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Design of a solid hydrogen target
The amount of H2 in the path of the particle beam is 0.08 g c m -2. In addition to the H2 along the beam path there are four organic glass windows, each 0.02 mm thick with 4 mm of insulation and 1 mm of liquid helium, amounting to 0.02 g c m -2 in total. By comparison with a polyethylene target of the same length, usually used in such experiments, a hydrogen target gives five times fewer background events for each true one.
Fig. 2
Target with H 2 gas space
w h i c h corresponds to a c o n s u m p t i o n o f ~ 2 cm 3 s -1 o f
liquid helium. It can be seen that the helium consumption should not exceed 250 cm 3 per freezing cycle. So to make the target design simple, the helium is let out into the atmosphere after flowing round the hydrogen space, and is not collected in a gas holder. An electric heater is used to help provide a helium flow to the target.
Tests carried out showed that the time for condensing H2 and the consumption of helium correspond to that expected, and the target is suitable for use in experiments to measure the magnetic moments of hyperons in high magnetic fields.
Reference 1
Baxkov,L.M., Zolotorev, M.S., Okhapkin, V.S. lnt Conf on Apparatus in High Energy Physics, Dubna, Vol 2 (1971) 603
Design of a solid hydrogen target Yu. V. Gorodkov, G.P. Eliseev, V.A. Lyubimov, and A.S Mileshkin
We have designed and built a hydrogen (deuterium) target in which liquid helium is used to cool the working volume. Since the normal boiling point of helium is below the melting temperatures of hydrogen and deuterium, the material in the working volume is solid. The general appearance of the target is shown in Fig. 1. The design is shown in Fig. 2. All the components of the target, except the nitrogen shield, are made of stainless steel. The working volume 10, placed inside the helium bath 9 (diameter 115, wall-thickness 0.3 mm), is a thin-walled cylinder (diameter 60, wall thickness 0.3 mm) with elliptical flanged end-plates, made of 0.3 mm thick foil, soldered on to it. The ends of the working volume are protected by copper thermal shields 11, soldered to the helium bath. The rims of the helium vessel and of the working volume are connected by corrugations.
fins for strength. There is multilayer thermal insulation of SBR glass fibre and M-foil between the jacket and the nitrogen shield. The target is protected by the A1 foil bursting membrane 3. The pressure in the working space is monitored with a
Copper shield, 8, cooled by liquid nitrogen was used to reduce the heat influx. The helium space is fixed inside the nitrogen shield with multiple supports of stainless steel and teflon. The inner surface of the nitrogen shield is carefully polished. The side faces of the helium bath are covered with 0.01 mm thick copper foil. Charcoal absorbent 12 is fixed to the surface of the nitrogen space. The target jacket, a stainless steel tube (220 mm diameter, 0.5 mm wallthickness) is provided with corrugations and longitudinal Prib i Tekh ~ksper No 1 (1976) 20. Received 13 January 1976.
C R Y O G E N ICS . A U G U S T 1976
Fig. 1 General appearance of the target
503
~
removed from the helium space, and the helium space is filled with liquid helium. 3 2 14 4 5 6 7
8
t3,
9
I0
(I
I
Fig, 2 Design of the target 1 - - helium return line; 2 - tube for filling the w o r k i n g volume; 3 - - safety membrane; 4 - helium filling tube; 5 - - helium vapour line; 6 - - nitrogen filling tube; 7 - nitrogen vapour line; 8 - nitrogen shield; 9 - helium space; 10 - - w o r k i n g volume; 11 - thermal shield; 12 -- charcoal absorbent; 13 - - pressure gauge; 14 - - tube connecting the helium vapour line to the gas-holder
vacuum gauge. The design of the target is such as to allow both horizontal (operating) and vertical (for convenience in transporting) positions.
Technical data of the target The length of the working space is 1000 mm, its diameter is 60 mm, and the volume 2.7 1. There is 0.65 g cm -2 of constructional material along the axis of the target. The helium bath volume is 7 1. The heat influx to the target in the steady state (determined experimentally) is 0.09 W (3.1 I of liquid He per day). The overall dimensions (without the neck) are 220 x 220 x 1110 m m a n d the mass 25 kg. Before filling the thermal insulating space of the target, it is pumped for a day to a vacuum of 3 x 10 -3 torr. Afterwards the vacuum is maintained with the help of the charcoal absorbent. The absence of evaporation from the working space after it has been cooled by helium, converts the target into a transportable apparatus and allows it to be filled in a specially fitted room. We used the Institute of Theoretical and Experimental Physics liquefier, which has hydrogen and helium draw-offs to fill the target without transport dewars. The target is filled by condensing gaseous hydrogen or deuterium in the working space at the boiling point of liquid hydrogen. During the condensation, the helium space is connected to the hydrogen draw-off of the liquefier. The amount of condensed gas is determined with a GKF gas meter; the gas pressure in the" working space during condensation is maintained at 1.5 atm and the time spent on condensation is 1.5 hours. After the condensation has finished, the filling valve of the working space is shut, the hydrogen is
504
If all operations are carried out accurately, the filling proceeds without any surges in pressure in the working volume (Pmax ~ 1.3 atm). If the pressure increases to more than 1.5 atm, part of the gas can be returned to the gasholder through the gas-meter. At the end of the filling the target is ready for operation. The rate of helium evaporation from the helium space decreases with time (on the second day after condensing it is 5.1 1 of liquid helium per day and on the 15th day, 3.1 I per day). The change in evaporation rate agrees satisfactorily with the change in the ortho-para composition of the hydrogen in the working space. 1 The filled target can be taken to the place where it is to be used by any form of transport without any special shock-absorbing devices; the increase in liquid helium consumption in moving it in a lorry is not more than 0.1 1 for a 100 km journey. The experimental set-up in which the target is exposed is fitted with a blow-out line through which the hydrogen is discharged from the target at the end of the operation or in case of an emergency increase in pressure in the working space. RefiUing with helium can be carried out without shutting off the counting apparatus, which is important for reducing the time to collect the statistics. Refilling is from a SD-10G dewar. The end of the filling is determined from the indications of a sensitive manometer, measuring the pressure in the helium space. The evaporating helium fills a gas holder and is then compressed into a cylinder. The target was used in experiments to study rr'R scattering 2 and lr'p scattering at large momentum transfer. By reconstruc ting with the aid of a computer, the geometrical picture for interaction of primary rr' mesons with the working material of the target, the distribution of interaction points over the target volume was obtained. Analysing this distribution, we did not find any departure in the density and configuration of the working material from the values expected for the given amount of condensate at liquid helium temperature. The target made according to the design given is most suitable for studying reactions in which the angles of emission of secondary particles are close to 0 or 180 °. After changing the shield configurations, the target with solid hydrogen can be used for a wide range of experiments in which the accuracy in measuring cross-sections is not above a few percent. A change in the amount of condensed material and carrying out freezing of the condensate with the target in the vertical position can change the effective length of the working material. The target is reliable in operation, as shown by its use from 1969 to the present time.
The authors are very grateful to A.V. Belonogov and V.M. Dobrov for discussing the target design, and also to the staff of the ITEF hydrogen liquefier for its reliable operation.
References
1 2
The Properties of Liquid and Solid Hydrogen (Standards Publishing House, 1969) Babaev,A., Brachman, E., et al Phys Lett 33B (1970) 615
CRYOGENICS. AUGUST 1976
Fig. 2
Target with H 2 gas space
w h i c h corresponds to a c o n s u m p t i o n o f ~ 2 cm 3 s -1 o f
liquid helium. It can be seen that the helium consumption should not exceed 250 cm 3 per freezing cycle. So to make the target design simple, the helium is let out into the atmosphere after flowing round the hydrogen space, and is not collected in a gas holder. An electric heater is used to help provide a helium flow to the target.
Tests carried out showed that the time for condensing H2 and the consumption of helium correspond to that expected, and the target is suitable for use in experiments to measure the magnetic moments of hyperons in high magnetic fields.
Reference 1
Baxkov,L.M., Zolotorev, M.S., Okhapkin, V.S. lnt Conf on Apparatus in High Energy Physics, Dubna, Vol 2 (1971) 603
Design of a solid hydrogen target Yu. V. Gorodkov, G.P. Eliseev, V.A. Lyubimov, and A.S Mileshkin
We have designed and built a hydrogen (deuterium) target in which liquid helium is used to cool the working volume. Since the normal boiling point of helium is below the melting temperatures of hydrogen and deuterium, the material in the working volume is solid. The general appearance of the target is shown in Fig. 1. The design is shown in Fig. 2. All the components of the target, except the nitrogen shield, are made of stainless steel. The working volume 10, placed inside the helium bath 9 (diameter 115, wall-thickness 0.3 mm), is a thin-walled cylinder (diameter 60, wall thickness 0.3 mm) with elliptical flanged end-plates, made of 0.3 mm thick foil, soldered on to it. The ends of the working volume are protected by copper thermal shields 11, soldered to the helium bath. The rims of the helium vessel and of the working volume are connected by corrugations.
fins for strength. There is multilayer thermal insulation of SBR glass fibre and M-foil between the jacket and the nitrogen shield. The target is protected by the A1 foil bursting membrane 3. The pressure in the working space is monitored with a
Copper shield, 8, cooled by liquid nitrogen was used to reduce the heat influx. The helium space is fixed inside the nitrogen shield with multiple supports of stainless steel and teflon. The inner surface of the nitrogen shield is carefully polished. The side faces of the helium bath are covered with 0.01 mm thick copper foil. Charcoal absorbent 12 is fixed to the surface of the nitrogen space. The target jacket, a stainless steel tube (220 mm diameter, 0.5 mm wallthickness) is provided with corrugations and longitudinal Prib i Tekh ~ksper No 1 (1976) 20. Received 13 January 1976.
C R Y O G E N ICS . A U G U S T 1976
Fig. 1 General appearance of the target
503
~
removed from the helium space, and the helium space is filled with liquid helium. 3 2 14 4 5 6 7
8
t3,
9
I0
(I
I
Fig, 2 Design of the target 1 - - helium return line; 2 - tube for filling the w o r k i n g volume; 3 - - safety membrane; 4 - helium filling tube; 5 - - helium vapour line; 6 - - nitrogen filling tube; 7 - nitrogen vapour line; 8 - nitrogen shield; 9 - helium space; 10 - - w o r k i n g volume; 11 - thermal shield; 12 -- charcoal absorbent; 13 - - pressure gauge; 14 - - tube connecting the helium vapour line to the gas-holder
vacuum gauge. The design of the target is such as to allow both horizontal (operating) and vertical (for convenience in transporting) positions.
Technical data of the target The length of the working space is 1000 mm, its diameter is 60 mm, and the volume 2.7 1. There is 0.65 g cm -2 of constructional material along the axis of the target. The helium bath volume is 7 1. The heat influx to the target in the steady state (determined experimentally) is 0.09 W (3.1 I of liquid He per day). The overall dimensions (without the neck) are 220 x 220 x 1110 m m a n d the mass 25 kg. Before filling the thermal insulating space of the target, it is pumped for a day to a vacuum of 3 x 10 -3 torr. Afterwards the vacuum is maintained with the help of the charcoal absorbent. The absence of evaporation from the working space after it has been cooled by helium, converts the target into a transportable apparatus and allows it to be filled in a specially fitted room. We used the Institute of Theoretical and Experimental Physics liquefier, which has hydrogen and helium draw-offs to fill the target without transport dewars. The target is filled by condensing gaseous hydrogen or deuterium in the working space at the boiling point of liquid hydrogen. During the condensation, the helium space is connected to the hydrogen draw-off of the liquefier. The amount of condensed gas is determined with a GKF gas meter; the gas pressure in the" working space during condensation is maintained at 1.5 atm and the time spent on condensation is 1.5 hours. After the condensation has finished, the filling valve of the working space is shut, the hydrogen is
504
If all operations are carried out accurately, the filling proceeds without any surges in pressure in the working volume (Pmax ~ 1.3 atm). If the pressure increases to more than 1.5 atm, part of the gas can be returned to the gasholder through the gas-meter. At the end of the filling the target is ready for operation. The rate of helium evaporation from the helium space decreases with time (on the second day after condensing it is 5.1 1 of liquid helium per day and on the 15th day, 3.1 I per day). The change in evaporation rate agrees satisfactorily with the change in the ortho-para composition of the hydrogen in the working space. 1 The filled target can be taken to the place where it is to be used by any form of transport without any special shock-absorbing devices; the increase in liquid helium consumption in moving it in a lorry is not more than 0.1 1 for a 100 km journey. The experimental set-up in which the target is exposed is fitted with a blow-out line through which the hydrogen is discharged from the target at the end of the operation or in case of an emergency increase in pressure in the working space. RefiUing with helium can be carried out without shutting off the counting apparatus, which is important for reducing the time to collect the statistics. Refilling is from a SD-10G dewar. The end of the filling is determined from the indications of a sensitive manometer, measuring the pressure in the helium space. The evaporating helium fills a gas holder and is then compressed into a cylinder. The target was used in experiments to study rr'R scattering 2 and lr'p scattering at large momentum transfer. By reconstruc ting with the aid of a computer, the geometrical picture for interaction of primary rr' mesons with the working material of the target, the distribution of interaction points over the target volume was obtained. Analysing this distribution, we did not find any departure in the density and configuration of the working material from the values expected for the given amount of condensate at liquid helium temperature. The target made according to the design given is most suitable for studying reactions in which the angles of emission of secondary particles are close to 0 or 180 °. After changing the shield configurations, the target with solid hydrogen can be used for a wide range of experiments in which the accuracy in measuring cross-sections is not above a few percent. A change in the amount of condensed material and carrying out freezing of the condensate with the target in the vertical position can change the effective length of the working material. The target is reliable in operation, as shown by its use from 1969 to the present time.
The authors are very grateful to A.V. Belonogov and V.M. Dobrov for discussing the target design, and also to the staff of the ITEF hydrogen liquefier for its reliable operation.
References
1 2
The Properties of Liquid and Solid Hydrogen (Standards Publishing House, 1969) Babaev,A., Brachman, E., et al Phys Lett 33B (1970) 615
CRYOGENICS. AUGUST 1976