Thin Scintillating Polarized Targets for Spin
Physics
B. van den Brandt 1, E.I. Bunyatova‡, P. Hautle and J.A. Konter
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
‡Joint Institute for Nuclear Research, Dubna, Head P.O. Box 79, 10100 Moscow, Russia
Abstract. At PSI polarized scintillating targets are available since 1996. Proton polarizations of
more than 80%, and deuteron polarizations of 25% in polystyrene-based scintillators can be reached
under optimum conditions in a vertical dilution refrigerator with optical access, suited for nuclear
and particle physics experiments. New preparation procedures allow to provide very thin polarizable
scintillating targets and widen the spectrum of conceivable experiments.
INTRODUCTION
Polarized scintillating targets are instruments, in which nuclei, present in scintillating
organic substances, can be polarized and the light produced in the scintillator by scattering particles is forwarded to a photo-multiplier at room temperature. In the past years
scintillator blocks of 5x18x18 mm were successfully polarized [1, 2, 3] and used in np
experiments to measure the neutron-proton spin correlation parameter at forward angles
at 68 MeV [4] and in π p scattering to measure the analyzing powers at 45-87 MeV [5].
Polarized scintillating targets offer the unique capability of coincident in situ detection
of low energy recoil protons (or other nuclei) in the target itself, and the possibility to
suppress background scattering by time-of-flight methods or by deposited energy level
discrimination.
In this communication we report on the preparation of thin polarizable scintillating foils
with an embedded NMR coil. A possible configuration is proposed for an instrument in
which a scintillating thin foil, surrounded by an extremely small quantity of material,
can be polarized.
SAMPLE PREPARATION
Samples were prepared by dissolving powder or small pieces of scintillating polymer
(PMMA) in methyl-ethyl-ketone, warming the mixture to 120 ◦ C during 1-2 hours.
Subsequently the free radical (2,2,6,6-tetramethyl-4-acetooxypiperidine-1-oxyl: "acetoTEMPO") was added, the mixture was stirred during 10 minutes, and filled into a
syringe. A mould made of a glass substrate and a mask of PTFE plate (2 mm thickness)
1
e-mail: [email protected]
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
907
was prepared (see figure 1). The PTFE plate was shaped to form little rectangular
"bays", and in each of them a 0.3 mm diameter copper wire (the NMR coil) was
placed. Using the syringe the solution was carefully injected to form a liquid layer of
ca. 2 mm thickness, held in place by the PTFE mask on the glass substrate and by
surface tension. After drying at room temperature during 2-4 hours and further drying
in an oven at 60 ◦ C, the solvent was completely evaporated, as could be concluded by
weighing the samples. The concentration of the free radical in the remaining polymer
was estimated by EPR measurements. Scintillating polymer films with an integrated
NMR coil were produced with a thickness between 20 and 100 micrometers. Their
scintillation properties were tested with an UV lamp.
In a similar way combinations of other scintillating polymers, solvents and free radicals
FIGURE 1. The mould (left) to produce scintillating foils. On the right a 70 micrometer foil of PMMA,
doped with aceto-TEMPO.
were prepared and tested, as reported elsewhere [6].
RESULTS AND OUTLOOK
The foil shown in figure 1 was placed in the mixing chamber of our dilution refrigerator
(lowest temperature 50 mK) and polarized at 2.5 T. A maximum proton polarization of
+77% resp. -76% was obtained. It took 20 min to reach 60% and 40 min to reach 70%
polarization. The relaxation time was 200 sec at 1.19 K and 2.5 T. The sensitivity of the
NMR coil is demonstrated in figure 2, showing the T.E. signal of this 0.025 g sample at
2.17 K.
In scattering experiments background scattering from materials surrounding the polarized target should be in any case minimized, but in some experiments it is not permitted
to have more than nothing around the target. In the past we have shown [7] that it is
possible to polarize thin polymer films cooled by a 0.12 µ m thin layer of superfluid
4 He. This suggests to build a target consisting of two parallel polarizable scintillating
foils, held in place by the embedded NMR coil and glued together at the rims to form
a rectangular box, covered on the inner sides with a layer of superfluid 4 He (see figure
3). The two foils should be glued together without adding glue that contains unwanted
protons, e.g. using pure solvent. The helium layer should be in thermal contact with the
908
FIGURE 2. NMR signal of 0.025 g PMMA doped with 2,2,6,6-tetramethyl-4-acetooxypiperidine-1oxyl free radical ( Thermal equilibrium signal taken at T = 2.17 K and 2.5 T).
mixing chamber via a suitable heat exchanger, while the scintillating "sandwich" would
be hanging free in the vacuum chamber of the polarized target cryostat, with close to
nothing surrounding it. The scintillation light could be coupled to the lower end of the
lightguide via an optical window of quartz in the bottom of the mixing chamber. A configuration consisting of a transparent disc of 3 mm thickness, glued with Stycast 2850 in
a stainless steel 316L flange appeared to be reliable at low temperatures.
mixing chamber
lightguide
optical window
copper wire (NMR coil)
incoming beam
2 scintillating foils
inside covered with
superfluid 4He film
FIGURE 3. A possible configuration of a thin target consisting of 2 scintillating foils, cooled via a
superfluid 4 He film, that is cooled in turn by a 3 He-4 He dilution refrigerator.
909
CONCLUSIONS
Thin scintillator foils with an embedded NMR coil have been produced and polarized.
A new scintillating polarized target, in principle suited for experiments with heavy ions,
radioactive beams etc. has been proposed. A further study of the specific technical
requirements is necessary.
ACKNOWLEDGEMENTS
The continuous interest of Dr. S. Mango is gratefully acknowledged.
REFERENCES
1. B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter, S. Mango, Proc. of SPIN96, 12th Internat.
Symp. on High-Energy Spin Physics, Sept. 10-14, 1996, Amsterdam, (World Scientific, Singapore,
1997), p. 238
2. B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter, S. Mango, Nucl. Instr. and Meth. A446
(2000) 592-599 and references therein
3. B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter, S. Mango and I. Nemchonok, Proc.
SPIN2000, 14h Internat. Spin Phys. Symp., 16-21 oct 200, Osaka, AIP conf. Proc. 570 (2001) 866.
4. M. Hauger, Measurement of the neutron-proton spin correlation parameter Azx , Inauguraldissertaion,
Universität Basel, Basel 2002.
5. R. Bilger et al., PSI Annual Rep. 1 (1997) 22
6. B. van den Brandt, E.I. Bunyatova, P. Hautle and J.A. Konter, Proc. of the GDH 2002 conference,
Genova, Italy, July 3-6, 2002, World Scientific Publishing Co., to be published.
7. B. van den Brandt, P. Hautle, Yu. Kisselev, J.A. Konter and S. Mango, Nucl. Instr. and Methods A 381
(1996) 219.
910