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DEVELOPMENT OF AN IN-BEAM POLARIMETER FOR INTERMEDIATE ENERGY PROTONS USING
p+d ELASTIC SCATTERING
S. Wissink, S. Wells, A. Opper, T. Rinckel, E. Stephenson
To cite this version:
S. Wissink, S. Wells, A. Opper, T. Rinckel, E. Stephenson. DEVELOPMENT OF AN IN-BEAM POLARIMETER FOR INTERMEDIATE ENERGY PROTONS USING p+d ELASTIC SCATTER- ING. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-557-C6-560. �10.1051/jphyscol:1990672�.
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COLLOQUE DE PHYSIQUE
Colloque C6, suppl6ment au n022, Tome 51, 15 novembre 1990
DEVELOPMENT OF AN IN-BEAM POLARIMETER FOR INTERMEDIATE ENERGY PROTONS USING p+d ELASTIC SCATTERING
S.W. WISSINK, S.P. WELLS, A.K. OPPER, T. RINCKEL and E.J. STEPHENSON Indiana University Cyclotron Facility, Bloomington, Indiana 47405.
U.S.A.
Rksumk
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Nous avons construit, install&, et calibrk un polarimhtre de transmission dont on utilise la riaction diffusion 6lastique p+d pour la mksure de la polarisation verticale et horizontale d'un faisceau de protons de haute 6nergie d'une fason indestructible. Considirations expirimentales et dktails techniques sont discutes, et les spectres reprksentatifs sont inclus.Abstract We have constructed, installed, and calibrated a transmission-type polarimeter in which we use p+d elastic scattering to measure the normal and sideways polarization components of a high-energy proton beam in a non-destructive manner. Experimental design considerations and technical details are discussed, and representative spectra are presented.
1 - INTRODUCTION
Spin physics and the study of polarization phenomena play a major role in the physics program a t the Indiana University Cyclotron Facility (IUCF). Because the beam polarization can be oriented in an arbitrary direction through the use of two high-energy spin precession solenoids, it is crucid that one be able to determine d three polarization components of the accelerated proton beam in a non-destructive way. In particular, the ($,a) spin-transfer program on the K600 spectrometer, the (6,n') program on the beam swinger,, and the recent successful demonstration of the Siberian Snake concept on the IUCF Cooler Ring have all required use of in-plane (horizontally) polarized proton beams.
The primary motivation for the current work stems from problems encountered with existing devices for proton polarimetry at IUCF. One such device, a low-energy p+'He polarimeter, is located in the injection line to the main cyclotron. Though very reliable, the actual beam polarization may change significantly during the final acceleration process. Moreover, because the beam is stopped in the polarimeter, polarization measurements tend to be widely separated in time. Recently, we have relied on high-energy polarimeters based on p+12C elastic scattering a t Blab = 20°, using either NaI stopping detectors or various combinations of AE-E pairs of plastic scintillator. Because one is essentially performing four singles measurements in this case (detecting protons scattered left, right, up, or down from the beam), either the energy resolution of the detector system must be good enough to c l e d y separate the 12C ground state from the first excited state a t 4.44 MeV, or the background must be low enough that one can sum over a well-defined region of excitation in a precisely reproducible way. The NaI system offers occasionally excellent resolution, but is extremely sensitive to beam halo and scintillator activation, and can be rendered useless by a relatively short period of beam instability. Plastic scintillator schemes, though much more rugged, have never produced satisfactory resolution to allow reliable extraction of beam polarizations at better than the 2-3% uncertainty level. ' For these reasons, we concluded that we could not depend on any polarimeter scheme based solely on singles detection, and various coincidencedetection techniques were considered. There were several significant advantages in choosing p+d scattering over p+p. Perhaps most importantly, one could use the scattered deuteron t o separate the elastic events of interest from the quasifree knockout processes that would occur in anything other than a pure hydrogen target. Also, for incident proton energies of 100 to 200 MeV, the p+d analyzing powers are generally larger (greater than 0.5 in magnitude / l / ) than those for p+p scattering, especially in the kinematic regimes that are most accessible experimentally, i.e., those in which each of the outgoing particles contains roughly half of the total available kinetic energy. (Note that the p+p analyzing power is exactly zero a t this point.) Since CD2 targets are fairly easy to produce in thicknesses of a few hundred pg/cm2 to a few mg/cm2, we were able to perform preliminary measurements very quickly.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990672
C6-558 COLLOQUE DE PHYSIQUE
The primary objectives of these tests were to: i) measure the p+d relative cross sections and analyzing powers at several incident proton energies and over a reasonable angular range; ii) locate the maxima in the analyzing power distribution to determine the optimum angle for polarimeter operation; and iii) find a relatively simple and efficient means of unambiguously identifying the scattered proton-deuteron pair such that left-right (or up-down) asymmetries could be reliably extracted even under fairly harsh experimental conditions. This last item involved testing several different types and sizes of scintillator, varying the detector geometry, and examining the effects of various hardware and software cuts.
2 - POLARIMETER DESIGN CONSIDERATIONS
Because detection of a deuteron would be a strong signature for a valid p+d elastic event, we decided to concentrate on the "dn arm. By choosing a scintillator thickness only slightly larger than the deuteron range, we would detect the f d energy of the deuteron, while no proton (of normal incidence) could deposit an equivalent amount of energy. For example, a 100 MeV deuteron will stop in -4.3 cm of plastic scintillator, yet protons can deposit a maximum of only 74 MeV in a scintillator this size. A pulse-height spectrum should therefore show a gap between the deuteron peak of interest and the knock-out proton continuum. We also chose to use the collimated "dn arm to set the lab scattering angle and coincident solid angle. The "p" arm, on the other hand, was a transmission scintillator, large enough in area to extend beyond the allowed p+d coincidence range, yet thin enough to keep the total volume of the detector small.
Based on these considerations, our final configuration for a p+d polarimeter is shown schematically below (Fig. 1). For our 200 MeV measurements, the setup consisted of two left-right symmetric detector pairs:
two "d" detectors, 5.08 cm thick and collimated to a diameter of 1.43 cm, and two "p" detectors, 0.64 cm thick and approximately 5 cm vertically and horizontally. The distance of the "dn detector from the target was varied, but most data were taken at about 60 cm, for a lab And = 0.47 msr. Analyzing powers and relative cross sections were measured for lab Bd values between 37' and 49' (with Bp = 74"
-
54'). For asecond run at 120 MeV, a similar geometry was used, but the "dn detectors were only 2.54 cm thick, and Bd was varied from 32" to 47'. At both energies, a 1.2 mg/cm2 CD2 target was used throughout the run.
Our acquisition electronics was also kept as simple as possible. The I'd" and "p" anode signals were fanned out to integrating ADC's and through CFD's, to generate logic signals for event definition, ADC gating, and TDC timing. In keeping with our philosophy of maximizing information on the deuteron, the "d" signals set
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Schematic diagram of the detector configuration used for detection of p+d coincidence events. A similar system was used for up-down scattering. The drawing is not to scale.the timing for the ADC gates and started the TDC's for time-of-flight data. In software, we used gating conditions on AEp and the relative
TOF
to select p+d elastic events out of the deuteron energy spectrum.The spectra shown in Fig. 2 were taken at Tp = 200 MeV, at laboratory angles of Bd = 40" and 8, z 69'.
Typical pulse height spectra for the LLpn (upper left) and "dn (upper right) arms, prior to imposition of any software conditions, are shown for the dL-PR pair. As expected, one sees a small gap between the deuteron peak (centered near channel 720) and the proton edge near channel 660, which already allows for fairly unambiguous peak summing. By adding a gate condition on the coincident "pn detector, as suggested by the vertical dashed lines, the deuteron spectrum is cleaned up considerably. A much more stringent requirement, however, is shown in the corresponding TDC spectrum of the particle time-of-flight difference (lower left), in which the p+d elastic events, with typical F W H M resolutions of 250 ps, are easily distinguished from the broad, predominantly (p,2p) background. By using the gates shown as conditions on the "dn pulse height, we obtain an essentially background-free elastic deuteron spectrum (lower right). The few counts remaining between channels 400 and 680 are predominantly p+d elastic events in which the deuteron suffered some
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Fig. L - Spectra taken at 200 MeV for Blsb = 40'. Experimental details are provided in the text. The histograms correspond to the ungated pulse-height spectra for the LLp" (upper left) and LLd" (upper right) arms, TOF between the two signals (lower left), and the LLd" pulse-height gated by the conditions suggested by the dashed vertical lines (lower right).
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C6-560 COLLOQUE DE PHYSIQUE
energy loss, such as in slit-edge scattering from the collimator. These counts therefore exhibit a left-right asymmetry comparable to that of the primary peak.
The angular distributions of the p+d analyzing powers are shown in Fig. 3 for incident proton energies of 200 and 120 MeV. Error bars are purely statistical, and do not reflect any contribution from uncertainty in the beam polarization normalization (estimated to be about 3-490, as determined by the low-energy p+4He polarimeter).
Guided by these results, two complete polarimeters have been built and installed thus far, each containing four pairs of scintillators for detection of p+d coincident events in which the outgoing deuterons scatter left, right, down, or up. Three complete sets of deuteron detectors have been constructed for each device, so that as the incident proton energy is varied, scintillators of the optimum thickness can be placed at the optimum angle. Because the polarimeters are placed both upstream and downstream of a 45' horizontal bending magnet, the normal and sideways polarization information from each can be combined to yield complete knowledge of the proton spin orientation at all points in the high-energy beamlines. Typically, with -100 nA on a 0.4 mg/cmz CD2 target, the proton polarization can be measured to f 0.01 in about 45 minutes. We have also demonstrated that targets up to 1.3 mg/cmZ thick are compatible with maintaining high resolution (<30 keV) on the K600 spectrometer.
We have completed preliminary absolute normalization measurements by cross-calibrating the p+d polarime- ters against the K600 spectrometer and focal plane polarimeter, using a reference analyzing power very close to 1. Using this technique, we estimate our effective analyzing power to be 0.524 f 0.010 for Tp = 200 MeV, where the error is dominated by the statistical precision achieved in the p+d polarimeters. More extensive calibration measurements will be performed in the near future.
In summary, we feel that these polarimeters have greatly improved our ability to accurately and continuously monitor the polarization of high-energy proton beams, using apparatus that is inexpensive and easy to construct, with minimal hardware and electronics requirements, and greatly decreased sensitivity to problems associated with high-background or unstable beam conditions.
- p+d Analyzing Power, 200 MeV
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REFERENCES
, I , ,
/l/ Arvieux, J. and Cameron, J.M., Adv. Nucl. Phys., Vol.
18
(1987) 107.1 1 t 1 ~ 1 1 1 1
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Fig. 3 - Deduced values for the p+d analyzing power at 200 and 120 MeV, as a function of Bd in the laboratory frame. Error bars represent only the statistical uncertainties.
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