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The 100 MeV electron scattering facility at Amsterdam
NUCLEAR
INSTRUMENTS
AND
METHODS
T H E 100 M e V E L E C T R O N
74 (I969)
5--12;
©
NORTH-HOLLAND
PUBLISHING
COo
SCATTERING FACILITY AT AMSTERDAM
C. D E VRIES and P. J. T. B R U I N S M A
Institute for Nuclear Physics Research, Amsterdam, The Netherlands Received 2 June 1969
The facility for electron scattering experiments with the beam from the 85 MeV linear electron accelerator at Amsterdam (EVA) is described. The beam handling system allows research to be carried out with a purified beam of well defined energy and energyspread. Up to 15 /~A average current is achieved within an energyband of 0.3%. The current at the target is monitored to an absolute accuracy of a few tenths of one percent. The "magic-angle" spectrometer (p0 = 65 cm) allows measurements at high resolution (fwhm < 0.2%) and yet with a relatively large
solid angle (6 mster). The counting system consists of ten partially overlapping plastic scintillators, and detects scattered electrons simultaneously in 19 narrow channels. Experiments can be performed at scattering angles between 35° and 150°. In addition a special magnet arrangement permits scattering experiments at 180°. Experimental data to illustrate the overall performance are given in subsequent articles which describe the main components of the system in more detail.
1. Introduction
e. Beam spot size at the target: a few mm. This set o f specifications is first reflected in the design and construction o f the accelerator. A brief description o f the 85 MeV linear electron accelerator is given by Bruinsma and de Vries 1) and a m o r e detailed description o f its c o n s t r u c t i o n a n d p e r f o r m a n c e is presented in the preceding article2). The p e r f o r m a n c e o f the machine is a g o o d m a t c h for the p r o g r a m outlined above. In fig. 1 a floor plan is shown o f the entire facility. The d i m e n s i o n s of the different areas and their shielding walls are indicated. The shielding walls are m a d e o f blocks o f b a r i t e - l o a d e d concrete (3.6 g/cm2). The experimental areas are accessible t h r o u g h rolling d o o r s which are interlocked with the o p e r a t i o n conditions o f the machine. D u r i n g s t r a i g h t - t h r o u g h b e a m i r r a d i a t i o n s with h i g h - Z targets, the electron scattering area is accessible. Interlocks exist between the accelerator o p e r a t i o n , the p o w e r supply o f the second 45 ° b e n d i n g m a g n e t a n d a 25 cm thick retractible lead shield. The experimental areas have a r o o f of 0.5 m high concrete beams covered with at least 0.5 m thick concrete blocks. Part o f the concrete r o o f a b o v e the s p e c t r o m e t e r can be rolled away. In this m a n n e r the local shielding a r o u n d the c o u n t i n g system m a y be m o v e d using an overhead crane (3 ton). The deflection system a r e a is provided with a ventilation system to exhaust r a d i o a c t i v e gases a n d ozone t h r o u g h a vent tower (10 m a b o v e g r o u n d level). A d d i t i o n a l heavy local shielding is placed near the collimator, the slits a n d the F a r a d a y cup. W o r k with p r i m a r y b e a m intensities in the o r d e r o f 100/~A average b e a m can be carried out u n d e r p r o p e r personnel safety conditions. The different c o m p o n e n t s o f the electron scattering facility are discussed in the following sections. The
The main p r o g r a m to be carried out with the present facility is : 1. accurate d e t e r m i n a t i o n of the electromagnetic structure o f nuclear g r o u n d states; 2. investigation of discrete energy levels a n d o f structure in the giant resonance region. This p r o g r a m requires the precise m e a s u r e m e n t o f elastic and inelastic differential cross sections at various p r i m a r y energies and scattering angles. In particular, 180 ° experiments will be undertaken. The scope o f this p r o g r a m d e p e n d s strongly on the energy and intensity o f the p r i m a r y beam a n d on the resolving p o w e r o f the analyzing instrumentation. High p r i m a r y b e a m intensities are specifically required for the study o f b a c k w a r d scattering from discrete levels, because o f the very low cross sections involved ( ~ 10 _32 cm2). High resolution is needed for elastic scattering in those cases where low lying excited states are present a n d for inelastic scattering m e a s u r e m e n t s to separate closely-spaced individual levels. In the latter case high resolution is also highly desirable to enhance inelastic peaks in the r a d i a t i o n tail of d o m i n a t i n g elastic peaks. F o r high resolution experiments a well focused m o n o c h r o m a t i c b e a m at the target, an app r o p r i a t e l y thin target a n d a g o o d s p e c t r o m e t e r are imperative. In a d d i t i o n the b e a m and the analyzing i n s t r u m e n t a t i o n ( s p e c t r o m e t e r and counting device) m u s t be stable d u r i n g long periods needed to o b t a i n sufficient statistics. F r o m these c o n s i d e r a t i o n s the following p r i m a r y b e a m specifications are established: a. Energy r a n g e : easily variable from 15-85 M e V ; b. Energy setting: accurate to a p p r o x i m a t e l y 0 . 1 % ; c. Energy s p r e a d : variable from 0 . 1 - 0 . 5 % ; d. Intensity: average target currents ranging from 1 n A to 10 kLA;
6
C. D E
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P. J. T.
BRUINSMA
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ELECTRON
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most important parts of the system are treated in full detail in subsequent articles.
2. Deflection system An achromatic two-magnet system satisfies the requirements for beam analysis. The system allows : a. precise energy setting corresponding to the magnetic field of the first bending magnet; b. the capability for choice of energy band desired for the analyzed beam by means of slits in the focal plane of that magnet; c. purification of the analyzed beam. This second magnet refocuses the dispersed electrons and removes those electrons which do not have the proper energy (due to slit scattering). An additional shielding benefit is obtained in that bremsstrahlung photons emerging from the slits are then not directed toward the target or spectrometer. 2.1. BEAMOPTICS For the present facility a deflection system with two 45 ° magnets has been chosen. The system originally proposed by K. L. Brown and described by Penner 3) contains uniform-field magnets with edge focusing to fulfill the double focusing conditions, X/Xo = 0 and Y/Yo = 0 in the symmetry plane of the system. In addition the third symmetry plane condition, x / 2 = O, needed for complete optical symmetry is satisfied using From L nac
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FACILITY
7
a quadrupole magnet in the centre of the system. The matrix notation is that of Penner. The Amsterdam system (fig. 2) differs slightly from that in the original proposal. Vertical focusing is obtained with quadrupole magnets (nr. 3 and nr. 6). The quadrupole in the symmetry plane is replaced by quadrupole magnets nr. 4 and nr. 5 in order to provide sufficient space for the rather elaborate slit system (section 2.2). The quadrupole doublet nr. 1 and nr. 2 determine the location of the virtual object for the beam handling system. Downstream of this quadrupole doublet a retractable collimator limits the diameter of the beam to 5 mm. Comparison of current readings from two toroid current monitors, one located between the two accelerator sections and the other downstream of the collimator, has shown that the full beam emerging from the first section can be made to pass through this collimator. The effective radius of curvature of the central orbit through the first bending magnet as determined with floating wire techniques 4) is p = 58.47 + 0.06 cm. The careful alignment of magnets and slits with optical methods allows determination of the energy of the electrons passing through the slits to an accuracy of better than av , 2oj , o *• The beam optics have been studied with the computer program T R A N S P O R T from Stanford University*. These studies also included the action of the quadrupole triplet and the three bending magnets in the end station, the latter in the "normal" scattering position as well as in the 180° position (section 6). Theoretical beam spot sizes at the target for an energy spread of the beam of 0 •4°//o were less than 1 mm in both directions. The dispersion at the symmetry plane ( A p / p ) ( A E / E ) = 2.21. The momentum distribution of the deflected beam is determined and displayed by a ten-channel secondary emission monitor situated just in front of the slits• This monitor, which covers a 3% momentum bite is described by Piceni and de VriesS). A position monitor in the straight-through direction beyond the first bending magnet further enhances the updating of the beam parameters• One reference voltage controls the current setting of all beam handling magnets (including those in the * This has been confirrned by independent calibration measurements with the spectrometer. * We thank Mr. H. S. Butler for kindly supplying this program to us. The calculations were performed at the Deutsches Rechen-Zentrum at Darmstadt. The opportunity to use this facility is gratefully acknowledged.
8
C. DE V R I E S A N D P. J. T. B R U I N S M A
scattering area). Hence one-knob manipulation suffices for proper delivery of the beam to the target for all energies. Fine adjustments on the quadrupole supplies, to improve the quality of the spot at the target may be needed only for large variations of the energy. Experimentally, spot sizes less than 2 mm diameter are regularly obtained.
energy and removed without undue production of high intensity bremsstrahlung. They are thin-walled stainless steel buckets, 10 cm long, through which water flows at high speed. The front surfaces of the buckets are shaped according to the envelope of a beam with small values of AE/E (conditions for the analyzed beam are less stringent for large values of A E/E). Most of the bremsstrahlung and the energy-degraded electrons emerging from the slits are stopped by a lead brick cast in a stainless steel housing and placed immediately beyond the slits. A hole in the lead brick, 1.5 cm diameter, allows the passage of a beam with an energy spread of 1°/~. All walls of the vacuum chamber of the first bending magnet are shielded from improperly steered beams by thin-walled stainless steel watercooled pipes.
2.2. COLLIMATORAND SLIT CONSTRUCTION The construction of the beam handling system is determined not only by beam optical considerations but by the high power of electron beams involved (up to 10 kW). Special care has been taken in the design and construction of the collimator and of the slits (fig. 3a and 3b, respectively). The 10 cm long tapered doublewalled cylinder made of stainless steel is designed to allow high fluxes of water for cooling purposes. During the first high intensity beam tests, the beam (not properly steered!) burned a hole in the thin stainless steel wall, causing water damage to the vacuum system in the deflecting area. The graphite lining was added to prevent direct bombardment of the stainless steel wall on the beam. A molybdenum plate was added to further reduce the energy of those electrons which traverse the graphite. Average beam currents of up to 200 pA are now handled. Directly beyond this collimator a 25 cm long water-cooled double walled stainless steel tube (20 mm inner diameter) is included in the vacuum tubing system. This pipe is encased in 15 cm thick lead to shield the bremsstrahlung emerging from the collimator. Hence the production of radioactive gases and ozone is suppressed. The slits have been designed so that electrons with energies outside the desired band are degraded in
2.3. VACUUM AND COOLING SYSTEMS Vacuum pressure of 10 -8 Torr is maintained in the accelerator by ion-getter pumps. The accelerator vacuum system is common with that of the beam handling system. Hence the requirements for the vacuum conditions of the latter system are predominantly determined by those for the accelerator. A pressure of 1 0 - 7 - 1 0 - 6 Torr is maintained in the deflection area by means of a 100 1/s ion-getter pump near the collimator and a 200 1/s turbo-molecular pump at the vacuum chambers of the bending magnets and the slit box (fig. 2). Vacuum conditions (10 -5 Tort) in the experimental area are maintained by another 200 l/s turbo-molecular pump near the Faraday cup. Stainless steel is used for the collimator, the slitboxes and the vacuum chambers of the magnets. All other construction is of aluminum. bgam
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9
ELECTRON SCATTERING FACILITY Cooling is provided for structures which can be hit by the beam. This system circulates deionized water at a pressure of 5 kg/cm 2 and has a capacity of 80 l/rain. In case of emergency characterized by poor vacuum the vacuum gauges provide the information needed to shut off the machine, close valves and bypass water flow in the internal cooling loops. Dry air is then forced
into these loops to remove water and to strongly limit damage to the vacuum system should there be a water leak.
3. Current monitoring system The accuracy with which the beam intensity at the target can be measured is crucial for the final precision
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C. D E V R I E S
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to be obtained in the cross sections. A major attempt has been made to monitor the initial target current with an accuracy of better than lC)/o. A properly designed Faraday cup is well suited as an absolute monitor. We use such an instrument placed far from the detection equipment to reduce the high background of photons and neutrons from the stopped beam. The beam becomes divergent after passing through the target due to the multiple scattering processes. Nevertheless we have kept the Faraday cup size of reasonable dimensions, by refocusing the beam beyond the target with two large quadrupole magnets (gap 30 cm). The distance from the quadrupole magnets to the target can be changed to accommodate different types of experiments. The remaining intensity losses are measured using the method of Hague et al.6). In this method the loss is measured by a differential toroid monitor installed upstream of the target. The primary beam current is balanced against the current fed back from the Faraday cup plus an adjustable fraction of the latter current. Design, constructional details and performance of the toroid monitor, the Faraday cup and the electronic system are given in full detail in a following paperY). It is shown there that the accuracy with which the losses can be determined is highly satisfactory. Even under unfavourable conditions of large loss and low current the primary beam current can be determined with an accuracy of better than 0.5°,,o. With the system in use the charge-collecting efficiency of the Faraday cup has been measured to be 99.8% at high energies, in excellent agreement with the calculated efficiency7) of better than 99.7%.
4. Scattering chamber The scattering chamber is shown in fig. 4. The material chosen for construction was aluminum to reduce background due to multiply scattered electrons. For the same reason the diameter of the chamber (60 cm) is chosen as large as is allowed by the optical consideration of target to spectrometer distance. It was required that the spectrometer rotates precisely around the virtual vertical axis determined by the intersection of the target plane and the vertical plane through the beam center line and that an evacuated path be provided between the entrance to the spectrometer and the scattering chamber. The latter condition permits the elimination of vacuumfoil windows which would interfere with the scattered electrons before their analysis. One side of the cylindrical chamber is provided with a sliding foil to permit rotation of the spectrometer without interrupting the vacuum con-
P. J. T. B R U I N S M A
ditions. Measurements are possible at all angles in the range 30°-150 ° . The construction of the seal is indicated in the drawing. The foil is 0.5 mm thick and is made of nickel plated spring steel. Portholes at the opposite side of the chamber enable observation of the targets by means of closed-circuit television. The figure shows the targets mounted on the circumference of a target wheel. The entire target assembly can be rotated independently about the vertical chamber axis. Hence different positions of the target plane relative to the beam centerline can be obtained. The target plane angle is adjustable to within an accuracy of 0.1 c. The scattering chamber is also used for those cases where excessive heating of the target due to the beam must be avoided. Then, a target holder assembly is used which permits continuous vibration and rotation of the target.
5. Detection channel Differential cross-section measurements require the determination of: a. the energy and angle at which the electrons are scattered; b. the solid angle into which the electrons are allowed to enter the analyzing equipment; c. the number of scattered electrons relative to the initial number of electrons impinging on the target. The analyzing instrument in use is a magic angle (169.7 °) double focusing ( n = - ½ , ] ~ = l ) magnetic spectrometer 8) with a radius of curvature of 65 cm. The instrument is capable of focusing electrons with a momentum of up to 240 MeV/c. The dimensions of the aluminum vacuum chamber allow for a solid angle of 6 roster. The magnetic field strength is monitored with a rotating coil fluxmeter. The reproducibility of the spectrometer field setting is about l:105. In the near future the magnetic field strength will be set automatically by means of an on-line computer (PDP-8). The distance between spectrometer and target can be changed, to accommodate 180° experiments where the beam optics are different because of the additional bending magnet involved. The spectrometer is provided with a platform to support the local shielding around the counter array. This shielding consists of at least 15 cm of lead on all sides, an extra 15 cm layer of concrete between target and counters and a 25 cm thick layer of boron-loaded paraffin directly surrounding the lead shielding. This shielding is adequate for low cross-section measurements when used in conjunction with the special
ELECTRON
SCATTERING
telescopic counter arrangement (scintillators backed by one Cerenkov counter). Further constructional details and the performance of the spectrometer will be discussed in a subsequent paperg). The remotely controlled entrance slits which determine the solid angle for the electrons to be analyzed consist of two pairs of lead plates (thickness 8 ram). The solid angle is known with a precision of 0.5%. Slit scattering corrections are under study. The detection in the focal plane of the spectrometer takes place in an array of ten scintillation counters. These counters partly overlap each other, to allow simultaneous counting in 19 channels (momentum width 0.1%). The counter ladder is followed by a single 12erenkov counter for background rejection. The events are registered by standard logic circuitry and the results processed by a small on-line computer (PDP-8). A full description of the system is given in a following paper1°). 6. 180 ° scattering facility
The electron scattering equipment includes a 180 ° scattering facility to enable the study of: a. ground state magnetic moment distributions of nuclei ; b. properties of inelastic levels which are excited mainly by magnetic transitions.
11
FACILITY
The original work in this field, reported by Peterson and Barber~l), has clearly shown the importance of such experiments. They used a bending magnet upstream of the target which d e f l e c t s - a n d s e p a r a t e s the primary and the 180 ° beam. Since then the use of a circular bending magnet with uniform field has been proposed for this purpose 12) and several instances of its use reported13). The advantage of this arrangement is that the scattered electrons emerge radially from such a magnet independent of their energy. To measure a 180 ° spectrum, the spectrometer field and the spectrometer angle around the vertical axis of the circular bending magnet are simultaneously adjusted for each momentum bite. In order to use the complex arrangement for stopping and monitoring the beam (section 3) for 180 ° as well as for " n o r m a l " scattering experiments, the incoming beam direction must be changed. Fig. 5b shows a three magnet system with the desired features. The three-magnet system as a whole is displaced upstream along the beam line when scattering experiments at other angles are to be carried out (fig. 5a). The vacuum chamber of the third magnet is provided with a sliding foil similar to the one described in section 4. It allows rotation of the spectrometer to cover a momentum range from P0 to 0.4 Po. Optical considerations, based on those made by Rand ~3) have
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12
¢. DE VRIES
AND
been given by van Niftrik et al.'4). The 180 ° scattering facility will be described in more detail in a forthcoming article.
P. J. T. B R U 1 N S M A
Netherlands O r g a n i z a t i o n for the A d v a n c e m e n t of Pure Research (Z.W.O.).
Referenees 7. Final remarks With the facility described in the foregoing sections a variety of problems are under study. Preliminary results indicate that the facility is well matched to the high quality of the primary beam. This is shown in the subsequent articles, where the m a i n c o m p o n e n t s of the i n s t r u m e n t a t i o n are treated in more detail. The authors wish to t h a n k Prof. Dr. R. van Lieshout for his stimulating support. M a n y thanks are due to Prof. Dr. H. M. H o o g e n b o o m and Dr. C. D a u m who helped considerably in the design of the described facility. M a n y thanks are due to Dr. S. P e n n e r for his invaluable advice for the design of the deflecting system of the e q u i p m e n t needed to handle intense electron beams. Enthusiastic support has been given d u r i n g all stages of the construction period by Messrs C. W. de Jager, J. A. Jansen, G. J. C. van Niftrik, H. de Vries a n d P. K. A. de Witt Huberts. This work is part of the research p r o g r a m of the Institute for Nuclear Physics Research ( I . K O.), made possible by financial s u p p o r t from the F o u n d a t i o n for F u n d a m e n t a l Research on Matter (F.O.M.) and the
') P. J. T. Bruinsma and C. de Vries, Ned. Tijdschr. Natuurk 34 (1968) 338. o) p. j. T. Bruinsma, J. G. Noomen and C. de Vries, Nucl. Instr. and Meth. 74 (1969) 1. 3) S. Penner, Rev. Sci. Instr. 32 (1961) 150. 4) G. J. C. van Niftrik, G. J. Veenhof and F. Th. Douma, Internal Report IKO (Jan. 1967). 5) H. A. L. Piceni and C. de Vries, Nucl. Instr. and Meth. 51 (1967) 87. 6) j. F. Hague, R. E. Jennings and R. E. Rand, Nucl. Instr. and Meth. 24 (1963) 456. 7) j. A. Jansen, G. J. Veenhof and C. de Vries, Nucl. Instr. and Meth. 74 (1969) 20. 8) S. Penner and J. M. Lightbody, Proc. I st Intern. Syrup. Magnet technology (Stanford, 1965) 154. 9) C. W. de Jager, F. Th. Douma, P. J. T. Bruinsma and C. de Vries, Nucl. Instr. and Meth. 74 (1969) 13. 10) p. K. A. de Witt Huberts, H. de Vries, G. J. C. van Niftrik and G. A. Peterson, Nucl. Instr. and Meth. 74 (1969) 27. 11) G. A. Peterson and W. C. Barber, Phys. Rev. 128 (1962) 812. 12) C. de Vries, Summer Study Report SLAC 25-I (Stanford, 1963) 160. 13) R. E. Rand, Nucl. Instr. and Meth. 39 (1966) 45; G. A. Proca, Thesis LAL 1150 (Orsay, 1966); D. Ganichot, D. Benaksas, B. Grosset~te, J. L. Lorrain and D. B. Isabelle, Nucl. Instr. and Meth. 60 (1968) 151. 14) G. J. C. van Niftrik, H. de Vries and C. de Vries, M.I.T. Summer Study TID-24667 (1967) 201.
INSTRUMENTS
AND
METHODS
T H E 100 M e V E L E C T R O N
74 (I969)
5--12;
©
NORTH-HOLLAND
PUBLISHING
COo
SCATTERING FACILITY AT AMSTERDAM
C. D E VRIES and P. J. T. B R U I N S M A
Institute for Nuclear Physics Research, Amsterdam, The Netherlands Received 2 June 1969
The facility for electron scattering experiments with the beam from the 85 MeV linear electron accelerator at Amsterdam (EVA) is described. The beam handling system allows research to be carried out with a purified beam of well defined energy and energyspread. Up to 15 /~A average current is achieved within an energyband of 0.3%. The current at the target is monitored to an absolute accuracy of a few tenths of one percent. The "magic-angle" spectrometer (p0 = 65 cm) allows measurements at high resolution (fwhm < 0.2%) and yet with a relatively large
solid angle (6 mster). The counting system consists of ten partially overlapping plastic scintillators, and detects scattered electrons simultaneously in 19 narrow channels. Experiments can be performed at scattering angles between 35° and 150°. In addition a special magnet arrangement permits scattering experiments at 180°. Experimental data to illustrate the overall performance are given in subsequent articles which describe the main components of the system in more detail.
1. Introduction
e. Beam spot size at the target: a few mm. This set o f specifications is first reflected in the design and construction o f the accelerator. A brief description o f the 85 MeV linear electron accelerator is given by Bruinsma and de Vries 1) and a m o r e detailed description o f its c o n s t r u c t i o n a n d p e r f o r m a n c e is presented in the preceding article2). The p e r f o r m a n c e o f the machine is a g o o d m a t c h for the p r o g r a m outlined above. In fig. 1 a floor plan is shown o f the entire facility. The d i m e n s i o n s of the different areas and their shielding walls are indicated. The shielding walls are m a d e o f blocks o f b a r i t e - l o a d e d concrete (3.6 g/cm2). The experimental areas are accessible t h r o u g h rolling d o o r s which are interlocked with the o p e r a t i o n conditions o f the machine. D u r i n g s t r a i g h t - t h r o u g h b e a m i r r a d i a t i o n s with h i g h - Z targets, the electron scattering area is accessible. Interlocks exist between the accelerator o p e r a t i o n , the p o w e r supply o f the second 45 ° b e n d i n g m a g n e t a n d a 25 cm thick retractible lead shield. The experimental areas have a r o o f of 0.5 m high concrete beams covered with at least 0.5 m thick concrete blocks. Part o f the concrete r o o f a b o v e the s p e c t r o m e t e r can be rolled away. In this m a n n e r the local shielding a r o u n d the c o u n t i n g system m a y be m o v e d using an overhead crane (3 ton). The deflection system a r e a is provided with a ventilation system to exhaust r a d i o a c t i v e gases a n d ozone t h r o u g h a vent tower (10 m a b o v e g r o u n d level). A d d i t i o n a l heavy local shielding is placed near the collimator, the slits a n d the F a r a d a y cup. W o r k with p r i m a r y b e a m intensities in the o r d e r o f 100/~A average b e a m can be carried out u n d e r p r o p e r personnel safety conditions. The different c o m p o n e n t s o f the electron scattering facility are discussed in the following sections. The
The main p r o g r a m to be carried out with the present facility is : 1. accurate d e t e r m i n a t i o n of the electromagnetic structure o f nuclear g r o u n d states; 2. investigation of discrete energy levels a n d o f structure in the giant resonance region. This p r o g r a m requires the precise m e a s u r e m e n t o f elastic and inelastic differential cross sections at various p r i m a r y energies and scattering angles. In particular, 180 ° experiments will be undertaken. The scope o f this p r o g r a m d e p e n d s strongly on the energy and intensity o f the p r i m a r y beam a n d on the resolving p o w e r o f the analyzing instrumentation. High p r i m a r y b e a m intensities are specifically required for the study o f b a c k w a r d scattering from discrete levels, because o f the very low cross sections involved ( ~ 10 _32 cm2). High resolution is needed for elastic scattering in those cases where low lying excited states are present a n d for inelastic scattering m e a s u r e m e n t s to separate closely-spaced individual levels. In the latter case high resolution is also highly desirable to enhance inelastic peaks in the r a d i a t i o n tail of d o m i n a t i n g elastic peaks. F o r high resolution experiments a well focused m o n o c h r o m a t i c b e a m at the target, an app r o p r i a t e l y thin target a n d a g o o d s p e c t r o m e t e r are imperative. In a d d i t i o n the b e a m and the analyzing i n s t r u m e n t a t i o n ( s p e c t r o m e t e r and counting device) m u s t be stable d u r i n g long periods needed to o b t a i n sufficient statistics. F r o m these c o n s i d e r a t i o n s the following p r i m a r y b e a m specifications are established: a. Energy r a n g e : easily variable from 15-85 M e V ; b. Energy setting: accurate to a p p r o x i m a t e l y 0 . 1 % ; c. Energy s p r e a d : variable from 0 . 1 - 0 . 5 % ; d. Intensity: average target currents ranging from 1 n A to 10 kLA;
6
C. D E
VRIES
AND
P. J. T.
BRUINSMA
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ELECTRON
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most important parts of the system are treated in full detail in subsequent articles.
2. Deflection system An achromatic two-magnet system satisfies the requirements for beam analysis. The system allows : a. precise energy setting corresponding to the magnetic field of the first bending magnet; b. the capability for choice of energy band desired for the analyzed beam by means of slits in the focal plane of that magnet; c. purification of the analyzed beam. This second magnet refocuses the dispersed electrons and removes those electrons which do not have the proper energy (due to slit scattering). An additional shielding benefit is obtained in that bremsstrahlung photons emerging from the slits are then not directed toward the target or spectrometer. 2.1. BEAMOPTICS For the present facility a deflection system with two 45 ° magnets has been chosen. The system originally proposed by K. L. Brown and described by Penner 3) contains uniform-field magnets with edge focusing to fulfill the double focusing conditions, X/Xo = 0 and Y/Yo = 0 in the symmetry plane of the system. In addition the third symmetry plane condition, x / 2 = O, needed for complete optical symmetry is satisfied using From L nac
~ IE•Q
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Fig. 2. Deflecting system.
FACILITY
7
a quadrupole magnet in the centre of the system. The matrix notation is that of Penner. The Amsterdam system (fig. 2) differs slightly from that in the original proposal. Vertical focusing is obtained with quadrupole magnets (nr. 3 and nr. 6). The quadrupole in the symmetry plane is replaced by quadrupole magnets nr. 4 and nr. 5 in order to provide sufficient space for the rather elaborate slit system (section 2.2). The quadrupole doublet nr. 1 and nr. 2 determine the location of the virtual object for the beam handling system. Downstream of this quadrupole doublet a retractable collimator limits the diameter of the beam to 5 mm. Comparison of current readings from two toroid current monitors, one located between the two accelerator sections and the other downstream of the collimator, has shown that the full beam emerging from the first section can be made to pass through this collimator. The effective radius of curvature of the central orbit through the first bending magnet as determined with floating wire techniques 4) is p = 58.47 + 0.06 cm. The careful alignment of magnets and slits with optical methods allows determination of the energy of the electrons passing through the slits to an accuracy of better than av , 2oj , o *• The beam optics have been studied with the computer program T R A N S P O R T from Stanford University*. These studies also included the action of the quadrupole triplet and the three bending magnets in the end station, the latter in the "normal" scattering position as well as in the 180° position (section 6). Theoretical beam spot sizes at the target for an energy spread of the beam of 0 •4°//o were less than 1 mm in both directions. The dispersion at the symmetry plane ( A p / p ) ( A E / E ) = 2.21. The momentum distribution of the deflected beam is determined and displayed by a ten-channel secondary emission monitor situated just in front of the slits• This monitor, which covers a 3% momentum bite is described by Piceni and de VriesS). A position monitor in the straight-through direction beyond the first bending magnet further enhances the updating of the beam parameters• One reference voltage controls the current setting of all beam handling magnets (including those in the * This has been confirrned by independent calibration measurements with the spectrometer. * We thank Mr. H. S. Butler for kindly supplying this program to us. The calculations were performed at the Deutsches Rechen-Zentrum at Darmstadt. The opportunity to use this facility is gratefully acknowledged.
8
C. DE V R I E S A N D P. J. T. B R U I N S M A
scattering area). Hence one-knob manipulation suffices for proper delivery of the beam to the target for all energies. Fine adjustments on the quadrupole supplies, to improve the quality of the spot at the target may be needed only for large variations of the energy. Experimentally, spot sizes less than 2 mm diameter are regularly obtained.
energy and removed without undue production of high intensity bremsstrahlung. They are thin-walled stainless steel buckets, 10 cm long, through which water flows at high speed. The front surfaces of the buckets are shaped according to the envelope of a beam with small values of AE/E (conditions for the analyzed beam are less stringent for large values of A E/E). Most of the bremsstrahlung and the energy-degraded electrons emerging from the slits are stopped by a lead brick cast in a stainless steel housing and placed immediately beyond the slits. A hole in the lead brick, 1.5 cm diameter, allows the passage of a beam with an energy spread of 1°/~. All walls of the vacuum chamber of the first bending magnet are shielded from improperly steered beams by thin-walled stainless steel watercooled pipes.
2.2. COLLIMATORAND SLIT CONSTRUCTION The construction of the beam handling system is determined not only by beam optical considerations but by the high power of electron beams involved (up to 10 kW). Special care has been taken in the design and construction of the collimator and of the slits (fig. 3a and 3b, respectively). The 10 cm long tapered doublewalled cylinder made of stainless steel is designed to allow high fluxes of water for cooling purposes. During the first high intensity beam tests, the beam (not properly steered!) burned a hole in the thin stainless steel wall, causing water damage to the vacuum system in the deflecting area. The graphite lining was added to prevent direct bombardment of the stainless steel wall on the beam. A molybdenum plate was added to further reduce the energy of those electrons which traverse the graphite. Average beam currents of up to 200 pA are now handled. Directly beyond this collimator a 25 cm long water-cooled double walled stainless steel tube (20 mm inner diameter) is included in the vacuum tubing system. This pipe is encased in 15 cm thick lead to shield the bremsstrahlung emerging from the collimator. Hence the production of radioactive gases and ozone is suppressed. The slits have been designed so that electrons with energies outside the desired band are degraded in
2.3. VACUUM AND COOLING SYSTEMS Vacuum pressure of 10 -8 Torr is maintained in the accelerator by ion-getter pumps. The accelerator vacuum system is common with that of the beam handling system. Hence the requirements for the vacuum conditions of the latter system are predominantly determined by those for the accelerator. A pressure of 1 0 - 7 - 1 0 - 6 Torr is maintained in the deflection area by means of a 100 1/s ion-getter pump near the collimator and a 200 1/s turbo-molecular pump at the vacuum chambers of the bending magnets and the slit box (fig. 2). Vacuum conditions (10 -5 Tort) in the experimental area are maintained by another 200 l/s turbo-molecular pump near the Faraday cup. Stainless steel is used for the collimator, the slitboxes and the vacuum chambers of the magnets. All other construction is of aluminum. bgam
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P
9
ELECTRON SCATTERING FACILITY Cooling is provided for structures which can be hit by the beam. This system circulates deionized water at a pressure of 5 kg/cm 2 and has a capacity of 80 l/rain. In case of emergency characterized by poor vacuum the vacuum gauges provide the information needed to shut off the machine, close valves and bypass water flow in the internal cooling loops. Dry air is then forced
into these loops to remove water and to strongly limit damage to the vacuum system should there be a water leak.
3. Current monitoring system The accuracy with which the beam intensity at the target can be measured is crucial for the final precision
Primary beam
Jingfoil
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l0
C. D E V R I E S
AND
to be obtained in the cross sections. A major attempt has been made to monitor the initial target current with an accuracy of better than lC)/o. A properly designed Faraday cup is well suited as an absolute monitor. We use such an instrument placed far from the detection equipment to reduce the high background of photons and neutrons from the stopped beam. The beam becomes divergent after passing through the target due to the multiple scattering processes. Nevertheless we have kept the Faraday cup size of reasonable dimensions, by refocusing the beam beyond the target with two large quadrupole magnets (gap 30 cm). The distance from the quadrupole magnets to the target can be changed to accommodate different types of experiments. The remaining intensity losses are measured using the method of Hague et al.6). In this method the loss is measured by a differential toroid monitor installed upstream of the target. The primary beam current is balanced against the current fed back from the Faraday cup plus an adjustable fraction of the latter current. Design, constructional details and performance of the toroid monitor, the Faraday cup and the electronic system are given in full detail in a following paperY). It is shown there that the accuracy with which the losses can be determined is highly satisfactory. Even under unfavourable conditions of large loss and low current the primary beam current can be determined with an accuracy of better than 0.5°,,o. With the system in use the charge-collecting efficiency of the Faraday cup has been measured to be 99.8% at high energies, in excellent agreement with the calculated efficiency7) of better than 99.7%.
4. Scattering chamber The scattering chamber is shown in fig. 4. The material chosen for construction was aluminum to reduce background due to multiply scattered electrons. For the same reason the diameter of the chamber (60 cm) is chosen as large as is allowed by the optical consideration of target to spectrometer distance. It was required that the spectrometer rotates precisely around the virtual vertical axis determined by the intersection of the target plane and the vertical plane through the beam center line and that an evacuated path be provided between the entrance to the spectrometer and the scattering chamber. The latter condition permits the elimination of vacuumfoil windows which would interfere with the scattered electrons before their analysis. One side of the cylindrical chamber is provided with a sliding foil to permit rotation of the spectrometer without interrupting the vacuum con-
P. J. T. B R U I N S M A
ditions. Measurements are possible at all angles in the range 30°-150 ° . The construction of the seal is indicated in the drawing. The foil is 0.5 mm thick and is made of nickel plated spring steel. Portholes at the opposite side of the chamber enable observation of the targets by means of closed-circuit television. The figure shows the targets mounted on the circumference of a target wheel. The entire target assembly can be rotated independently about the vertical chamber axis. Hence different positions of the target plane relative to the beam centerline can be obtained. The target plane angle is adjustable to within an accuracy of 0.1 c. The scattering chamber is also used for those cases where excessive heating of the target due to the beam must be avoided. Then, a target holder assembly is used which permits continuous vibration and rotation of the target.
5. Detection channel Differential cross-section measurements require the determination of: a. the energy and angle at which the electrons are scattered; b. the solid angle into which the electrons are allowed to enter the analyzing equipment; c. the number of scattered electrons relative to the initial number of electrons impinging on the target. The analyzing instrument in use is a magic angle (169.7 °) double focusing ( n = - ½ , ] ~ = l ) magnetic spectrometer 8) with a radius of curvature of 65 cm. The instrument is capable of focusing electrons with a momentum of up to 240 MeV/c. The dimensions of the aluminum vacuum chamber allow for a solid angle of 6 roster. The magnetic field strength is monitored with a rotating coil fluxmeter. The reproducibility of the spectrometer field setting is about l:105. In the near future the magnetic field strength will be set automatically by means of an on-line computer (PDP-8). The distance between spectrometer and target can be changed, to accommodate 180° experiments where the beam optics are different because of the additional bending magnet involved. The spectrometer is provided with a platform to support the local shielding around the counter array. This shielding consists of at least 15 cm of lead on all sides, an extra 15 cm layer of concrete between target and counters and a 25 cm thick layer of boron-loaded paraffin directly surrounding the lead shielding. This shielding is adequate for low cross-section measurements when used in conjunction with the special
ELECTRON
SCATTERING
telescopic counter arrangement (scintillators backed by one Cerenkov counter). Further constructional details and the performance of the spectrometer will be discussed in a subsequent paperg). The remotely controlled entrance slits which determine the solid angle for the electrons to be analyzed consist of two pairs of lead plates (thickness 8 ram). The solid angle is known with a precision of 0.5%. Slit scattering corrections are under study. The detection in the focal plane of the spectrometer takes place in an array of ten scintillation counters. These counters partly overlap each other, to allow simultaneous counting in 19 channels (momentum width 0.1%). The counter ladder is followed by a single 12erenkov counter for background rejection. The events are registered by standard logic circuitry and the results processed by a small on-line computer (PDP-8). A full description of the system is given in a following paper1°). 6. 180 ° scattering facility
The electron scattering equipment includes a 180 ° scattering facility to enable the study of: a. ground state magnetic moment distributions of nuclei ; b. properties of inelastic levels which are excited mainly by magnetic transitions.
11
FACILITY
The original work in this field, reported by Peterson and Barber~l), has clearly shown the importance of such experiments. They used a bending magnet upstream of the target which d e f l e c t s - a n d s e p a r a t e s the primary and the 180 ° beam. Since then the use of a circular bending magnet with uniform field has been proposed for this purpose 12) and several instances of its use reported13). The advantage of this arrangement is that the scattered electrons emerge radially from such a magnet independent of their energy. To measure a 180 ° spectrum, the spectrometer field and the spectrometer angle around the vertical axis of the circular bending magnet are simultaneously adjusted for each momentum bite. In order to use the complex arrangement for stopping and monitoring the beam (section 3) for 180 ° as well as for " n o r m a l " scattering experiments, the incoming beam direction must be changed. Fig. 5b shows a three magnet system with the desired features. The three-magnet system as a whole is displaced upstream along the beam line when scattering experiments at other angles are to be carried out (fig. 5a). The vacuum chamber of the third magnet is provided with a sliding foil similar to the one described in section 4. It allows rotation of the spectrometer to cover a momentum range from P0 to 0.4 Po. Optical considerations, based on those made by Rand ~3) have
rna~lic-on~lespactrornetcr -0.5,~=0.25p,~65cm c
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Fig. 5. 180° three-magnet system, a. Normal scattering equipment; b. 180° scattering arrangement.
12
¢. DE VRIES
AND
been given by van Niftrik et al.'4). The 180 ° scattering facility will be described in more detail in a forthcoming article.
P. J. T. B R U 1 N S M A
Netherlands O r g a n i z a t i o n for the A d v a n c e m e n t of Pure Research (Z.W.O.).
Referenees 7. Final remarks With the facility described in the foregoing sections a variety of problems are under study. Preliminary results indicate that the facility is well matched to the high quality of the primary beam. This is shown in the subsequent articles, where the m a i n c o m p o n e n t s of the i n s t r u m e n t a t i o n are treated in more detail. The authors wish to t h a n k Prof. Dr. R. van Lieshout for his stimulating support. M a n y thanks are due to Prof. Dr. H. M. H o o g e n b o o m and Dr. C. D a u m who helped considerably in the design of the described facility. M a n y thanks are due to Dr. S. P e n n e r for his invaluable advice for the design of the deflecting system of the e q u i p m e n t needed to handle intense electron beams. Enthusiastic support has been given d u r i n g all stages of the construction period by Messrs C. W. de Jager, J. A. Jansen, G. J. C. van Niftrik, H. de Vries a n d P. K. A. de Witt Huberts. This work is part of the research p r o g r a m of the Institute for Nuclear Physics Research ( I . K O.), made possible by financial s u p p o r t from the F o u n d a t i o n for F u n d a m e n t a l Research on Matter (F.O.M.) and the
') P. J. T. Bruinsma and C. de Vries, Ned. Tijdschr. Natuurk 34 (1968) 338. o) p. j. T. Bruinsma, J. G. Noomen and C. de Vries, Nucl. Instr. and Meth. 74 (1969) 1. 3) S. Penner, Rev. Sci. Instr. 32 (1961) 150. 4) G. J. C. van Niftrik, G. J. Veenhof and F. Th. Douma, Internal Report IKO (Jan. 1967). 5) H. A. L. Piceni and C. de Vries, Nucl. Instr. and Meth. 51 (1967) 87. 6) j. F. Hague, R. E. Jennings and R. E. Rand, Nucl. Instr. and Meth. 24 (1963) 456. 7) j. A. Jansen, G. J. Veenhof and C. de Vries, Nucl. Instr. and Meth. 74 (1969) 20. 8) S. Penner and J. M. Lightbody, Proc. I st Intern. Syrup. Magnet technology (Stanford, 1965) 154. 9) C. W. de Jager, F. Th. Douma, P. J. T. Bruinsma and C. de Vries, Nucl. Instr. and Meth. 74 (1969) 13. 10) p. K. A. de Witt Huberts, H. de Vries, G. J. C. van Niftrik and G. A. Peterson, Nucl. Instr. and Meth. 74 (1969) 27. 11) G. A. Peterson and W. C. Barber, Phys. Rev. 128 (1962) 812. 12) C. de Vries, Summer Study Report SLAC 25-I (Stanford, 1963) 160. 13) R. E. Rand, Nucl. Instr. and Meth. 39 (1966) 45; G. A. Proca, Thesis LAL 1150 (Orsay, 1966); D. Ganichot, D. Benaksas, B. Grosset~te, J. L. Lorrain and D. B. Isabelle, Nucl. Instr. and Meth. 60 (1968) 151. 14) G. J. C. van Niftrik, H. de Vries and C. de Vries, M.I.T. Summer Study TID-24667 (1967) 201.