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

Eads, A.

Opper, A.K.

Rinckel, T.

Sawafta, R.

Schwandt, P.

Solberg, K.A.

Stephenson, E.J.

Wissink, S.W.

English

IUScholarWorks

line, relocation of the dump shielding and several auxiliary components and supply line. 
The ion optics for the beam transmission mode for the K600 at O0 is established. In 
this mode the beam is transmitted through the spectrometer end exits into a special port 
into a dump. Mounting fixtures are presently installed in order to mount the existing focal 
plane detector in the high dispersion mode. A crucial test will be conducted in August 
in order to test if the detectors can be operated so close to the transmission dump and if 
beam halo and background sources can be eliminated. 
THE K600 FOCAL PLANE POLARIMETER 
S.W. Wissink, A. Eads, A.K. Opper, T. Rinckel, R. Sawafta, 
P. Schwandt, K.A. Solberg, and E.J. Stephenson 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 7408 
During 1988, we completed the construction and calibration of a high-efficiency po- 
larimeter for detection of intermediate-energy charged particles, currently mounted down- 
stream from the medium-dispersion focal plane of the K600 spectrometer system. All 
components of the dedicated electronics are installed and tested, acquisition and prelimi- 
nary analysis software exists, and two extended running periods of approximately ten days 
each (for initial calibration of the polarimeterl and for experiment E306~) have resulted in 
detailed measurements of the polarimeter's efficiency, momentum bite, effective analyzing 
power, and count rate capabilities which have met or exceeded all design goals. 
As shown schematically in Fig. 1, the focal plane polarimeter (FPP) consists of a thick 
carbon block, followed by two sets of paired x-y multiwire proportional chambers, and two 
planes of plastic scintillator. The high-density (1.78 g/cm3) graphite target serves as the 
polarization analyzer; thicknesses of 1.27 cm, 3.81 cm, or 5.08 cm are currently available. 
For incident proton energies between 120 and 200 MeV, the p-12c analyzing powers are 
large in magnitude, and have been carefully studied for both elastic and inelastic scattering 
processes.3 The graphite block is carefully counterbalanced, and can be moved vertically in 
or out of the reaction plane in a matter of seconds. The entire analyzer mounting system 
can also be completely removed with relative ease if need be for calibration or alignment 
studies. 
After extensive bench testing, the two sets of x-y chambers were installed in their 
final location in July 1988. A complete set of spare chambers has also been built and 
tested. The smaller x and y chambers have active areas that measure approximately 
70 cm horizontally and 30 cm vertically, while the corresponding numbers for the larger 
chambers are 100 cm and 50 cm, respectively. Readout of the wire chamber information 
occurs via the LeCroy PCOS I11 system, which offers several advantages over either direct 
coincidence register readout or highly multiplexed configurations. A feature crucial for 
our application is the rapid (< 500 ns) encoding of the first wire hit in real-time, with 
presentation of the encoded output at an ECLport. This information is used as input 
to a second-level trigger processor for on-line rejection of events in which the detected 
POLARIMETER MWPC'S 
FOCAL PLANE VDC'S GRAPHITE ANALYZER I I 
POLARIMETER DE 
Figure 1. Schematic representation of the K600 spectrometer focal plane and polarimeter 
systems, as viewed from above. The functions of the various elements are described in the 
text. The scattered beam enters from the left of the drawing, and typical trajectories are 
indicated by the dashed lines. 
particle underwent no significant scattering in the analyzer (carbon) target (see below 
for details). We estimate that, under optimum conditions, this could increase our data 
acquisition rate capability by approximately a factor of twenty; factors of ten to fifteen 
have already been demonstrated, though our inclusion of prescaled "straight-through" 
events reduces this somewhat. Other PCOS I11 features include remotely programmable 
thresholds for each chamber card, computer control of delay lengths ("software cable- 
cutting") for each latching module, clusterization of adjacent wire hits within the encoder 
for each chamber, and minimal word transfer over the (slow) CAMAC dataway. Under 
typical running conditions, full information from all four MWPC's can be compacted into 
less than ten 16-bit words for readout by the MBD. 
The scintillator planes consist of 0.64 cm and 7.62 cm thick NE102 stock. Depending 
on the incident particle type and energy, these can serve as either a AE-E pair or as a 
AE-high-resolution-AE combination. The 0.64 cm scintillator is a single sheet, viewed 
by four low-gain 5 cm photomultiplier tubes (Hamamatsu R329), while the 7.62 cm thick 
plastic has been segmented into six vertical non-overlapping sect ions, each viewed by 
two similar 5 cm tubes. This segmentation was considered necessary in light of the high 
background rates that may arise under certain spectrometer running conditions and the 
large volumes of scintillator involved. All of the thick plastic segments were equipped with 
rectangular light guides at each end, using a geometry chosen (via Monte Carlo response 
simulations) to make light collection over the scintillator area as uniform as possible. 
After fan-out, the sixteen anode signals are discriminated and fed into another second- 
level trigger processor, which allows user-selectable logic levels to be generated for all 
possible combinations of the 16 inputs. Thus, for the scintillator system, the high-voltage 
bias applied to the photomultiplier tubes, the discriminator thresholds, and even the 16- 
fold scintillator coincidence logic are all under remote computer control. After software 
correction for varying angles of incidence, resolutions of 3 4 %  have been obtained in the 
thick scintillator, with a uniformity over the entire active surface of better than 2% in 
response to monoenergetic particles. 
Electronically, a first-level trigger is generated by a coincidence between the S1 mean- 
timer (used for timing in the focal plane VDC's) and a four-fold AND of the A E signals. 
If this trigger is not vetoed by the wire chamber high voltage alarm or a "busy" signal 
from the event trigger module, then all focal plane and polarimeter ADC's and TDC's are 
gatedlstarted, and the four PCOS encoders are enabled. The focal plane multiplexers are 
then checked to be sure that there was at least one struck wire in both the front and rear 
VDC's. If so, we have a "good focal plane" event, a prescaled fraction of which (typically 
5%) are always sent to the event trigger module. At the same time, the first-hit information 
from each of the four polarimeter wire chambers is sent from the encoder ECLports to a 
pair of LeCroy Memory Look-up Units (MLU's). If the correlation between the two x 
chambers and the correlation between the two y chambers indicate that the proton went 
essentially straight through the carbon analyzer, then there is no useful information to be 
gained from this event concerning the proton polarization, and the event can be discarded. 
(These correlations are determined empirically by collecting data with the carbon analyzer 
removed, drawing a window around the locus of unscattered events, then down-loading 
this information into the LRS 2372 MLU, which performs the same bit test in hardware 
as does our window in software.) Good focal plane events which are not vetoed by the 
MLU conditions are then OR'ed with the prescaled focal plane event stream to provide 
our final event definition. Trigger signals from events which do not meet these criteria are 
then used to clear and reset all CAMAC and NIM modules to await the next event. The 
total time required to make these decisions and reset all modules is just under 5 ps. 
After a few short periods of split beam for initial hardware tests, we received our first 
dedicated beam during November 1988 for a polarimeter calibration run that is described 
in detail elsewhere in this report.' We also developed a "first-pass" set of software con- 
ditions that could be easily appended to the existing focal plane analysis package; these 
conditions are also discussed in the context of the calibration measurement.' During this 
run (approximately 16 shifts of beam) we were able to thoroughly test all aspects of the 
FP/FPP electronics (concentrating especially on the fast reject and clear electronics men- 
tioned above), debug and substantially improve our increasingly complex acquisition and 
sorting software, and add some much-needed on-line diagnostic checks to our analysis 
programs. We accomplished essentially all of the above goals in our allocated beam-time, 
with no time lost to FPP hardware problems or electronics optimization, having performed 
most of these tasks with a few shifts of split beam from the Cooler prior to receiving ded- 
icated beam. At each of the three incident proton energies required, only 2 hours or so 
were required to achieve .-40 keV resolution, more than adequate for our purposes. 
It is useful to point out the difficulties involved in trying to calculate the effective 
parameters of the polarimeter from measured values for cross sections and analyzing pow- 
ers. Each of these quantities must be averaged, with appropriate weighting, over incident 
energy (since the protons interact at different depths within the carbon analyzer), over ex- 
citation energy (due to the relatively poor energy resolution achievable in the polarimeter 
scintillators), and over scattering angle (due to multiple scattering, largely in the carbon 
block). All of these factors, and the relative weightings, are also affected as much by soft- 
ware choices as by hardware. It is imperative, therefore, that one arrive at  a reasonable, 
reproducible procedure by which one sets these hardware and software cuts, and that this 
procedure can be easily transferred from calibration running to production running. Our 
preliminary analysis of the calibration data shows no systematic dependence of either the 
polarimeter efficiency or effective analyzing power on the position of the scattered proton's 
trajectory along the focal plane, and all false asymmetry terms were determined to be zero 
within error. ' 
In January 1989 we received a second block of dedicated beam-time for E306, a 
measurement of the spin observables DNNt and P for selected unnatural-parity transitions 
in 12c and 160. This work is also described earlier in this report.2 Prior to this run, we 
had made several changes and improvements to the focal plane polarimeter system, each 
of which had to be thoroughly tested. We incorporated more sophisticated algorithms 
for determining proton trajectories and energy losses in the polarimeter wire chambers 
and scintillators. "Intelligent" diagnostic software used both cumulative and incremental 
scaler values to calculate spin-dependent wire chamber efficiencies, system dead-times, 
sampling fractions, and hardware-rejection factors which were crucial in rapidly noting and 
identifying possible hardware or analysis problems. Electrically isolating the scintillator 
high voltage power supply greatly reduced noise in the scintillator signals, while switching 
from twisted-pair to co-axial cables for our ADC delay lines eliminated cross-talk problems 
noted in replay of the calibration data. As a final check on the whole system, we began 
our production run by repeating a calibration point, which compared favorably with our 
earlier work. 
To increase our acquisition rate for selected data, we employed several schemes to 
eliminate events outside the excitation energy region of primary interest. We intercepted 
the elastically scattering protons with a copper block mounted on a movable track inside 
the K600 vacuum box, thus preventing these events from producing even first-level triggers, 
while disabling particular pre-amp cards on the VDC's of the K600 focal plane effectively 
vetoed regions of either very low or very high excitation at a second-level in hardware. 
For future runs, we are investigating the use of veto paddles mounted directly behind 
the focal plane S1 scintillator, or the possibility of replacing the existing S1 scintillator 
(approximately 1 m in length) with a much shorter version, though the effects of this latter 
technique on the VDC drift time spectra would need to be studied in some detail. 
Though not directly related to the operation of the focal plane polarimeter, we are also 
commissioning much of the additional equipment that will be required for full polarization 
transfer capabilities at IUCF. This includes the successful installation and beam-optics 
tests of two superconducting spin-precession solenoids in the high-energy beamlines, de- 
velopment and calibration of associated in-beam transmission polarimeters, the addition 
of two horizontal ("y") drift chambers to the K600 focal plane system for complete verti- 
cal trajectory information, and structural rearrangement of the southern shielding wall in 
the K600 experimental area to allow for rotation of the spectrometer system to angles as 
large as 44O to the right of the beam. Each of these projects is described in detail else- 
where in this document, and is proceeding essentially on schedule. We are fairly confident 
that all of these tasks will be completed shortly, and that the first in-plane spin transfer 
measurements on the K600 will be carried out before the end of 1989. 
1. E.J. Stephenson et al., "Calibration of the K600 Focal Plane Polarimeter", IUCF 
Scientific and Technical Report, 1988. 
2. A.K. Opper et  al., "Measurements of the Spin Observables DNNt and P in Inelastic 
Proton Scattering from 12C and '% at 200 MeV", IUCF Scientific and Technical 
Report, 1988. 
3. see, for example, IUCF Scientific and Technical Report, 1982, p. 182-188. 
TARGET LAB TECHNICAL STATUS 
W. Lozowski and J. Hudson 
Target preparations for 1988 included: 697~i, 1 0 j l l ~  9 H 3 l0BO 3 3 1 ° ~ 2 0 3 ,  CD2, 
CD2/CH2 mix, l2tl3c, CF2, ~ i ~ ~ s ~ ~ 0 ~ ,  Si, 3 4 ~ ,  39KF, CO, 87988~r, Ag, l18sn, Ag, and 
2 0 8 ~ b .  
Target development work for 1988 included: 
1) An evaporated ' O B ~ O ~  target of 3.5 mg/cm2 was prepared for an experiment with 
the K600. The oxide was obtained by thermal decomposition of isotopically enriched boric 
acid. To remove all hydrogen (a serious contaminate at some angles for the experiment), 
the decomposition had to be carefully controlled in high vacuum over a period of ten 
hours. A small tube furnace, for use within the high-vacuum evaporator, was constructed 
in the lab for this purpose. The oxide was then evaporated onto 260 pg/cm2 Au leaf. The 
details of this procedure are contained in a contribution to the December 1988 issue of the 
International Nuclear Target Development Society (INTDS) Newsletter. 
2) Much effort was devoted to achieving 2-3.5 mg/cm2 208Pb targets with 28 keV 
resolution and 5 0.001 Atom % 0 2  and C. The difficulty was due to the chemical reactivity 
of pure Pb, the need to conserve the expensive isotope, and the need to exclude all heavy 
elements from the target. Although great targets were not achieved until the first week of 
February, 1989, the progress in 1988 was: a) 208Pb metal, more pure than that supplied by 
ORNL, was obtained from 2 0 8 ~ b  CO3 with an efficiency of 98.36%. The method developed 


The K600 Focal Plane Polarimeter

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The K600 Focal Plane Polarimeter

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