Polarized Internal Target Experiments
(PINTEX) at the Indiana Cooler
B. v.Przewoski
IUCF, Milo B. Sampson Lane, Bloomington, IN 47405, USA
Abstract. The PINTEX 1 facility at the IUCF Cooler consists of a polarized, internal target an an
azimuthally symmetric detection system for charged particles. The polarized atomic beam source
can be used to either produce a hydrogen target or a deuterium target. The target thickness obtained
with a 12mm diameter and 25cm long storage cell is on the order of 10 13 atoms/cm2 . The variable
gradient of the two transition units facilitates changeover from hydrogen to deuterium. The ABS
is capable of producing pure deuteron vector polarization with a theoretical maximum value of
+2/3 and pure tensor polarization of theoretical maximal values of +/-1. The direction of the
vector polarization can be reversed by reversing the direction of the holding field at the target.
A hydrogen target was used for measurements of spin correlation coefficients in pp elastic sattering
and pion production. The deuterium target served to measure spin correlation coefficients in pd
elastic scattering and pd breakup.
THE ATOMIC BEAM SOURCE
The atomic beam source (ABS)[1] has been operational since 1993. In 2000 it was upgraded by installing two remotely controlled medium-field transition units to produce
either polarized deuterium or hydrogen. Atoms from an 18 MHz dissociator emerge
through an aluminum nozzle which is kept at liquid nitrogen temperature. The atoms
then pass along the axis of a set of sextupole magnets where they are separated according to their electron polarization. In the following medium-field transition unit (MFT-1),
transitions between hyperfine states are induced. The atoms pass along the axis of a second set of sextupole magnets whereby in the case of hydrogen an atomic beam in a pure
spin state is prepared. At last, depending on which polarization state is desired, another
transition between hyperfine states is induced in a second medium field transition unit
(MFT-2). The atoms are then injected into the storage cell which is located in a weak
holding field generated by a set of Helmholtz coils.
Medium Field Transitions
An MFT operates in magnetic fields of B 01Bc to B 02Bc , where Bc is the
hyperfine interaction field of 50.7 mT for hydrogen and 11.7 mT for deuterium. An
appropriate field gradient along the beam direction is required to satisfy the condition
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Polarized INternal Target EXperimets
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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of adiabatic passage at a given, fixed RF frequency. Multiple transitions can be made by
adjusting the static field so that the beam passes in sequence through field regions where
the populations of different pairs of hyperfine states are interchanged.
Originally, the atomic beam source was equipped with a single, fixed-gradient MFT
located after the first set of sextupole magnets. In order to facilitate switching between a
proton or a deuteron target, two new transition units with variable gradient and variable
static field were installed. The linearity of the gradient field over the transition region as
well as the homogeneity of the static field were determined prior to installation of the
units in the ABS. For deuterium the gradient field is set to +0.2 mT/cm. The RF coil of
each MF unit consists of 12-turn solenoids of 1.6 mm diameter wire with a length of
70 mm and an I.D. of 34 mm. The units are water cooled and operated at 60.5 MHz for
hydrogen and 30 MHz for deuterium. The currents used to drive the offset and gradient
coils are controlled remotely . This makes it possible to quickly change, during the
experiment, between vector-, positive tensor- and negative tensor polarization.
Since the operation of an ABS with hydrogen has been discussed extensively
elsewhere[2], we limit the following discussion to deuterium. After the first set of
sextupoles the atomic beam consists of states 1+2+3, where the states are labeled in
order of decreasing energy in a non-zero magnetic field[3]. One or more transitions can
be made sequentially in MFT-1.
Transitions are selected by changing the static field while the gradient field is kept
constant. For small static fields no transitions are made. When the static field is increased, the atoms first undergo a 3 4 transition. When the field is further increased,
the atoms undergo the 3 4 transition followed by the 2 3 transition. When the static
field is increased even further, the atoms undergo the 3 4, 2 3 and 1 2 transitions sequentially. The second set of sextupoles eliminates state 4, so that one is left with
states 1+2+3, 1+2, 1+3 and 2+3 depending on how many of the sequential transitions
are made. The corresponding maximum polarizations of the atomic beam, before entering MFT-2, are Pz Pzz 13 13, Pz Pzz 23 0, Pz Pzz 13 0
and Pz Pzz 0 1, where Pz Pzz are vector and tensor polarization. MFT-2 is only
needed to produce positive tensor polarization. After passing MFT-1 the beam contains
states 1+3 with polarizations Pz Pzz 13 0. If the parameters of MFT-1 are set
for the the 3 4 transition, the atomic beam contains states 1+4 with polarizations
Pz Pzz 0 1 when it is injected into the target cell.
OPERATION
In order to minimize systematic errors of measured polarization observables, it is desirable to be able to alternate between target states frequently, preferably while beam is
stored in the Cooler so that data in different target states are taken with the same stored
beam. This was accomplished by remotely controlling the currents to the transition units.
A sinusoidal function with exponentially decreasing amplitude was programmed to reproducably degauss each unit before every change to a new current. Fig. 1 shows the
static field current through MFT-1 (trace a), the beam current (trace b) and the trigger
rate (trace c) as a function of time. Trace (a) shows how MFT-1 is degaussed prior to
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FIGURE 1. MF1 static current (a), stored beam current (b) and trigger rate (c) as function of time.
each new current setting. Trace (b) shows the Cooler being filled, a flattop at 135 MeV
during which data are accumulated, a ramp to 200 MeV (which corresponds to an increase in beam current), another data taking flattop and the reset of all Cooler magnets
during which the stored beam is discarded before the ring is filled with protons of the
opposite spin state. Note that the trigger rate (trace (c) in Fig. 1 increases during degaussing, because the target density increases by 1/3. Events during degaussing are ignored in
the final analysis.
SILICON DETECTORS IN AN ATOMIC HYDROGEN
ENVIRONMENT
The target cell is surrounded by 18 position sensitive silicon detectors (see elsewhere in
these proceedings). When we comissioned this setup we found that the ambient atomic
deuterium or hydrogen gas apparently "poisons" silicon detectors. Even short exposures
(30 min) to ambient hydrogen in the region surrounding the target caused an increase
in leakage current that rendered the detectors useless for data acquisition. Fortunately,
we found that the detectors recovered once they were removed from atomic deuterium.
Deuterium poisoning of the silicon detectors was eventually avoided by shielding them
from ambient hydrogen using aluminized mylar foil. In addition, copper recombination
baffles were placed around the feedtube and the ends of the storage cell. In this way,
atomic deuterium was quickly recombined into harmless molecular deuterium. Although
the problem was solved by isolating the detectors from the harmful atomic deuterium,
we conducted an offline study in an attempt to understand the problem. Several 300µ m
thick detectors were tested. A detector was exposed to atomic hydrogen from the ABS,
while another served as the reference detector under otherwise identical conditions. In
this way changes in leakage current due to temperature change could be distinguished.
Fig. 2 shows exposure and recovery curves for two different detectors. Although both
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FIGURE 2. Silicon detector leakage current as a function of time. The left figure shows the increase in
leakage current due to exposure to atomic hydrogen while the right figure shows the decrease in leakage
current after the exposure. The two curves correspond to two apparently identical detectors.
detectors are seemingly identical and have the same initial leakage current of 0.05µ A,
their leakage currents after exposure to atomic hydrogen differ significantly. It thus
appears that the susceptibility to atomic hydrogen poisoning also depends on some
(unidentified) intrinsic property of the individual silicon wafer.
SPIN EXCHANGE
When we began using the deuterium target, we noted that the tensor polarization was
significantly lower than expected. We investigated obvious causes for the deficiency,
such as insufficient background subtraction, incomplete rejection of unwanted states by
the sextupoles, wall depolarization and inefficiency of the transition units. None of these
mechanisms quantitatively explained the low tensor polarization.
Spin exchange between deuterium atoms has a significant effect only on the tensor
polarization. It also depends on the target density, in such a way that the polarization
grows larger as the target density is decreased. We performed a series of measurements
where we determined the tensor polarization as a function of target thickness by reducing
the gas flow in the dissociator compared to its normal level. For these measurements we
operated the ABS without medium-field transitions in order to be independent from
inefficiencies of the transition units. In this case the target polarization is a mixture
of vector and tensor polarization Pz Pzz 13 13. The tensor component is
shown in Fig. 3 as a function of relative target thickness. The data are normalized
with a common, arbitrary factor. The curve shown is from Walker at al.[5] Note that
the tensor polarization is negative and that polarizations of larger magnitude are indeed
found at lower target densities. It can be seen from Fig. 3 that the calculation explains
the observed relative change in target tensor polarization with target density.
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FIGURE 3. Target tensor polarization as a function of target thickness. The curve is the calculated effect
from spin exchange.
CONCLUSIONS
We have commissioned a vector and tensor polarized deuterium target for the IUCF
Cooler. To date, we have used this target to measure beam and target analyzing powers
as well as spin correlation coefficients in pd elastic scattering at 135 and 200 MeV.
ACKNOWLEDGMENTS
This work was supported by NSF grants PHY-9602872, PHY-9722556, PHY-9901529
and DOE grant DOE-FG02-88ER40438.
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Haeberli, W. et al. Phys. Rev. C, 55, 597 (1997).
Haeberli, W. Ann. Rev. Nucl. Sci., 17, 373 (1967).
Roberts, A.D. et al.Nucl. Instr. Meth. A, 322, 6 (1992).
Walker, T. et al. Nucl. Instr. Meth. A, 334, 313 (1993).
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