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A flowing gas target system for precision electron scattering measurements
Nuclear instruments and Methods 203 (1982) 97-100 North-Holland Publishing Company
97
A FLOWING GAS TARGET SYSTEM FOR PRECISION EL'.'CTRON SCATTERING MEASUREMENTS W.J . STAPOR and Hall CRANNELL Department of Physics, Catholic University of America,
Washington, D.C. 20064. USA
J.W . LIGHTBODY, Jr., and X.K . MARUYAMA National Bureau of Standards, Washington. D. C. 10134, USA
J.T . O'BRIEN
Department of Physics, Montgomery College, Rockville, Maryland 10850, USA
Received 19 April 1982 We describe a (lowing gas target system which can be used in various electron scattering experiments that require gasdensity stabilisation to onepart in 103. The main advantage of this system is that both target pressure and temperature are directly monitored during the data taking process. This eliminates the need to use extra beam time to measure the dependence of local target density ost beam current. The system has been successfully used in an elastic electron scattering experiment to obtain a precise measurement of the nuclear ground state rmscharge radius of 4He.
1. f nrothtction The passage of a well-focused electron beam through a gas target causes heating along the beam axis . This heating induces density gradients, which may be time dependent, that affect local target thickness . For gas mixtures, beam-induced heating also contributes to different; al gas diffusion (the Sorel effect) and molecular dissociation, thereby altering the effective target composition. Thesecomplications are enhanced in a sealed or static gas target since there is no effective way to alleviate temperature increase or to replenish a depleted gas mixture in the beam region. In precision cross section measurements it is important to reduce and/or accurately monitor the effects of the beam on the gas density. There have been previous investigations of this matter in electron scattering experiments with gas targets [1-41. Such efforts involved measuring the elastic peak area as a function of beam current. Although these methods have been successful, they can be tedious . consuming much valuable beam time. We describe a flowing gas system that both reduces these beam heating effects anddirectly monitors the gasdensity during the data taking, thus eliminating the need for extra beam time to determine the dependence of the local target density on beam current . The system described has been tested and used with hydrogen and with a gas mixture of 30% methane and 70% helium in an elastic scattering experiment to obtain 0167-5087/82/0000-0000/$02.75
0
1982 North-Holland
a precise measurement of the nuclear ground state charge radius of `He. Fcst we describe the target system and means to measure pressure and temperature- They we discuss the method of determining temperature in the target region without interfering with thebeam . 2. Target system description A schematic of the target system is shown in fig. I . All parts are connected with 0.635 cm inter diameter high-pressure rubber tubing. Under normal operatic, target gas flows at some selected pressure above os~ atmosphere from agas supply tank through a ftowmeter into ~ target cell mounted in the scattering chamber. rice Then it flows through a gasdensity stabilization called a pycuostat [51 andoutof the system . Thevacuum pump allows for sub-atmosphere andempty cell Option, and is used to purgethesystem when changing to a different target gas. The pressure throughout the target system is Mguschematically is lated by the pycnostat which is shown fig. 2. Target gas flows into the pyenostat at some pressure P, and out through an aperture of 1-S diameter. Pressure regulation is due to the action of a rubber piston on top of a bellows near the owllow aperture. The distance between the rubber piston aae! the aperture is related to the flow impedance andtegulutes the flow rate . Areference pressure. P is contained
98
If. J. Smpor rr al. / A
flowiRg gac mrgrt .st".wrm
rARET LELL
BEAM_LINE
TRERMISTORS
Fig. 1. Schematic of target system. ® indicates location of a valve, - indicates direction of flow when system is in operation . by the bellows. There is an equilibrium position of the bellows such that the target pressure P, = PZ . Any change in target uressumcauses small excursions of the bellows to ine.--ase or decrease the flow until P, becomes equal to PZ again. The range of flow rates within whichthepyrnostat will operate properly is determined by the size of the aperture. Larger flow rates require larger apertures. We have tested apertures with diameters ranging from 0.25 to 2.0 mm, The 1 .5 mm aperture enabled us to obtain a volume rate of 350 cm'/min. with pressure regulation to better than 0.1% at 125 kPa . The fact that the pycnostat requires gas flow to operate is an advantage. It is assumed that flow will
SIDE
Fig . 3. Schematic side view and top view of larget cell interior for experiment. Thermistor positions 1, 2, 5and6 are indicated along with three ports on top of cell. IN or OUT tells direction of gas flow relative to the target cell. reduce effects due to differential diffusion and molecular dissociation. These effects were calculated [6j and were small (<0.1%) in our experiment. Therefore, no quantitative data on the effect of flow wascollected. We use a high-precision (0.1%), wide-range (0-5000 Tort)capacitive manometer to monitor the pressure of the gas at a point between the target cell and the pycnostat. In this configuration we must consider the hydrostatic and viscous pressure differences (OPII and AP,) between the target cell and manometer. Forany mean pressure P, API ,/Pwascalculated to be <0.02% and is clearly negligible . However, I)Pv/P, which can be accurately calculated from Poiseuilleslaw, wasfound to be 0.5% and notas negligible. This correction could be applied to the pressure reading . The OPvcorrection could be substantially reduced by using larger diameter tubing between the target cell and manometer. The target cell in the system is a copy of the one described by Webb et al. [I] except that there have been two major modifications as shown in fig. 3. One is the construction of three ports on top of thecell to accomodate the pipes necessary for flow. Target gas enters through the two outer ports and exits through the middle port. The other modification is the installation of a four thermistor network to directly -sure the temperature profile of the gas. 3. Temperatureprofile measurement
Fig.2. Schematic of pycnostat. Arrows indican; direction of flow under normal operation . dis the distance between aperture (A) and robber piston (H). P, is the target system pressure. Pzis the'operating refemac"_ pressure set with valve (C) inside bellows (D).
A well-focused electron beam that passes through themiddle of the target cell acts as a line source of heat. It hasbeen shown that the variation in local gasdensity is insensitive to thetime structure of the electron beam [4J. We have measured the characteristics of such a heat
W.J .
Stapor et al.
/ A flowing gas
3 AODITIONAL THERMISTORS
SIDE
Fig. 4. Schematic side view of larget cell interior for calibrations. Additional thermistor positions 3, C, and 4 are shown with =24 awg nichrome wire orientation . source in the cell by installing a thin gauge (#24) nichrome heater wire along the beam axis and measuring temperature profiles forvarious gases versus power input . For this bench testing, the thermistor network included seven thermistors as shown in fig. 4. In this way we were able to obtain temperature measurements of the gas at various points to within I mm of the heat source. A typical set of temperature profiles is shown in 23 5
TOP
CENTER
BOTTOM
I CH.-He qas-turc ot 250 230
YI
22 .5
xPo
e A
-om -I4mw
Ô
- 25mw
zz o 2L5 F Q
a s w
W
2Io
I
205
2o
o
I95
Iso I .,
203mm III
12Tmm 121
I SImmlOmm 13) ICI
~Imm .1
POSITION OFTHERMISTOR
I2Tmn 151
203mm I61
Fig. 5. Temperature profiles for 0, 14, 56 and 12.° IoW power inputs. These correspond to tae energy deposit from 0. 0.5, 2, and 4 pA average beam current, respectively, through die CH,-He gas mixture al 250kPa.
target srsterw
fig. 5 for several power levels into the CH,--He gas mixture at 250 kPa with avolume rate of 350cmj/minThe temperature increase in the top part of the cell above the temperature at zero-power input is duemostly to convection. This was verified by measuring profiles for alternate orientations of the target cell . The region above the source always showed thegreater temperature increase, while the region below showed very little change, with the bottom always near the zero-power temperature. We found that the temperature profiles were independent of gas pressure for pressures up to 300 kPa and for power inputs up to 1 .4W. which corresponds to the energy deposited from 40 )eA average beam current through the CHsHe gas mixture. The effect of the thermistors heat capacities on the temperature measurements was negligible. We verified this by measuring temperature profiles with two different sizes of thermistors. No appreciable differences in the temperature profiles were found. From profiles like these we were alite to develop a method for determining the temperature of the CH,He gas mixture near the target region knowing only the temperatures monitored by the outer thermistors- Such a method is essential since the three central thermistors must be removed from the cell to avoid interference with the electron beam (cf. fig. 3) during thescattering experiment. Since the bottom thermistor (position 6 in i&4) always registers a temperature close to the zero-power temperature, we use it to derive a baseline. The offset above the baseline temperature at any other thennistcr position is related to thepower input. This shown in fig. 6 where we have plotted the temperature offset above the baseline . A, versus position forthemeasured profiles of fig. 5. If beam power dissipation is 20 mW, then one expects the corm .pouding A profile to he between the 14 and the 56 mW curves. Therefore, we can interpolate to predict thetemperature offset above the baseline in the center of thecell. Ac . for anypower input . When we plot pt- versus A, (the temperature offset abovethebaseline at themstor positiorcs i = 1, 2) or power inputs up to 502 mW,as sMTwn in fig.?, we see that the data exhibit a relationship of theform AE_=C,A,+K where C, and K, are constants determialed by a least squares straight line fit to the me-rcd data. Saint fits were performed for each of the top two th. sto-, (positions 1, 2) in the ceIL 13y measuririx the tetttperature at the top two thermistor po itiovtc and at the baseline position, we can use these calibration fits to obtain the temperature of the target in the region .fthe beam . Both the target temperature and prc- anti Bitsu ; " d periodically while accumulating scatterin data. From these measurements we determine the detsity tit
W.J. Stapor et al. / A flowing gas targe-ystent
100
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OS
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En
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a
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25
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(BASELINE OEFINEO By T-111111 IT TNEBMISTON POSITION 6)
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POSITION OF
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5.1 141
T'Jt:astlsTOR
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Fig. 6. A at each thermistor position in target ell . A is the temperature difference between anythermistor and the base.The baseline is defined as the temperature at thermistor 6m position 6. the target region that is viewed by the detectors. We use this measurement to generate adensity correction to the scattered energy spectrum so that the scattered electron counts arenormalized to the average target density over the whole data run. For our case, the corrections are typically less than 0.1%, indicating density stabilization better than 0.1%. 71ie technique described can be applied to situations where the'effects of the beam on the gas density are much`larger as in experiments requiring higher beam currents andhigher target pressures. We would like to thank machinists, Mr . Bill Wartiuwö" - -., ï:UA, and Mr. Donald Bryant, NBS, for their skillful aid . We are especially grateful to Dr. Steve
Ai (i- 1 .21 ('C) Fig. 7. A c vs. A(, beamline temperature difference versus thermistor i temperature difference. Subscript i indicates thermistor positions 1 and 2. Dashed lines illustrate best fits to A,=C,A,+K, with C1 =4.65, Kh =0 .67o, C2=4.22 . and KZ = -0 .12°. Domen, NBS, for his helpful discussion and use of his lab and equipment in calibrating thermistors, to Dr. R.D. Deslatto, NBS, for lending us a pycnostat, and to Prof. G.A. Peterson, U. Mass., who provided us with a gas target cell of ref. 1 . '1his work was supported in part by the National &:ience Foundation under grant PHY7923968. References [11 D.V. Webb. G.A. Petersen, Z.M. Szalata and P.T. Kan. Nucl. Instr. andMeth. 120 (1974) 359 . [2) R.P. Singhal, H. Purdie, A. Cave, E. Pearce and H.S. Caplan, Nucl. Instr . andMeth . 73 (1969) 237 . [31 H. Frank, Ch. Schmidt, W. Schutz and H. Thei-, ZAED-Conf. 71-400-013-H13,(1972) 177. 141 G.G. Simon, Ch . Schmidt, F. Borkowski, C. Ostermann, V.H. Wallher, D. Bender and A. Von Gunten, Nuct. Instr. and Meth. 158 (1979) 185. [51 R.D. Deslattes, B.G . Simpson and R.E . LaVilla, Rev. of Sci. Instr . 37 (1966) 596. [61 J .O. Hirschfelder, C.F. Curtiss and R. ByronBird. Molecular theory of gases and liquids (John Wiley, New York. 1954) ch. 8.
97
A FLOWING GAS TARGET SYSTEM FOR PRECISION EL'.'CTRON SCATTERING MEASUREMENTS W.J . STAPOR and Hall CRANNELL Department of Physics, Catholic University of America,
Washington, D.C. 20064. USA
J.W . LIGHTBODY, Jr., and X.K . MARUYAMA National Bureau of Standards, Washington. D. C. 10134, USA
J.T . O'BRIEN
Department of Physics, Montgomery College, Rockville, Maryland 10850, USA
Received 19 April 1982 We describe a (lowing gas target system which can be used in various electron scattering experiments that require gasdensity stabilisation to onepart in 103. The main advantage of this system is that both target pressure and temperature are directly monitored during the data taking process. This eliminates the need to use extra beam time to measure the dependence of local target density ost beam current. The system has been successfully used in an elastic electron scattering experiment to obtain a precise measurement of the nuclear ground state rmscharge radius of 4He.
1. f nrothtction The passage of a well-focused electron beam through a gas target causes heating along the beam axis . This heating induces density gradients, which may be time dependent, that affect local target thickness . For gas mixtures, beam-induced heating also contributes to different; al gas diffusion (the Sorel effect) and molecular dissociation, thereby altering the effective target composition. Thesecomplications are enhanced in a sealed or static gas target since there is no effective way to alleviate temperature increase or to replenish a depleted gas mixture in the beam region. In precision cross section measurements it is important to reduce and/or accurately monitor the effects of the beam on the gas density. There have been previous investigations of this matter in electron scattering experiments with gas targets [1-41. Such efforts involved measuring the elastic peak area as a function of beam current. Although these methods have been successful, they can be tedious . consuming much valuable beam time. We describe a flowing gas system that both reduces these beam heating effects anddirectly monitors the gasdensity during the data taking, thus eliminating the need for extra beam time to determine the dependence of the local target density on beam current . The system described has been tested and used with hydrogen and with a gas mixture of 30% methane and 70% helium in an elastic scattering experiment to obtain 0167-5087/82/0000-0000/$02.75
0
1982 North-Holland
a precise measurement of the nuclear ground state charge radius of `He. Fcst we describe the target system and means to measure pressure and temperature- They we discuss the method of determining temperature in the target region without interfering with thebeam . 2. Target system description A schematic of the target system is shown in fig. I . All parts are connected with 0.635 cm inter diameter high-pressure rubber tubing. Under normal operatic, target gas flows at some selected pressure above os~ atmosphere from agas supply tank through a ftowmeter into ~ target cell mounted in the scattering chamber. rice Then it flows through a gasdensity stabilization called a pycuostat [51 andoutof the system . Thevacuum pump allows for sub-atmosphere andempty cell Option, and is used to purgethesystem when changing to a different target gas. The pressure throughout the target system is Mguschematically is lated by the pycnostat which is shown fig. 2. Target gas flows into the pyenostat at some pressure P, and out through an aperture of 1-S diameter. Pressure regulation is due to the action of a rubber piston on top of a bellows near the owllow aperture. The distance between the rubber piston aae! the aperture is related to the flow impedance andtegulutes the flow rate . Areference pressure. P is contained
98
If. J. Smpor rr al. / A
flowiRg gac mrgrt .st".wrm
rARET LELL
BEAM_LINE
TRERMISTORS
Fig. 1. Schematic of target system. ® indicates location of a valve, - indicates direction of flow when system is in operation . by the bellows. There is an equilibrium position of the bellows such that the target pressure P, = PZ . Any change in target uressumcauses small excursions of the bellows to ine.--ase or decrease the flow until P, becomes equal to PZ again. The range of flow rates within whichthepyrnostat will operate properly is determined by the size of the aperture. Larger flow rates require larger apertures. We have tested apertures with diameters ranging from 0.25 to 2.0 mm, The 1 .5 mm aperture enabled us to obtain a volume rate of 350 cm'/min. with pressure regulation to better than 0.1% at 125 kPa . The fact that the pycnostat requires gas flow to operate is an advantage. It is assumed that flow will
SIDE
Fig . 3. Schematic side view and top view of larget cell interior for experiment. Thermistor positions 1, 2, 5and6 are indicated along with three ports on top of cell. IN or OUT tells direction of gas flow relative to the target cell. reduce effects due to differential diffusion and molecular dissociation. These effects were calculated [6j and were small (<0.1%) in our experiment. Therefore, no quantitative data on the effect of flow wascollected. We use a high-precision (0.1%), wide-range (0-5000 Tort)capacitive manometer to monitor the pressure of the gas at a point between the target cell and the pycnostat. In this configuration we must consider the hydrostatic and viscous pressure differences (OPII and AP,) between the target cell and manometer. Forany mean pressure P, API ,/Pwascalculated to be <0.02% and is clearly negligible . However, I)Pv/P, which can be accurately calculated from Poiseuilleslaw, wasfound to be 0.5% and notas negligible. This correction could be applied to the pressure reading . The OPvcorrection could be substantially reduced by using larger diameter tubing between the target cell and manometer. The target cell in the system is a copy of the one described by Webb et al. [I] except that there have been two major modifications as shown in fig. 3. One is the construction of three ports on top of thecell to accomodate the pipes necessary for flow. Target gas enters through the two outer ports and exits through the middle port. The other modification is the installation of a four thermistor network to directly -sure the temperature profile of the gas. 3. Temperatureprofile measurement
Fig.2. Schematic of pycnostat. Arrows indican; direction of flow under normal operation . dis the distance between aperture (A) and robber piston (H). P, is the target system pressure. Pzis the'operating refemac"_ pressure set with valve (C) inside bellows (D).
A well-focused electron beam that passes through themiddle of the target cell acts as a line source of heat. It hasbeen shown that the variation in local gasdensity is insensitive to thetime structure of the electron beam [4J. We have measured the characteristics of such a heat
W.J .
Stapor et al.
/ A flowing gas
3 AODITIONAL THERMISTORS
SIDE
Fig. 4. Schematic side view of larget cell interior for calibrations. Additional thermistor positions 3, C, and 4 are shown with =24 awg nichrome wire orientation . source in the cell by installing a thin gauge (#24) nichrome heater wire along the beam axis and measuring temperature profiles forvarious gases versus power input . For this bench testing, the thermistor network included seven thermistors as shown in fig. 4. In this way we were able to obtain temperature measurements of the gas at various points to within I mm of the heat source. A typical set of temperature profiles is shown in 23 5
TOP
CENTER
BOTTOM
I CH.-He qas-turc ot 250 230
YI
22 .5
xPo
e A
-om -I4mw
Ô
- 25mw
zz o 2L5 F Q
a s w
W
2Io
I
205
2o
o
I95
Iso I .,
203mm III
12Tmm 121
I SImmlOmm 13) ICI
~Imm .1
POSITION OFTHERMISTOR
I2Tmn 151
203mm I61
Fig. 5. Temperature profiles for 0, 14, 56 and 12.° IoW power inputs. These correspond to tae energy deposit from 0. 0.5, 2, and 4 pA average beam current, respectively, through die CH,-He gas mixture al 250kPa.
target srsterw
fig. 5 for several power levels into the CH,--He gas mixture at 250 kPa with avolume rate of 350cmj/minThe temperature increase in the top part of the cell above the temperature at zero-power input is duemostly to convection. This was verified by measuring profiles for alternate orientations of the target cell . The region above the source always showed thegreater temperature increase, while the region below showed very little change, with the bottom always near the zero-power temperature. We found that the temperature profiles were independent of gas pressure for pressures up to 300 kPa and for power inputs up to 1 .4W. which corresponds to the energy deposited from 40 )eA average beam current through the CHsHe gas mixture. The effect of the thermistors heat capacities on the temperature measurements was negligible. We verified this by measuring temperature profiles with two different sizes of thermistors. No appreciable differences in the temperature profiles were found. From profiles like these we were alite to develop a method for determining the temperature of the CH,He gas mixture near the target region knowing only the temperatures monitored by the outer thermistors- Such a method is essential since the three central thermistors must be removed from the cell to avoid interference with the electron beam (cf. fig. 3) during thescattering experiment. Since the bottom thermistor (position 6 in i&4) always registers a temperature close to the zero-power temperature, we use it to derive a baseline. The offset above the baseline temperature at any other thennistcr position is related to thepower input. This shown in fig. 6 where we have plotted the temperature offset above the baseline . A, versus position forthemeasured profiles of fig. 5. If beam power dissipation is 20 mW, then one expects the corm .pouding A profile to he between the 14 and the 56 mW curves. Therefore, we can interpolate to predict thetemperature offset above the baseline in the center of thecell. Ac . for anypower input . When we plot pt- versus A, (the temperature offset abovethebaseline at themstor positiorcs i = 1, 2) or power inputs up to 502 mW,as sMTwn in fig.?, we see that the data exhibit a relationship of theform AE_=C,A,+K where C, and K, are constants determialed by a least squares straight line fit to the me-rcd data. Saint fits were performed for each of the top two th. sto-, (positions 1, 2) in the ceIL 13y measuririx the tetttperature at the top two thermistor po itiovtc and at the baseline position, we can use these calibration fits to obtain the temperature of the target in the region .fthe beam . Both the target temperature and prc- anti Bitsu ; " d periodically while accumulating scatterin data. From these measurements we determine the detsity tit
W.J. Stapor et al. / A flowing gas targe-ystent
100
ToT_
OS
T_aOTT,rn_.._.__
En
40
35/
é ..
a
1 CN4-Xe " as misfun el ESOKP I e -e lo, 14mw ~I o -elo,5emw O -e l01)25 mw 1
25
20
(BASELINE OEFINEO By T-111111 IT TNEBMISTON POSITION 6)
i
3
U
15
Y5 5
Io OS
1
-
o I
f0.3 U1
I
1Y.7 12)
~
5.1 131
li
1.0 (O)
POSITION OF
I
,
5.1 141
T'Jt:astlsTOR
I
~~i 20.3 (61
12.' (5
Fig. 6. A at each thermistor position in target ell . A is the temperature difference between anythermistor and the base.The baseline is defined as the temperature at thermistor 6m position 6. the target region that is viewed by the detectors. We use this measurement to generate adensity correction to the scattered energy spectrum so that the scattered electron counts arenormalized to the average target density over the whole data run. For our case, the corrections are typically less than 0.1%, indicating density stabilization better than 0.1%. 71ie technique described can be applied to situations where the'effects of the beam on the gas density are much`larger as in experiments requiring higher beam currents andhigher target pressures. We would like to thank machinists, Mr . Bill Wartiuwö" - -., ï:UA, and Mr. Donald Bryant, NBS, for their skillful aid . We are especially grateful to Dr. Steve
Ai (i- 1 .21 ('C) Fig. 7. A c vs. A(, beamline temperature difference versus thermistor i temperature difference. Subscript i indicates thermistor positions 1 and 2. Dashed lines illustrate best fits to A,=C,A,+K, with C1 =4.65, Kh =0 .67o, C2=4.22 . and KZ = -0 .12°. Domen, NBS, for his helpful discussion and use of his lab and equipment in calibrating thermistors, to Dr. R.D. Deslatto, NBS, for lending us a pycnostat, and to Prof. G.A. Peterson, U. Mass., who provided us with a gas target cell of ref. 1 . '1his work was supported in part by the National &:ience Foundation under grant PHY7923968. References [11 D.V. Webb. G.A. Petersen, Z.M. Szalata and P.T. Kan. Nucl. Instr. andMeth. 120 (1974) 359 . [2) R.P. Singhal, H. Purdie, A. Cave, E. Pearce and H.S. Caplan, Nucl. Instr . andMeth . 73 (1969) 237 . [31 H. Frank, Ch. Schmidt, W. Schutz and H. Thei-, ZAED-Conf. 71-400-013-H13,(1972) 177. 141 G.G. Simon, Ch . Schmidt, F. Borkowski, C. Ostermann, V.H. Wallher, D. Bender and A. Von Gunten, Nuct. Instr. and Meth. 158 (1979) 185. [51 R.D. Deslattes, B.G . Simpson and R.E . LaVilla, Rev. of Sci. Instr . 37 (1966) 596. [61 J .O. Hirschfelder, C.F. Curtiss and R. ByronBird. Molecular theory of gases and liquids (John Wiley, New York. 1954) ch. 8.