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

Li, P.

Stephenson, E.J.

Wells, S.P.

Wissink, S.W.

English

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THE K600 FOCAL PLANE POLARIMETER 
S.W. Wissink, P. Li, E.J. Stephenson, and S.P. Wells 
In 1989 the capability of performing complete sets of polarization transfer measure- 
ments at IUCF was achieved for the first time. This involved only minor upgrades of the 
K600 focal plane and focal plane polarimeter (FPP) hardware, but significant installation 
and calibration efforts on devices located in several other areas of the laboratory. These 
projects centered largely on the equipment and facilities necessary to be able to orient the 
polarization of the primary beam in an arbitrary direction, to measure precisely all three 
components of the beam polarization, and to have sufficient knowledge of the beamline 
optics to be able to determine the polarization at any subsequent point along the beam's 
path. Our goal was to develop a system flexible enough to supply proton bea8ms of any 
desired polarization orientation to as many target areas as possible. Particular emphasis 
was given to the K600 spectrometer, the bea.m swinger area, and injection into the Cooler 
ring, as each of these programs has already demonstrated a need for in-plane (horizontally) 
polarized proton beams. 
Descriptions of the primary components and physical arrangements of the K600 focal 
plane and focal plane polarimeter scintillator detectors and wire chambers are provided 
in Ref. 1. In addition to the equipment described therein, two horizontal drift chambers 
(HDC's) have since been installed directly behind (downstream) of the vertical drift cham- 
bers (VDC's) in the focal plane. Due to various technical problems with the HDC's, which 
provide y-axis or vertical trajectory information, we are currently in the process of replac- 
ing these with more standaxd MWPC's. By using two adjacent cha~nbers with staggered 
wires, we will achieve an effective wire spacing of 1.0 mm. The large separation of the two 
sets of chambers (in excess of 20 cm for most particle trajectories) will provide extremely 
accurate vertical position and angle determinations. 
In November 1989 we completed the first measurements on the K600 which required 
that the polarization vector of the incident proton beam lie in the (horizontal) scattering 
plane. Technical details and results from this experiment (E290) can be found elsewhere 
in this report.2 Prior to the production running phase of this experiment, a series of 
calihra.tion measurements was performed which utilized Loth of the superconducting spin 
precession solenoids (located just before and after the BL3 energy analysis magnet), the 
two permanently installed p+d polarimeters3 associated with each solenoid, and an older 
proton polarimeter, based on p+'2C elastic scattering, thak was temporarily positioned 
just downstream of the K600 scattering chamber. By systema.tica1ly varying the two 
solenoid currents and detecting the vertical and sideways polarizations in each of the three 
polarimeters, it was possible to deduce both the conversion factors from solenoid current 
to precession angle for each solenoid, and the bend angle of the proton's trajectory in 
the energy analysis magnet (BM1 BL3) and in the QDDM (BM3 BL8). The locations of 
these elements are illustrated in Figure 1. Because some of these calibration schemes rely 
on the stability of the proton polarization orientation out of the main stage cyclotron, all 
measurements were repeated several times in relatively rapid succession, so that slow, time- 
dependent variations in the polarizatio~l direction and magnitude could be accomodated 
in the analysis. For example, data would be taken with a particu1a.r solenoid alternately 
off or energized to a certain value, from which one could interpolate the needed beam 
polarization parameters. 
K600 spectrometer and 
focal plane polarimeter 
Figure 1. Schematic floor plan of the IUCF high energy beam lines, indica.ting the locations 
of the principal magnetic elements (dipoles and solenoids) that affect the orientation of the 
proto11 polarization vector, and the polarimeters used to measure the normal and sideways 
polarization components. 
As a result of these procedures, the horizontal bend in the beam line between the two 
p+d polaximeters was found to be 44.8" f 0.6', which is consistent with the assunlption 
that the beam undergoes a single 45" bend in the energy analysis magnet (BMI BL3, see 
Fig. 1). Similarly, the net horizontal bend between the second (BL5) p+d polaximeter 
and the K600 beam line (BL8) had an average value of 130.4" f 0.4', also in agreement 
with the geometrically determined bend angle of 130" for the QDDM magnet system tha.t 
serves as bending magnet 3 in bean1 line 8. Previous estimates that magnet currents of 
a.pproxima,tely 48.4 A would be required to produce 90' precession of a 200 MeV prolor1 
Learn in the spin precession solenoids were also verified. 
With these calibrations complete, we could reliably determine all comyo~lents of the 
proton polariza.tion at the K600 target based on the asymmetries measured in the two p+d 
pola.rimeters and the known currents in the two spi11 precession solenoids. To perform 
in-plane polarization transfer measurements, two additional pieces of information were 
required: a means of determining the bend angle of the scattered proton's tra,jectory 
within the K600 spectrometer, and an accurate determina,tion of the effective analyzing 
power of the focal plane polarimeter for sideways polarization. Because the focal plane 
x-chambers (VDC's) provide angular information, the first task reduced to a determinakion 
of the angle of the focal plane chambers with respect to the central ray at the entrance 
to the K600 spectrometer. This angle was deduced geometrically to be 77.9" f 0.05" 
through two independent sets of measurements, which agreed with each other within their 
estimated uxlcert ainties. 
To determine the effective analyzing power of the focal plane polarimeter for the 
sideways polarization component, we again exploited some of the simplifying relationships2 
which exist among the reaction spin observables for + + O --+ + + 0 spin configurations. In 
particulax, if one investigates proton elastic scattering from O+ nuclei in kinemastic regimes 
where the reaction analyzing power Ay is close to zero, then the effect of the nuclear 
sca.tteri11g is to maintain the magnitude of the in-plane polarization but to rotate it by 
a.n angle ,d which is independent of the initial orientation. Thus, if one were to perform a 
series of measurements with a purely in-plane polarization vector lying at several different 
orientations, one would map out an up-down asymmetry in the focal plane thak should 
vary sinusoidally with the angle of this orientation. More explicitly, if the polarization of 
the proton beam incident on the K600 target is expressed as 
PL = Po cos q5 (1) 
then for elastic scattering from a spin-0 target near Ay = 0, one would measure an up-down 
asymmetry in the focal plane polarimeter given by 
where AFpp  is the effective polarimeter analyzing power of interest, and d! is a. constant 
spin rotation due to contributions from both the nuclear scaktering ( P )  and additional spin 
precession within the spectrometer magnet system. Note that these two parameters are 
completely decoupled, so that lack of precise knowledge of either /? or the bend angle of 
the K600 do not affect the determination of A F p p .  
As part of experiment E290, AFPP was calibrated using the above procedure at pro- 
ton bombarding energies of a#pproximately 200, 190, and 180 MeV on a 12C target. In 
Figure 2 are shown the data at 190 MeV, which indicate a value of AFPP = 0.483 f 0.010. 
This is in reasonable agreement with earlier measurements4 of AFPP for normal compo- 
nent polarization, though precise comparisons are difficult since much of the pola.rime t er 
software has been rewritten to incorporate more sophistica.ted trajectory and energy loss 
caalcula.tions. It is useful to point out that the same procedures were used to analyze the 
E290 production data as were used to determine A F P P ,  and that the two data sets were 
taken at the same beam energy and with the same spectrometer angle calibration. 
For the next year, further development of the K600 spin transfer system will continue 
to be centered largely on equipment external to the y~la~rimeter itself, which is essentially 
complete. Data taken a8t either very srnall scattering angles ( Q l a b  < 1 6 O )  or on the right 
side of the beam presently require that the beam be stopped on a Faraday cup mounted 
Incident polarization angle q5 (deg) 
Figure 2. The up-down asymmetry measured in the K600 focal plane polarimeter, normal- 
ized by the magnitude of the incident polarization, plotted versus the angle of the incident 
polarization, as defined in equations 1 and 2 in the text. The data correspond to proton 
elastic scattering from a l2 C target at 188.9 MeV near the zero-crossing of the analyzing 
power at 24.85". 
within the K600 scattering chamber. The shielding on this cup is sufficiently poor that 
the room background generated limits operation of the focal plane polarimeter to a few 
na.noa,mperes of beam current. We are investigating several schemes for reducing this 
background radiation, either through use of a septum magnet to allow for transport to 
an external beam dump a.t significantly smaller scattering angles, or through extensive 
shielding of an internal Faraday cup that would be mounted downstream of the present 
scattering chamber. 
1. S.W. Wissink et al., IUCF Scientific and Technical Report, May 1988 - April 1989, 
p. 204. 
2. S.W. Wissink et al., "Precise  determination^ of Proton Analyzing Powers for 180-200 
Me V Elastic Scattering on l2 C and He ", IUCF Scientific and Technical Report, May 
1989 - April 1990. 
3. A.K. Opper et al., IUCF Scientific and Technical Report, May 1988 - April 1989, 
p. 89. 
4. E.J. Stephenson et al., IUCF Scientific a.nd Technical Report, May 1988 - April 1989, 
p. 94. 


The K600 Focal Plane Polarimeter

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The K600 Focal Plane Polarimeter

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