This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Bacher, A.D.

Betker, A.C.

Black, T.C.

Franklin, W.A.

Jiang, K.

Prout, D.

Schmitt, W.M.

Stephenson, E.J.

Wissink, S.W.

Yu, C.

English

IUScholarWorks

EXPERIMENTAL TECHNOLOGY 
PRECISE DETERMINATION OF PROTON SPIN-PRECESSION ANGLES 
IN THE K600 SPECTROMETER AND BEAMLINE 
W.A. Franklin, A.D. Bacher, A.C. Betker, T.C. Black, K. Jiang, 
D. Prout, W.M. Schmitt, E.J. Stephenson, S.W. Wissink, and C. Yu 
Indiana University Cyclotron Facility, Bloornington, Indiana 4 7408 
Recent spin-transfer measurements for (6, p") reactions, carried out on the IUCF K600 
Spectrometer and its Focal Plane Polarimeter (FPP), have provided much information 
on both the free nucleon-nucleon interaction and on its modifications inside the nuclear 
medium. These types of experiments require that the proton polarization be accurately 
determined immediately before and after the nuclear scattering. In practice, however, the 
polarimeters used to measure the initial (beam) and final (scattered) proton polarization 
are separated from the scattering chamber by intervening magnetic elements. The beam 
polarization, as measured by the High Energy Polarimeters (HEP's) located in BL3 and 
BL5, is affected primarily by transport through a QDDM magnet system in BL8 that is 
used for momentum dispersion. After the scattering, the proton flux must first pass through 
the K600 (a QDD magnet system) before its polarization is deduced in the FPP. For highly 
precise in-plane spin-transfer measurements, such as those of E383,' it is crucial that one 
know accurately the angles by which the proton spin is precessed by these two magnet 
systems, in order to be able to relate the asymmetries measured in these polarimeters to 
specific observables in the lab frame. 
If one considers only transverse magnetic fields, i. e., fields perpendicular to the proton 
momentum, then the angle by which the proton's spin is precessed with respect to  its 
momentum direction is given by: 
where y is the usual Lorentz factor (dependent on the proton energy), pp is the proton 
magnetic moment, and @bend is the angle by which the proton's trajectory has been bent 
due to the magnetic field. Because of this simple proportionality, an accurate determination 
of Obend is therefore sufficient to provide complete information on the spin-precession angle 
a. Using a combination of mechanical measurements and direct measurements of spin 
precession, as described below, we have thus extracted the bend angles BQ and BK for the 
beamline (QDDM) and spectrometer (K600) magnet systems, respectively, to an absolute 
accuracy of better than lo. 
The change in trajectory experienced by a proton passing through the K600 can 
be determined through knowledge of two angles: OFP, defined as the angle of incidence 
between the proton trajectory and the wire plane of the first vertical drift chamber (VDC) 
in the focal plane; and 00, the angle between this wire plane and the central ray for 
protons entering the spectrometer. With precise knowledge of the focal-plane chamber 
geometry, O F p  can be calculated very accurately for each event from VDC information; 
our mechanical measurements, therefore, focussed primarily on determining Bo. This angle 
will depend on the specific configuration of the spectrometer system, e.g., whether the 
septum magnet is being used, and whether the focal plane detectors are positioned at the 
medium- or low-dispersion exit port. Thus, several independent mechanical measurements 
were made during the past year to establish Bo for all modes of K600 operation. In some 
cases, the determination of Bo was fairly direct: these studies required that a transit 
situated behind the K600 sight along the wire plane, while a front-silvered mirror was 
carefully aligned at the center of the scattering chamber to determine the path of protons 
entering the spectrometer. For another, more indirect, determination, we measured the 
VDC wire-plane angle with respect to the upstream beamline, then relied upon the K600 
digital encoder to determine the (relative) angle between this beamline and the entrance 
to the K600. 
Our results are summarized in the first column of Table I, which also provides details 
on the spectrometer configuration and measurement technique employed. In order to check 
for consistency among these numbers, we rely on the values determined for two other angles, 
each independently measured to a high accuracy: the additional bend introduced by the 
septum magnet, found to be 8.54 f 0.05"; and the angle difference between the VDC wire 
planes when mounted at the medium- or low-dispersion exit port, which is 12.45 & 0.10". 
Using these values, one can convert each measurement to an effective determination of Bo 
for the non-septum, medium-dispersion mode configuration, as listed in the final column 
of Table I. The close agreement found among these numbers suggests that, for any K600 
configuration, Oo can be determined to within better than a few tenths of a degree. We also 
note that these results differ significantly from an earlier measurement2 of 77.9" for the 
"standard configuration". The origin of this discrepancy is not known, but the internal 
consistency seen among the current set of measurements casts significant doubt on the 
older number. 
Table I. Values of do determined for several configurations of the K600 (first 
column), plus values that take into account known additional bend angles (final 
column). 
Bo 
77.10" 
85.78" 
98.05" 
We also devised a method to measure directly the total spin precession angle experi- 
enced by protons in passing through the QDDM and K600 magnet systems, using p + 12C 
elastic scattering. For in-plane spin-transfer measurements in which a spin- 112 particle 
Septum 
No 
Yes 
Yes 
Dispersion 
Medium 
Medium 
Low 
Method 
Direct 
Direct 
Indirect 
Converted Value 
77.10" 
77.24" 
77.06" 
scatters elastically from a spin-0 target, the measured down-up yield asymmetry in the 
FPP (eFpp), normalized by the in-plane polarization magnitude of the beam (ph), can be 
written in the following manner: 
where AFpp is the effective analyzing power of the focal plane polarimeter, Ay is the ana- 
lyzing power for the reaction being studied, 4BL5 is the angle between the beam direction 
and the beam's in-plane polarization vector at the BL5 polarimeter, a~ and a~ are the 
spin precession angles in the QDDM and the K600, respectively, P is the spin rotation 
induced in the nuclear scattering, and B,, is the lab frame scattering angle. We note that 
changing the currents in the spin-precession solenoids located in the high-energy beamlines 
dows us to vary $hBL5 without altering any other parameter in this equation. By measur- 
ing E F P ~  for multiple values of qhBL5 (which are determined solely from HEP information), 
a sine wave can be mapped out. 
We now consider the effect on Eq. (2) of rotating the K600 from the left side of the 
beam to the same scattering angle on the right. In this case, a~ and a~ (and also AFPP 
are unchanged, but both ,O and B,, reverse sign. Hence, if one again maps out E ~ p p  as a 
function of $hBL5, one can in principle separate the nuclear-rotation component (P) from 
the magnetically-induced spin-precession terms (aQ + aK) .  In practice, of course, slight 
variations in B,, (and therefore also in A, and p) from run to run complicate the data 
analysis. For each run, we rely on being able to determine B,, absolutely to very high 
precision (within a few hundredths of a degree) using a relativistic kinematics program 
and information from the K600 focal plane. Because /3 changes very rapidly as a function 
of scattering angle, it is necessary to make corrections to the measured FPP asymmetries 
to account for these variations. In order to estimate the size of these corrections, measure- 
ments were made at three different scattering angles in 1" steps, and an iterative method 
was used to estimate the variation of ,f3 with B,,. 
Table 11. Values of the nuclear rotation angle ,O and the FPP effective analyzing 
power as a function of scattering angle for p+12 C elastic scattering at 198 MeV. 
Using this technique, and assuming that a~ can be calculated reliably based on Eq. (1) 
and the mechanical measurements described earlier, we find that BQ = 131.1" f 0.3". The 
results of these measurements, including our values for the FPP effective analyzing power, 
are summarized in Table 11. This new measurement of the QDDM bend angle is also in 
moderate agreement with the best value determined previously,2 130.4" k 0.4". The older 
number was determined using a completely different method, and the level of agreement 
found between the two numbers is encouraging. We expect the value of the QDDM bend 
angle to be somewhat sensitive to details of beam steering in BL5, an idea we plan to 
explore further in the future. 
1. W.A. Franklin et al., "A Complete Set of In-plane Spin-Transfer Coefficients for Small 
Angle pp Elastic Scattering at 200 MeV," contribution to this report. 
2. S.W. Wissink et al., IUCF Sci. and Tech. Rep., May 1989 - April 1990, p. 180. 
TRAP TARGET STUDIES 
R. Pollock, M. Schatten, A. Sarrazine, M. Pelath, D. Stoller, and T. Sloan 
Indiana University Cyclotron Facility, Bloornington, Indiana 4 7408 
An elongated electromagnetic trap of a modified Malmberg-Penning variety is under 
development to test the feasibility of using trapped particles for a storage ring target. 
Electrons are used for this target prototype, although the eventual conversion to hadronic 
matter is envisioned. During 1995, the trap geometry was modified to improve the method 
of applying torque to the trap contents. The resulting electron plasma could be stably 
confined (up to three weeks on two occasions) using controlled background gas ionization 
to maintain a const ant particle number of order 10'' particles. 
In March and April, 1996, the filled trap was exposed to a cooled 45-MeV proton beam 
in the IUCF Cooler. The coasting beam was observed to heat the trapped plasma at a rate 
consistent with energy transfer through individual particle collisions, while the bunched 
beam was found to exhibit enhanced heating at a rate that could be varied by changing 
the length of the plasma column. A preliminary interpretation is that a harmonic of the 
proton orbit frequency was brought into resonance with one of the standing charge-density 
wave (Gould-Trivelpiece) modes. The interacting beam-plasma system was found to be 
stable up a beam current of 1015 particles/s, corresponding to a luminosity in excess of 
~ m - ~ s - ' ,  which is sufficient f ~ :  application to atomic physics experiments. 
An unexpected difficulty was encountered in these first beam tests. The passage of 
beam through the trap gradually deteriorated the electrostatic cylindrical symmetry that 
is needed to prevent radial growth of the plasma. Apparently the beam interacted with the 
background gas, leading to an irregular deposit of surface charge on the trap wall, which 
remained in place at room temperature for periods in excess of one hour. By heating the 
vacuum chamber, the trap recovered more quickly, dowing the tests reported above to 
be performed. The trap is being modified to remove stainless-steel surfaces exposed to 
the plasma, and to coat the copper wall with a thin gold coating to improve the surface 
conductivity. 


Precise Determination of Proton Spin-Precession Angles in the K600 Spectrometer and Beamline

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Precise Determination of Proton Spin-Precession Angles in the K600 Spectrometer and Beamline

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