to be taken in properly aligning the beam as it passed through the chamber. This was done with the help of a retractable scintillator 12 cm in front of the chamber in conjunction with a "count minimizing" technique. That is, there is a small range of beam-steering settings where the wall
scattering is minimum and essentially constant, as reflected by a similar behaviour in the counting rate. The center of this range is determined and the beam is set there. This procedure is repeated at intervals during a run since the position of the spectrometer, through its fringe field, can affect slightly the direction of the incoming beam. An important concern in the use of the present system with the copper liner in place was that, despite careful beam alignment, the relative closeness of the geometry would lead to significant backscattering especially from the downstream portion of the gas chamber walls. As mentioned earlier, one source of this backscattering is that arising from the electrons scattered at a sufficiently large forward angle in the first foil and then backscattered by the walls. This contribution can be accounted for by taking a spectrum with the chamber evacuated, i.e. a spectrum of the foils alone. But subtracting this spectrum from that with gas in the chamber, although a necessary part of the experimental procedure, still does not account for the additional wall backscattering from electrons which are first scattered at large enough forward angles by the gas itself. An attempt to measure this contribution was made by determining the linearity of the intensity of the elastic peak of the gas as a function of the gas pressure. By means of this experiment, as well as similar ones on selected portions of the inelastic spectrum, we were unable to detect any such contribution. On the other hand, by removing the copper liner (see fig. 2) and thus increasing the radius of the gas cylinder from 1.1 to 3.8 cm, we found that a broad "ghost" peak in the neon spectrum at about 3 MeV excitation was virtually eliminated. With the liner inserted, this peak is observable in the spectrum obtained from 56 MeV electron bombardment (see fig. 4). However, the peak is more pronounced in the spectrum resulting from 39 MeV bombardment. This is the only wall backscattering effect we definitely detected with this technique. We believe there may be other such instrumental effects that have not yet been isolated. However, in principle these can always be corrected for by appropriate subtraction techniques using neighboring isotopes. As yet no such backscattering phenomena have been apparent in the helium experiments. Thus, as would be expected, at the same gas target pressure such effects increase with increasing density. Another concern affecting the experimental results was the accuracy with which the gas density could be determined through a knowledge of the gauge pressure
A REFRIGERATED
GAS
ELECTRON
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139
APPARATUS
SCATTERING FROM ~He AND4He Eo = 5 6 6 MeV, 8 = 178.9"
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1200 36 i
SCATTERED ELECTRON ENERGY (MeV) 45 50
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180°ELECTRON SCATTERING By 2ONe Eo=560
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o f e l e c t r o n s s c a t t e r e d at 180 ° f r o m 2 ° N e w i t h a n i n c i d e n t k i n e t i c e n e r g y o f 56 M e V .
of
140
L . W . FAGG et al.
and liquid nitrogen t e m p e r a t u r e alone. The gas pressure was observed to increase with b e a m current due to heating o f the n e o n by the b e a m , a n d the c o u n t i n g rate to decrease by r o u g h l y 2 % p e r / t a . Again, as w o u l d be expected, such effects were much w e a k e r with helium. A l t h o u g h calculations were m a d e to correct for this t e m p e r a t u r e increase, the p l a n n e d installation o f a t h e r m o c o u p l e should i m p r o v e the accuracy of this correction.
4. Use in experiments The a p p a r a t u s discussed here has thus far been successfully used in elastic and inelastic scattering experiments on 3He, 4He, a n d 2°Ne. The complete spectra 8) o b t a i n e d f r o m b o t h the 3He a n d CHe with an incident t o t a l electron energy o f 56.6 MeV is presented in fig. 3. The striking intensity difference in the two elastic peaks is explained by the c o m b i n a t i o n of the low Z and the zero g r o u n d state magnetic m o m e n t of 4He. The c o n s i d e r a b l e inelastic c o n t i n u u m structure in the 3He spectrum, due mostly to magnetic b r e m s s t r a h l u n g in the low excitation region a n d to 2- a n d 3 - b o d y b r e a k u p a b o v e a b o u t 5.5 a n d 7.7 MeV respectively, is largely the result of an M1 interactionS). The spectrum 9) o b t a i n e d from 2°Ne at 56 MeV incident electron kinetic energy is shown in fig. 4. The obvious o u t s t a n d i n g feature o f this s p e c t r u m is the very strong c o n c e n t r a t i o n o f the MI strength in the peak at 11.2 MeV, a r a t h e r singular e x a m p l e o f the t h e o r y o f K u r a t h 1°) a p p l i e d to self-conjugate s-d shell nuclei. As m e n t i o n e d a b o v e all of these spectra have had the foil spectrum subtracted from them.
5. Planned improved design We have now begun testing a revised gas target
c h a m b e r which will i n c o r p o r a t e m a n y i m p r o v e m e n t s which were implied as desirable from the a b o v e discussion. Firstly, a s o m e w h a t larger d i a m e t e r (1.9 cm) will be used in o r d e r to diminish the 3 M e V " g h o s t " peak. Secondly, the thickness of the c h a m b e r walls along the length o f the c h a m b e r , as well as the central region of the d o w n s t r e a m end plate, will be greatly diminished in o r d e r to further reduce unwanted backscattering. A t h e r m o c o u p l e will be installed in the gas just out o f the b e a m for t e m p e r a t u r e measurement. Larger d i a m e t e r t u b i n g for the liquid nitrogen delivery will be used in o r d e r to avoid occasional ice (water) blockages. Such blockages during the t r a p p i n g process in the high pressure t u b i n g will also be reduced by increasing ~he d i a m e t e r of the end section of the tubing (the p o r t i o n inserted into the liquid helium).
References l) R. w. McAllister and R. Hofstadter, Phys. Rev. 102 (1956) 851. 2) j. p. Repellin, P. Lehmann, J. Lefrancois and D. B. Isabelle, Phys. Letters 16 (1965) 169. 3) G. R. Burleson and H. W. Kendall, Nucl. Phys. 19 (1960) 68. 4) R.P. Singhal, H. Purdie, A. Cave, E. Pearce and H.S. Caplan, Nucl. Instr. and Meth. 73 (1969) 237. 5) H. Collard, R. Hofstadter, E.B. Hughes, A. Johansson, M. R. Yearian, R. B. Day and R. T. Wagner, Phys. Rev. 138 (1965) B57. 6) W. C. Barber, J. Goldemberg, G. A. Peterson and Y. Torizuka, Nucl. Phys. 41 (1963) 461. 7) L. W. Fagg, W. L. Bendel, R. A. Tobin, T. F. Godlove anti H. F. Kaiser, Report of NRL Progress (May 1966) p. I. ~) B.T. Chertok, E. C. Jones, W. L. Bendel and L. W. Fagg, Phys. Rev. Letters 23 (1969) 34. u) W. L. Bendel, L. W. Fagg, E. C. Jones, Jr., H. F. Kaiser and S. Numrich, Bull. Am. Phys. Soc. 13 (19681 1373. 10) D. Kurath, Phys. Rev. 130 (1963) 1525.
INSTRUMENTS
AND
M E T H O D S 77
(I97O) I36-14o;
©
NORTH-HOLLAND
PUBLISHING
CO.
A REFRIGERATED GAS TARGET A P P A R A T U S FOR 180 ° E L E C T R O N SCATTERING L. W. F A G G , E. C. J O N E S , Jr. and W. L. B E N D E L
Naval Research Laboratory, Washington, D.C. 20390, U.S.A. Received 16 September 1969 A gas target system for use with 180 ° electron scattering experiments is described. The target gas is cooled to 77 °K and maintained at about 4.4 atm with 6 l~m havar foils separating the gas from the v a c u u m of the main target chamber. A gas transfer
feature using liquid helium trapping is employed. The successful use of this apparatus in experiments with 2°Ne, ~He, and 4He is discussed. Salient features of an improved gas target design, now being tested, are also given.
1. Introduction
2. Gas transfer system
Although several gas target systems 1"5) have found use in experiments in electron scattering at angles less than 180 °, the first used in 180 ° electron scattering was an uncooled device developed by Barber et al.6). The characteristics of 180 ° scattering are sufficiently unique to necessitate a distinctly different design from that needed for other angles. An advantage of such a design is that, since the incident and 180°-scattered electron directions are along essentially the same line, a very simple cylindrical geometry with thin windows (foils) only at the cylinder ends can be used.
Among the primary considerations in the initial design of the presently operating system was the requirement that it be capable of transferring and storing valuable isotopically enriched gases with a minimum of loss. Since the gases are generally delivered to the laboratory at near atmospheric pressure and the desired cooled target pressure is usually about 4 or 5 arm, it is necessary to pressurize the gas. Also a favorable ratio of gas target volume to storage volume must be provided for. This can be accomplished by trapping out the gas from its original container into a low volume, high pressure container using liquid helium. In the present apparatus this container is a 2 m long, small bore (1 m m i.d.), piece of stainless steel tubing capped off at one end. The desired volume ratio is also enhanced by minimizing the i.d. of all the delivery tubing.
The length and diameter of the cylinder are governed by many considerations including 1)the acceptable gas target to foil thickness ratio, 2) the degree to which loss of accurate spectrometer focus can be tolerated as a result of the target elongation, 3) keeping double scatterings which add to 180 ° below a tolerable level, 4) whether the gas is to be cooled, and 5) the amount of gas available. A compromise generally has to be found between these criteria. The advantage of refrigeration is self-evident. With the liquid nitrogen cooling, a density gain of about a factor of three can be achieved at a given pressure. However, if cooling is undertaken the diameter of the cylinder cannot be too large; otherwise, the cooling will not be effective along the central axis where the beam passes. On the other hand, the diameter cannot be made too small because of double-scattering adding to 180 ° , primarily arising from generally forward scattering in the first foil and in the gas followed by generally backward scattering from the walls, particularly at the downstream end of the gas cylinder. In the succeeding sections we describe the apparatus that was originally designed and successfully tested v) four years ago at the Naval Research Laboratory and placed back in operation over a year ago. A discussion & i t s recent use with isotopes of helium and neon and of an improved design, now under construction, is given. 136
A semi-perspective diagram of the entire apparatus is presented in fig. 1. The procedure of transfer is quite straight forward. After the whole system except for the original gas container is evacuated, the junction manifold valves to the vacuum and to the target chamber are closed while the remaining two valves are opened. The end of the high pressure tubing is placed in a helium dewar and the gas from the original container is trapped into the tubing. After the original container and high pressure tubing valves are closed, the tubing is withdrawn from the dewar and the gas therein allowed to rise to room temperature and high pressure ( ~ 50 atm). Transfer to the target chamber is accomplished by first opening the manifold valve to the target chamber. Then the manifold valve to the high pressure tubing, which is a needle valve, is opened slightly, allowing gas to pass slowly to the target chamber. Both the tubing and target valves can be adjusted and the desired flow to fill the target chamber can be obtained. Clearly this must be done cautiously. When a data-
A REFRIGERATED
AUTOMATIC REFILL CIRCUIT 1
GAS TARGET
LEVEL ,,--.-INDICATOR
_ _
/CIRCUIT
I
TO
i ~ VACUUM L'
GAUGE I G I
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JUNCTION MANIFOLD
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GAS CONTAINER
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APPARATUS
IQUIO I HELIUM ] DEWAR L
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Fig. I. Semi-perspective diagram o f the entire gas target apparatus. T h e dashed lines represent electric wiring.The high pressure t u b i n g is s h o w n inserted into the liquid helium dewar.
7PRESSURE GAUGE
~- TO VACUUM,•L_r_T ~GL__ L ~ II_nA~ GAS
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Fig. 2. T w o views o f the gas target assembly proper. O n the left is a cross-sectional side view and on the right is an exterior (end) view (looking u p s t r e a m along the b e a m axis).
138
L . W . FAGG et al.
taking run is completed, the gas in the target chamber can again be trapped out using the helium dewar and stored at r o o m temperature in the high pressure tubing. It is generally considered advisable to let a little gas into the target chamber before it is cooled in order that the foils assume a shape approximating their ultimate bowed out configuration. However, this procedure has been occasionally neglected with no ill effects. The liquid nitrogen is furnished to the target chamber from the reservoir by means of gravity (see fig. I). The supply in the reservoir is replenished automatically from a large dewar by means of air pressure. The temperature sensitivity of a resistive element placed near the bottom of the reservoir furnishes the signal that actuates the solenoid air valve connected to the dewar. The current through another circuit containing two resistors (fig. 1) serves as an indication of how full the reservoir is by means of a calibrated ammeter on the control panel.
3. Gas target A cross-sectional diagram of the gas target assembly proper is shown in fig. 2. The present device was designed to yield target thicknesses comparable to those used at this laboratory for solid targets ( ~ 7 0 m g / c m 2) with an effective thickness ratio of target to foils of roughly about seven. Basing our calculations on neon, this results in a choice of 5 cm length for the target gas chamber. Since some of the gases to be bombarded were expensive, the original design diameter was effectively diminished to 1.1 cm by a copper liner. As it has turned out, most experiments using the apparatus thus far have been conducted with the liner in. The 6/tin foils are held between two copper gaskets. The gasket surfaces facing the foil are indium coated. The foils are operated with a safety factor of about 1.25 on the average since they burst at about 5.5 atm for a 1.1 cm chamber diameter. Of course, it is vital that the linac be equipped with an automatic safety valve to provide for foil rupture or other vacuum defects. However, in our experience foil ruptures have been rare. Since, in most cases, the chamber diameter was so small, considerable care had to be taken in properly aligning the beam as it passed through the chamber. This was done with the help of a retractable scintillator 12 cm in front of the chamber in conjunction with a "count minimizing" technique. That is, there is a small range of beam-steering settings where the wall
scattering is minimum and essentially constant, as reflected by a similar behaviour in the counting rate. The center of this range is determined and the beam is set there. This procedure is repeated at intervals during a run since the position of the spectrometer, through its fringe field, can affect slightly the direction of the incoming beam. An important concern in the use of the present system with the copper liner in place was that, despite careful beam alignment, the relative closeness of the geometry would lead to significant backscattering especially from the downstream portion of the gas chamber walls. As mentioned earlier, one source of this backscattering is that arising from the electrons scattered at a sufficiently large forward angle in the first foil and then backscattered by the walls. This contribution can be accounted for by taking a spectrum with the chamber evacuated, i.e. a spectrum of the foils alone. But subtracting this spectrum from that with gas in the chamber, although a necessary part of the experimental procedure, still does not account for the additional wall backscattering from electrons which are first scattered at large enough forward angles by the gas itself. An attempt to measure this contribution was made by determining the linearity of the intensity of the elastic peak of the gas as a function of the gas pressure. By means of this experiment, as well as similar ones on selected portions of the inelastic spectrum, we were unable to detect any such contribution. On the other hand, by removing the copper liner (see fig. 2) and thus increasing the radius of the gas cylinder from 1.1 to 3.8 cm, we found that a broad "ghost" peak in the neon spectrum at about 3 MeV excitation was virtually eliminated. With the liner inserted, this peak is observable in the spectrum obtained from 56 MeV electron bombardment (see fig. 4). However, the peak is more pronounced in the spectrum resulting from 39 MeV bombardment. This is the only wall backscattering effect we definitely detected with this technique. We believe there may be other such instrumental effects that have not yet been isolated. However, in principle these can always be corrected for by appropriate subtraction techniques using neighboring isotopes. As yet no such backscattering phenomena have been apparent in the helium experiments. Thus, as would be expected, at the same gas target pressure such effects increase with increasing density. Another concern affecting the experimental results was the accuracy with which the gas density could be determined through a knowledge of the gauge pressure
A REFRIGERATED
GAS
ELECTRON
TARGET
139
APPARATUS
SCATTERING FROM ~He AND4He Eo = 5 6 6 MeV, 8 = 178.9"
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o f e l e c t r o n s s c a t t e r e d at a n effective s c a t t e r i n g a n g l e o f 178.9 ~: f r o m 3 H e a n d 4 H e w i t h a n i n c i d e n t t o t a l e n e r g y 56.6 M e V .
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SCATTERED ELECTRON ENERGY (MeV) 45 50
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•
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o f e l e c t r o n s s c a t t e r e d at 180 ° f r o m 2 ° N e w i t h a n i n c i d e n t k i n e t i c e n e r g y o f 56 M e V .
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L . W . FAGG et al.
and liquid nitrogen t e m p e r a t u r e alone. The gas pressure was observed to increase with b e a m current due to heating o f the n e o n by the b e a m , a n d the c o u n t i n g rate to decrease by r o u g h l y 2 % p e r / t a . Again, as w o u l d be expected, such effects were much w e a k e r with helium. A l t h o u g h calculations were m a d e to correct for this t e m p e r a t u r e increase, the p l a n n e d installation o f a t h e r m o c o u p l e should i m p r o v e the accuracy of this correction.
4. Use in experiments The a p p a r a t u s discussed here has thus far been successfully used in elastic and inelastic scattering experiments on 3He, 4He, a n d 2°Ne. The complete spectra 8) o b t a i n e d f r o m b o t h the 3He a n d CHe with an incident t o t a l electron energy o f 56.6 MeV is presented in fig. 3. The striking intensity difference in the two elastic peaks is explained by the c o m b i n a t i o n of the low Z and the zero g r o u n d state magnetic m o m e n t of 4He. The c o n s i d e r a b l e inelastic c o n t i n u u m structure in the 3He spectrum, due mostly to magnetic b r e m s s t r a h l u n g in the low excitation region a n d to 2- a n d 3 - b o d y b r e a k u p a b o v e a b o u t 5.5 a n d 7.7 MeV respectively, is largely the result of an M1 interactionS). The spectrum 9) o b t a i n e d from 2°Ne at 56 MeV incident electron kinetic energy is shown in fig. 4. The obvious o u t s t a n d i n g feature o f this s p e c t r u m is the very strong c o n c e n t r a t i o n o f the MI strength in the peak at 11.2 MeV, a r a t h e r singular e x a m p l e o f the t h e o r y o f K u r a t h 1°) a p p l i e d to self-conjugate s-d shell nuclei. As m e n t i o n e d a b o v e all of these spectra have had the foil spectrum subtracted from them.
5. Planned improved design We have now begun testing a revised gas target
c h a m b e r which will i n c o r p o r a t e m a n y i m p r o v e m e n t s which were implied as desirable from the a b o v e discussion. Firstly, a s o m e w h a t larger d i a m e t e r (1.9 cm) will be used in o r d e r to diminish the 3 M e V " g h o s t " peak. Secondly, the thickness of the c h a m b e r walls along the length o f the c h a m b e r , as well as the central region of the d o w n s t r e a m end plate, will be greatly diminished in o r d e r to further reduce unwanted backscattering. A t h e r m o c o u p l e will be installed in the gas just out o f the b e a m for t e m p e r a t u r e measurement. Larger d i a m e t e r t u b i n g for the liquid nitrogen delivery will be used in o r d e r to avoid occasional ice (water) blockages. Such blockages during the t r a p p i n g process in the high pressure t u b i n g will also be reduced by increasing ~he d i a m e t e r of the end section of the tubing (the p o r t i o n inserted into the liquid helium).
References l) R. w. McAllister and R. Hofstadter, Phys. Rev. 102 (1956) 851. 2) j. p. Repellin, P. Lehmann, J. Lefrancois and D. B. Isabelle, Phys. Letters 16 (1965) 169. 3) G. R. Burleson and H. W. Kendall, Nucl. Phys. 19 (1960) 68. 4) R.P. Singhal, H. Purdie, A. Cave, E. Pearce and H.S. Caplan, Nucl. Instr. and Meth. 73 (1969) 237. 5) H. Collard, R. Hofstadter, E.B. Hughes, A. Johansson, M. R. Yearian, R. B. Day and R. T. Wagner, Phys. Rev. 138 (1965) B57. 6) W. C. Barber, J. Goldemberg, G. A. Peterson and Y. Torizuka, Nucl. Phys. 41 (1963) 461. 7) L. W. Fagg, W. L. Bendel, R. A. Tobin, T. F. Godlove anti H. F. Kaiser, Report of NRL Progress (May 1966) p. I. ~) B.T. Chertok, E. C. Jones, W. L. Bendel and L. W. Fagg, Phys. Rev. Letters 23 (1969) 34. u) W. L. Bendel, L. W. Fagg, E. C. Jones, Jr., H. F. Kaiser and S. Numrich, Bull. Am. Phys. Soc. 13 (19681 1373. 10) D. Kurath, Phys. Rev. 130 (1963) 1525.