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A high resolution photon tagging system in the GeV region
NUCLEAR
INSTRUMENTS
AND METHODS
II 5
0974) 465-469;
©
NORTH-HOLLAND
PUBLISHING
CO.
A H I G H R E S O L U T I O N P H O T O N TAGGING SYSTEM IN T H E GeV REGION G. R. B R O O K E S
University of Sheffield, Sheffield, $3 7RH, England E. G A B A T H U L E R , W. R. R A W L I N S O N , C. S H I E L and D. W. L. T O L F R E E
Daresbury Nuclear Physics Laboratory, Daresbury, Nr. Warrington, Lancs., England Received 28 June 1973 A p h o t o n tagging system which measures the energy of p h o t o n s in a bremsstrahlung spectrum is described. The apparatus covers a p h o t o n energy range o f 2 GeV starting 160 MeV below the energy of the incident electron beam. 192 counters provide a
nominal resolution between 6 MeV for a p h o t o n energy 160 MeV below the incident electron energy and 11 MeV for a p h o t o n energy 2160 MeV below the incident electron energy.
1. Introduction
associated with these beams usually use large solid angle detectors. An earlier tagged photon beam covering a 1.6 GeV range of photon energy with energy resolution of 35 MeV has already been used for experiments at Daresbury4). The system described here has been designed and built to have improved energy resolution and the capabilty of operating at higher beam intensities, and has been used at tagged photon rates of 106 S - 1 mean.
In experiments using high energy photons where it is possible to have neutral particles in the final state, it is desirable to know the photon energy. The photon energy in bremsstrahlung beams can be determined by tagging the recoil electronsl-4). The principle of this technique is to allow a monoenergetic electron beam to radiate in a thin target and to momentum analyse the recoiling electrons, from which the photon energy is the difference between the incident and recoiling electron energies. Events in experiments using a tagged photon beam are observed in coincidence with the recoil electron, so that the photon flux intensity is limited ( ~ 1 0 6 S - 1 mean), and therefore experiments
2. Beam
The electron beam, after extraction from NINA is transported by a system of quadrupoles and bending
1.0
0-5
I
1
I
I
I0
20
30
40
I
50 > x (cm)
Fig. 1. Typical field variation along x-direction for magnet excitation of 816 A.
465
466
G.R.
B R O O K E S et al.
magnets. The necessary m o m e n t u m analysis is achieved using two successive systems of quadrupole doublets and bending magnets with a symmetry quadrupole between the systems; the total angle of bend is about 18 °. A weak electron beam is focussed onto the experimental target which is situated about 9 m downstream from the tagging target. This means that at the tagging target the beam has a convergence of _+0.4mrad horizontally and _+0.4mrad vertically, and a spot size of _+_+3 m m horizontally and + 5 m m vertically.
TABLE 1 E s t i m a t e d effect on the energy resolution of multiple scattering
in the target and the angular convergence of the incident beam. Recoil electron energy (MeV)
250 1000 2000
Counter design resolution (MeV)
i3.0 4-4.0 4- 5.5
Counter energy resolution with effects (1) and (2) (MeV) 4- 3.0 4- 6.5 4- 10.0
3. Magnet measurements and calculations Because of the finite cross section of the incident electron beam at the tagging target, the m o m e n t u m analysing magnet was required to provide line-to-point focussing with m o m e n t u m dispersion along the focal line. There are two types of magnetic field which satisfy these requirements; a wedge-shaped constant field as used in an earlier design4), or a linear field gradient. Past experience has suggested that fringe field effects with the particular wedge-shaped field available would limit the m o m e n t u m resolution, so that in this case a linear field gradient magnet has been used which gives parallel-to-point focussing in the bending plane. To achieve a wide range of photon energies, the system has been designed to have a total energy range of 2 GeV consisting of 192 bins each covering nominally ___5MeV/c with the lowest recoil electron m o m e n t u m being 160 MeV/c. Using a Hall effect probe, measurements of the vertical component of the magnetic field have been made in the median plane of the magnet gap for a mesh with 5 m m intervals in the s-direction and 10 m m in the x-direction - the s-direction being along the magnet axis, and the x-direction being horizontal and perpendicular to the magnet axis. The measurements were converted into field values by a calibration of the Hall effect probe with a nuclear magnetic resonance system using a separate uniform field magnet. A typical field variation along the x-axis is shown in fig. 1. Measurements of the field as a function of magnet current have also been made. By using the magnet in the linear region for field and current, it is possible to have a variation of energy resolution of the tagging system, as well as altering the total photon energy range that can be covered, and still preserve the focussing properties of the magnet. Using these field measurements, a computer program performed ray tracing calculations to establish the
position of the focal line along which the scintillation counters were to be placed. The program used an iterative R u n g e - K u t t a integration to calculate the electron trajectories. The incident beam direction and the target position were adjusted to give an acceptable focal line for the 2 GeV photon energy range required. The focal line was approximately a straight line, and to achieve the optimum resolution for the system the scintillation counters were placed edge to edge along this line. The angle between the electron trajectories and the focal line varies with the electron m o m e n t u m from 20 ° for 200 MeV/c to 6 ° for 2 GeV/c. This means that recoil electrons pass, on average, through one counter for 200 MeV/c and two counters for 2 GeV/c. The system has been designed so that the front face of each scintillation counter lies on the focal line, and in this case it is the first counter that the recoil electron passes through which defines its momentum. The m o m e n t u m dispersion has been calculated and varies from 6 MeV/c at 200 MeV/c to 11 MeV/c at 2GeV/c, and the 192 bins have been arranged so that they are of equal physical width. Various factors have been considered which worsen the energy resolutions: 1) angular convergence of the incident electron beam; 2) multiple scattering of electrons in the target; 3) multiple scattering of electrons in the path between the target and the scintillation counters. The effect of (3) has been minimised by maintaining the magnet and all but the last few centimetres of the recoil electron flight path under vacuum, Additional ray tracing calculations have been performed to estimate the effects of (1) and (2) and these are shown in table 1.
4. Arrangement of apparatus The tagging radiator is situated at the entrance of a
PHOTON TAGGING SYSTEM field gradient S magnet of length 1 m. For a momentum acceptance of 2 GeV/c, the magnet current is set at 816 A which gives a field gradient of approximately 3.0 Tm-1. The acceptance can be altered by variation of the magnet current. The radiator can be inserted and removed from the beam by a remote control mechanism, so that target in-target out measurements can be readily taken. Currently a 1/200 radiation length of platinum foil is being used, which gives a target-out to target-in ratio of 1%. To ease manufacture and construction of the counter array, the scintillation counters were each 9 0 x l l x 2 r a m a of NE102A scintillator. Each counter was viewed (through its end face) by an 18 mm diameter RCA type C31005 R photo-multiplier. To achieve the necessary close packing each adjacent counter was viewed from opposite ends of the scintillator. The scintillators were machined and polished to an accuracy of 4-0.012 mm and each was wrapped with aluminised mylar of thickness 0.012mm to improve its light collection and prevent cross-talk between adjacent counters. The counter array is shown in fig. 2.
467
In order to achieve the positional tolerances the counter array and support system was mounted directly onto the S magnet. The support system was made to also act as a light-tight box for the counter array. For the 24 counters nearest the magnet, it was necessary to mount these off the focal line, so as to avoid the complication of mounting counters inside the magnet aperture. These counters were mounted so as to be parallel to the outside face of the magnet. To reduce the complexity of using each of the 192 counters to produce an experimental trigger, a series of backing counters have been used which were mounted behind the main counter array. To reduce accidentals due to backgrounds in these backing counters they consisted of pairs of counters to form a telescope. There were 9 such pairs of scintillators Xi Yi. For each pair Xi and Yi were later put into coincidence to define a recoil electron. These counters were mounted on a separate framework and were made of such a size to provide an adequate overlap between adjacent pairs of counters.
Fig. 2. Tagging system counter array.
468
G . R . BROOKES et al.
5. Electronics For each experimental trigger, it was required to encode the number of the electron counter(s) associated with the recoil electron. To achieve this the following electronic functions are necessary: discrimination, coincidence and buffer. These three functions are contained within one module, thereby achieving considerable savings in cost and size. The module used was the Lecroy 2340DCB, which is C A M A C compatible allowing direct readout by the D D P 516 computer used in experiments. The characteristics of this module are described elsewhereS). The trigger pulse formed between the summed coincidence of the backing counters ~ i Xi Yi and a trigger from the experiment forms a common strobe for all 192 electron counters, and a fast coincidence is made within the kecroy 2340DCB. This module has a minimum resolving time of about 1 ns, but in practical circumstances a resolving time of 4 or 5 ns is obtainable. Various techniques6'7), using the D D P 516 computer have been used to facilitate timing and plateauing the 192 counters. 6. Performance With a system of such high m o m e n t u m resolution, the problem of absolute calibration is a difficult one. It is necessary to know the m o m e n t u m resolution as well as the energy calibration. To check the latter, use has been made of the linearity of the magnetic field with current around the design value. Using a weak electron beam and the tagging radiator out, the variation of electron counter number with magnet current
was recorded. The results are shown in fig. 3, for several settings of S magnet current. The line shows the predicted electron counter number and there is good agreement with that observed. As a further check a 1 GeV electron beam from N I N A was deflected at the design current setting into the correct electron counter. Similarly when a 2 GeV electron beam was used the electrons were detected in the appropriate counter. The electron beam was also deflected into this same counter when the incident electron beam energy was l GeV and the S magnet current was set to half its design value. From these results the total energy range of the tagging system was determined. To check the resolution of individual counters small changes in magnet current were made and the corresponding variation in electron counter number recorded. These results for one such typical change are shown in fig. 4, and this corresponds to the predicted momentum resolution. An absolute energy calibration has also been made. This has been done using a weak electron beam which passed through a uniform field bending magnet placed downstream of the tagging system. The deflected electrons then passed through a scintillation counter and spark chamber telescope to determine the electron direction. The initial electron direction was found a)
816
Amps
700 600 500
Counts
400 300 200 100 i
i
I
i
i '91'o ' '
i
, ,
,,
,,,
,,
94
100
150 Counter Number
b)
812 Am
, , i,,
Counter Number
)s
700 100
600 500 : o u n t s 40C
5C
300 200! 100
0
I
t
400
800
I 1200
•
L i
i
i
i
i
i
90 Magnet
Current
i
i
i
i
94 95
,
i
i
,
i
~ ,
,
,
I
100 Counter N u m b e r
Amps
Fig. 3. Calibration of the tagging system.
Fig. 4. Spectrum across the tagging system for 1.00 GeM electron beam with magnet current at (a) 816 A, (b) 812 A.
PHOTON TAGGING SYSTEM using the same detector only with the m a g n e t i c field set to zero. The m o m e n t u m o f the electron b e a m was m e a s u r e d to _+ 5 MeV/c. This b e a m was then deflected by the tagging m a g n e t a n d the electron c o u n t e r t h r o u g h which this b e a m passed was recorded. This electron c o u n t e r c o r r e s p o n d e d to the design c o u n t e r number. W e should like to t h a n k the Engineering Services G r o u p , a n d in p a r t i c u l a r M r W. Smith, for work in m a n y aspects o f design and construction. W e are also grateful to the A c c e l e r a t o r G r o u p for provision o f a suitable extracted electron beam. F i n a l l y we are indebted to all o t h e r m e m b e r s o f the L A M P experimental g r o u p for assistance t h r o u g h o u t the project.
469
References
1) D. 0. Caldwell, J. P. Dowel, K. Heinloth and M. D. Rousseau, Rev. Sci. Inst. 36 (1965) 283. 2) j. Weber, DESY Internal Report F1-69/3 (July 1968). 3) D. O. Caldwell, V. B. Elings, W. P. Hesse, G. E. Jahn, R. J. Morrison, F. V. Murphy and D. E. Yount, Phys. Rev. Letters 23 (1969) 1256. 4) G. R. Brookes, S. Hinds, W. R. Rawlinson, M. D. Rousseau, D. W. L. Tolfree and A. G. Wardle, Nucl. Instr. and Meth. 85 (1970) 125. 5) D. Mercer and C. Shiel, Daresbury Laboratory Internal Report DNPL/TM. 94. 6) M. N. Atkinson, Daresbury Laboratory Preprint DNPL/P. 123. 7) M. N. Atkinson and C. Shiel, to be published.
INSTRUMENTS
AND METHODS
II 5
0974) 465-469;
©
NORTH-HOLLAND
PUBLISHING
CO.
A H I G H R E S O L U T I O N P H O T O N TAGGING SYSTEM IN T H E GeV REGION G. R. B R O O K E S
University of Sheffield, Sheffield, $3 7RH, England E. G A B A T H U L E R , W. R. R A W L I N S O N , C. S H I E L and D. W. L. T O L F R E E
Daresbury Nuclear Physics Laboratory, Daresbury, Nr. Warrington, Lancs., England Received 28 June 1973 A p h o t o n tagging system which measures the energy of p h o t o n s in a bremsstrahlung spectrum is described. The apparatus covers a p h o t o n energy range o f 2 GeV starting 160 MeV below the energy of the incident electron beam. 192 counters provide a
nominal resolution between 6 MeV for a p h o t o n energy 160 MeV below the incident electron energy and 11 MeV for a p h o t o n energy 2160 MeV below the incident electron energy.
1. Introduction
associated with these beams usually use large solid angle detectors. An earlier tagged photon beam covering a 1.6 GeV range of photon energy with energy resolution of 35 MeV has already been used for experiments at Daresbury4). The system described here has been designed and built to have improved energy resolution and the capabilty of operating at higher beam intensities, and has been used at tagged photon rates of 106 S - 1 mean.
In experiments using high energy photons where it is possible to have neutral particles in the final state, it is desirable to know the photon energy. The photon energy in bremsstrahlung beams can be determined by tagging the recoil electronsl-4). The principle of this technique is to allow a monoenergetic electron beam to radiate in a thin target and to momentum analyse the recoiling electrons, from which the photon energy is the difference between the incident and recoiling electron energies. Events in experiments using a tagged photon beam are observed in coincidence with the recoil electron, so that the photon flux intensity is limited ( ~ 1 0 6 S - 1 mean), and therefore experiments
2. Beam
The electron beam, after extraction from NINA is transported by a system of quadrupoles and bending
1.0
0-5
I
1
I
I
I0
20
30
40
I
50 > x (cm)
Fig. 1. Typical field variation along x-direction for magnet excitation of 816 A.
465
466
G.R.
B R O O K E S et al.
magnets. The necessary m o m e n t u m analysis is achieved using two successive systems of quadrupole doublets and bending magnets with a symmetry quadrupole between the systems; the total angle of bend is about 18 °. A weak electron beam is focussed onto the experimental target which is situated about 9 m downstream from the tagging target. This means that at the tagging target the beam has a convergence of _+0.4mrad horizontally and _+0.4mrad vertically, and a spot size of _+_+3 m m horizontally and + 5 m m vertically.
TABLE 1 E s t i m a t e d effect on the energy resolution of multiple scattering
in the target and the angular convergence of the incident beam. Recoil electron energy (MeV)
250 1000 2000
Counter design resolution (MeV)
i3.0 4-4.0 4- 5.5
Counter energy resolution with effects (1) and (2) (MeV) 4- 3.0 4- 6.5 4- 10.0
3. Magnet measurements and calculations Because of the finite cross section of the incident electron beam at the tagging target, the m o m e n t u m analysing magnet was required to provide line-to-point focussing with m o m e n t u m dispersion along the focal line. There are two types of magnetic field which satisfy these requirements; a wedge-shaped constant field as used in an earlier design4), or a linear field gradient. Past experience has suggested that fringe field effects with the particular wedge-shaped field available would limit the m o m e n t u m resolution, so that in this case a linear field gradient magnet has been used which gives parallel-to-point focussing in the bending plane. To achieve a wide range of photon energies, the system has been designed to have a total energy range of 2 GeV consisting of 192 bins each covering nominally ___5MeV/c with the lowest recoil electron m o m e n t u m being 160 MeV/c. Using a Hall effect probe, measurements of the vertical component of the magnetic field have been made in the median plane of the magnet gap for a mesh with 5 m m intervals in the s-direction and 10 m m in the x-direction - the s-direction being along the magnet axis, and the x-direction being horizontal and perpendicular to the magnet axis. The measurements were converted into field values by a calibration of the Hall effect probe with a nuclear magnetic resonance system using a separate uniform field magnet. A typical field variation along the x-axis is shown in fig. 1. Measurements of the field as a function of magnet current have also been made. By using the magnet in the linear region for field and current, it is possible to have a variation of energy resolution of the tagging system, as well as altering the total photon energy range that can be covered, and still preserve the focussing properties of the magnet. Using these field measurements, a computer program performed ray tracing calculations to establish the
position of the focal line along which the scintillation counters were to be placed. The program used an iterative R u n g e - K u t t a integration to calculate the electron trajectories. The incident beam direction and the target position were adjusted to give an acceptable focal line for the 2 GeV photon energy range required. The focal line was approximately a straight line, and to achieve the optimum resolution for the system the scintillation counters were placed edge to edge along this line. The angle between the electron trajectories and the focal line varies with the electron m o m e n t u m from 20 ° for 200 MeV/c to 6 ° for 2 GeV/c. This means that recoil electrons pass, on average, through one counter for 200 MeV/c and two counters for 2 GeV/c. The system has been designed so that the front face of each scintillation counter lies on the focal line, and in this case it is the first counter that the recoil electron passes through which defines its momentum. The m o m e n t u m dispersion has been calculated and varies from 6 MeV/c at 200 MeV/c to 11 MeV/c at 2GeV/c, and the 192 bins have been arranged so that they are of equal physical width. Various factors have been considered which worsen the energy resolutions: 1) angular convergence of the incident electron beam; 2) multiple scattering of electrons in the target; 3) multiple scattering of electrons in the path between the target and the scintillation counters. The effect of (3) has been minimised by maintaining the magnet and all but the last few centimetres of the recoil electron flight path under vacuum, Additional ray tracing calculations have been performed to estimate the effects of (1) and (2) and these are shown in table 1.
4. Arrangement of apparatus The tagging radiator is situated at the entrance of a
PHOTON TAGGING SYSTEM field gradient S magnet of length 1 m. For a momentum acceptance of 2 GeV/c, the magnet current is set at 816 A which gives a field gradient of approximately 3.0 Tm-1. The acceptance can be altered by variation of the magnet current. The radiator can be inserted and removed from the beam by a remote control mechanism, so that target in-target out measurements can be readily taken. Currently a 1/200 radiation length of platinum foil is being used, which gives a target-out to target-in ratio of 1%. To ease manufacture and construction of the counter array, the scintillation counters were each 9 0 x l l x 2 r a m a of NE102A scintillator. Each counter was viewed (through its end face) by an 18 mm diameter RCA type C31005 R photo-multiplier. To achieve the necessary close packing each adjacent counter was viewed from opposite ends of the scintillator. The scintillators were machined and polished to an accuracy of 4-0.012 mm and each was wrapped with aluminised mylar of thickness 0.012mm to improve its light collection and prevent cross-talk between adjacent counters. The counter array is shown in fig. 2.
467
In order to achieve the positional tolerances the counter array and support system was mounted directly onto the S magnet. The support system was made to also act as a light-tight box for the counter array. For the 24 counters nearest the magnet, it was necessary to mount these off the focal line, so as to avoid the complication of mounting counters inside the magnet aperture. These counters were mounted so as to be parallel to the outside face of the magnet. To reduce the complexity of using each of the 192 counters to produce an experimental trigger, a series of backing counters have been used which were mounted behind the main counter array. To reduce accidentals due to backgrounds in these backing counters they consisted of pairs of counters to form a telescope. There were 9 such pairs of scintillators Xi Yi. For each pair Xi and Yi were later put into coincidence to define a recoil electron. These counters were mounted on a separate framework and were made of such a size to provide an adequate overlap between adjacent pairs of counters.
Fig. 2. Tagging system counter array.
468
G . R . BROOKES et al.
5. Electronics For each experimental trigger, it was required to encode the number of the electron counter(s) associated with the recoil electron. To achieve this the following electronic functions are necessary: discrimination, coincidence and buffer. These three functions are contained within one module, thereby achieving considerable savings in cost and size. The module used was the Lecroy 2340DCB, which is C A M A C compatible allowing direct readout by the D D P 516 computer used in experiments. The characteristics of this module are described elsewhereS). The trigger pulse formed between the summed coincidence of the backing counters ~ i Xi Yi and a trigger from the experiment forms a common strobe for all 192 electron counters, and a fast coincidence is made within the kecroy 2340DCB. This module has a minimum resolving time of about 1 ns, but in practical circumstances a resolving time of 4 or 5 ns is obtainable. Various techniques6'7), using the D D P 516 computer have been used to facilitate timing and plateauing the 192 counters. 6. Performance With a system of such high m o m e n t u m resolution, the problem of absolute calibration is a difficult one. It is necessary to know the m o m e n t u m resolution as well as the energy calibration. To check the latter, use has been made of the linearity of the magnetic field with current around the design value. Using a weak electron beam and the tagging radiator out, the variation of electron counter number with magnet current
was recorded. The results are shown in fig. 3, for several settings of S magnet current. The line shows the predicted electron counter number and there is good agreement with that observed. As a further check a 1 GeV electron beam from N I N A was deflected at the design current setting into the correct electron counter. Similarly when a 2 GeV electron beam was used the electrons were detected in the appropriate counter. The electron beam was also deflected into this same counter when the incident electron beam energy was l GeV and the S magnet current was set to half its design value. From these results the total energy range of the tagging system was determined. To check the resolution of individual counters small changes in magnet current were made and the corresponding variation in electron counter number recorded. These results for one such typical change are shown in fig. 4, and this corresponds to the predicted momentum resolution. An absolute energy calibration has also been made. This has been done using a weak electron beam which passed through a uniform field bending magnet placed downstream of the tagging system. The deflected electrons then passed through a scintillation counter and spark chamber telescope to determine the electron direction. The initial electron direction was found a)
816
Amps
700 600 500
Counts
400 300 200 100 i
i
I
i
i '91'o ' '
i
, ,
,,
,,,
,,
94
100
150 Counter Number
b)
812 Am
, , i,,
Counter Number
)s
700 100
600 500 : o u n t s 40C
5C
300 200! 100
0
I
t
400
800
I 1200
•
L i
i
i
i
i
i
90 Magnet
Current
i
i
i
i
94 95
,
i
i
,
i
~ ,
,
,
I
100 Counter N u m b e r
Amps
Fig. 3. Calibration of the tagging system.
Fig. 4. Spectrum across the tagging system for 1.00 GeM electron beam with magnet current at (a) 816 A, (b) 812 A.
PHOTON TAGGING SYSTEM using the same detector only with the m a g n e t i c field set to zero. The m o m e n t u m o f the electron b e a m was m e a s u r e d to _+ 5 MeV/c. This b e a m was then deflected by the tagging m a g n e t a n d the electron c o u n t e r t h r o u g h which this b e a m passed was recorded. This electron c o u n t e r c o r r e s p o n d e d to the design c o u n t e r number. W e should like to t h a n k the Engineering Services G r o u p , a n d in p a r t i c u l a r M r W. Smith, for work in m a n y aspects o f design and construction. W e are also grateful to the A c c e l e r a t o r G r o u p for provision o f a suitable extracted electron beam. F i n a l l y we are indebted to all o t h e r m e m b e r s o f the L A M P experimental g r o u p for assistance t h r o u g h o u t the project.
469
References
1) D. 0. Caldwell, J. P. Dowel, K. Heinloth and M. D. Rousseau, Rev. Sci. Inst. 36 (1965) 283. 2) j. Weber, DESY Internal Report F1-69/3 (July 1968). 3) D. O. Caldwell, V. B. Elings, W. P. Hesse, G. E. Jahn, R. J. Morrison, F. V. Murphy and D. E. Yount, Phys. Rev. Letters 23 (1969) 1256. 4) G. R. Brookes, S. Hinds, W. R. Rawlinson, M. D. Rousseau, D. W. L. Tolfree and A. G. Wardle, Nucl. Instr. and Meth. 85 (1970) 125. 5) D. Mercer and C. Shiel, Daresbury Laboratory Internal Report DNPL/TM. 94. 6) M. N. Atkinson, Daresbury Laboratory Preprint DNPL/P. 123. 7) M. N. Atkinson and C. Shiel, to be published.