polarized scintillator targets

*Corresponding author.E-mail address: [email protected] (B. van den Brandt).

Nuclear Instruments and Methods in Physics Research A 446 (2000) 592}599

Polarized scintillator targets

B. van den Brandt!,*, E.I. Bunyatova", P. Hautle!, J.A. Konter!, S. Mango!

!Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland"Joint Institute for Nuclear Research, Dubna, Head P.O. Box 79, 101000 Moscow, Russia

Received 1 November 1999; accepted 16 November 1999

Abstract

The hydrogen nuclei in an organic scintillator have been polarized to more than 80% and the deuterons in its fullydeuterated version to 24%. The scintillator, doped with TEMPO, has been polarized dynamically in a "eld of 2.5 T ina vertical dilution refrigerator in which a plastic lightguide transports the scintillation light from the sample in the mixingchamber to a photomultiplier outside the cryostat. Sizeable solid samples with acceptable optical properties and lightoutput have been prepared and successfully operated as `livea polarized targets in nuclear physics experiments. ( 2000Elsevier Science B.V. All rights reserved.

Keywords: Scintillating polarized target; Polarized scintillator; DNP

1. Introduction

The method of dynamic nuclear polarization(DNP) requires paramagnetic centers (p.c.) in thevicinity of the nuclei to be polarized. They arenormally provided either by chemical doping or byirradiation of the target material, but quickly decayat room temperature. In the past TEMPO (2,2,6,6-methyl-piperidine-1-oxyl), a remarkably stable freenitroxyl radical, had been tested and discarded asunsuited to achieve high nuclear polarizations [1].TEMPO, which has a melting point of 36}400Cand remains unscathed even by much higher tem-peratures, can be introduced into hydrogen-richsubstances solid at room temperature either bysolution or by di!usion [2]. Recently, interestingnuclear polarizations could be obtained at dilution

refrigerator temperatures in powders, thin foils andthin-walled tubes of polyethylene, deuterated poly-ethylene, PTFE, polystyrene, deuterated polysty-rene and ethylene}propylene copolymer which hadbeen doped with TEMPO by di!usion [3}6].Believing that the availability of polarized nuclei ina scintillator would provide new interesting experi-mental possibilities for the measurement of spin-dependent observables in nuclear and particlephysics, we proceeded to introduce free nitroxylradicals in some commonly used scintillating ma-terials, which actually consist to about 98% ofpolymeric material and can be regarded as solidsolutions of luminescent additives in transparentvinyl-aromatic polymers. The additives are aro-matic compounds consisting of bound benzenerings, e.g. p-terphenyl, and oxazole derivatives ofbenzene, e.g. 1,4-di-(2-(5-phenyloxazolil))-benzene(POPOP), which when dissolved in an aromaticpolymer give rise to chemical structures with excessconjugation in benzene rings, double ethylene

0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 2 7 7 - 2

1BC-400 and BC-408, commercially available PVT basedscintillator by BICRON. and a PS based scintillator made inJINR, Dubna.

2Aldrich, Industriestr. 25, CH-9471, Buchs, Switzerland.

3BC-490, by Bicron, supplied in kits consisting of a partiallypolymerized resin (prepolymerized plastic phosphor), a catalyst(organic peroxide) and a solvent (vinyltoluene monomer).

bonds and other groups, producing molecularn-orbitals spread over the entire conjugationregion. It is the excitation of the electrons withinthese orbitals (n-electrons) by the energy releasedby nuclear radiations that results in luminescence.Now nitroxyl free radicals do not form complexeswith the benzene rings in the scintillator and can beconsidered a classical quenching impurity. Theluminescence e$ciency of a scintillator is furthergreatly reduced even by the smallest impurity, likecoloured substances or suspended particulates. Itwas therefore questionable that one could attainowing to the addition of free nitroxyl radicals satis-factory nuclear polarizations while conserving theluminescence e$ciency of the basis material.

2. Preparation of the samples

Readily available for us were plastic scintillatorsbased on polyvinyltoluene (PVT) and a scintillatorbased on polystyrene (PS).1 First a few sampleswere prepared by dissolving the scintillator in tol-uene together with the free radical and then lettingthe toluene evaporate completely from the solutionin appropriate containers. Proton polarizations upto 65% were achieved in the "lms so obtained, butthese could be produced transparent at "rst sightonly with a thickness below 80 lm, which we judgedimpractical to handle and not suited to make a work-ing scintillating detector of 18]18]5 mm, asrequired for a series of experiments at the PSIaccelerators [7].

We then proceeded to introduce TEMPO2 bydi!usion (details of the process have been givenelsewhere [8]) into 40]40]0.5 mm3 platelets ofBC-400, a polyvinyltoluene-based material. Thethickness chosen was the result of a trade o! be-tween a reasonable guess on the penetration depthof TEMPO in the polymer [2] and the possibilty toobtain reasonably regular objects after the thermaltreatment connected with the di!usion process. The

highest degree of proton polarization achieved was57%, possibly due to a gradient in free radicalconcentration in the platelets. The samples hadacquired after the treatment a yellow colour [9],while staying transparent and homogeneous.A quick test at room temperature with a 90Srsource showed that in these samples some degree ofscintillating properties had survived the introduc-tion of TEMPO in the material, and 10 of theseplatelets, cut down to a size of 18]18 mm, stackedand pressed together, have been the "rst polarizedscintillating target ever used in a nuclear physicsexperiment [10]. The e$ciency of the TEMPO-doped scintillator platelets' stack was about 5% ofa volume equivalent block of untreated scintillatorunder the experiment's conditions.

It was clear at this point that we had to "nda way to achieve a homogeneous distribution ofTEMPO in the scintillator for a better nuclearpolarizability and to produce ca. 20]20]5 mm3

solid blocks of polarizable scintillator with betterluminescence e$ciency and greater easiness of ma-nipulation and optical interfacing. Ideally, the poly-merization of a monomer (styrene, vinyl toluene)with luminescent additives and the free radical ne-cessary for DNP in a suitable mould would havesolved the problem, avoiding any processing of theready polymer which could deteriorate the qualityof the scintillator. Unluckily, almost all knownstable free radicals, including TEMPO, are poly-merization inhibitors, which readily react with ac-tive centres arising in the course of polymerizationand die out.

Nevertheless, the availability of a partially poly-merized plastic scintillator,3 which can be cast inany form and cured to full hardness at temper-atures not higher than 800C, moved us to introduceTEMPO in the reaction mass and carry out theprescribed curing procedure. The result wasa transparent and polarizable but soft solid, inwhich the TEMPO concentration as estimated byEPR spectroscopy was practically the same before

593B. van den Brandt et al. / Nuclear Instruments and Methods in Physics Research A 446 (2000) 592}599

and after polymerization. Several trials involvingthe variation of di!erent parameters of the curingprocedure had no success: either the polymeriz-ation was not complete, or the material was notpolarizable, or it was not luminescent or any com-bination of them.

We also tried to polymerize styrene in the pres-ence of TEMPO, which was introduced for a cal-culated concentration of 2]1019 paramagneticcenters per gramme (p.c./g) of the polymer in a solu-tion of styrene with p-terphenyl and POPOP andplaced in an ampoule from which air was evacu-ated. After one year at room temperature no tracesof polymerization could be detected: the pink solu-tion had remained liquid, TEMPO had worked asa classical inhibitor. The solution was then poly-merized under the thermal conditions used formaking PS-based scintillators. When the polym-erization was about 90% ready, the material turnedcloudy, and the "nal result was a solid, infusible,non-transparent, pink, weakly scintillating polymerin which the TEMPO concentration had decreasedby two orders of magnitude. Its infusibility in-dicated that the polymer chains were cross linkedat the end of the reaction. May be optimized ther-mal conditions will one day make it possible toproduce good scintillating and polarizable samplesby polymerization of styrene with luminescent ad-ditives and TEMPO.

We came back to the technique of dissolving thescintillator in toluene, adding the free radical andthen letting the toluene evaporate completely atroom temperature. Using polystyrene-based scintil-lator and TEMPO we obtained again bubble free,at "rst sight transparent and scintillating, not morethan 0.1 mm thick "lms in which proton polariza-tions of up to 80% could be achieved. We were alsoconfronted with the fact that clearly the methodprovided a uniform distribution of free radical mol-ecules in the sample, allowed to achieve a corre-spondingly high degree of polarization but did notallow to produce reasonably sized polarizable scin-tillators, not even as stacks of platelets, a con"gura-tion which anyway operational experience hadshown to be a poor solution for an e$cient lightcollection and transmission.

Finally, owing to the extraordinary toughness ofTEMPO, a method to produce thicker samples was

found in warm pressing a set of the thin "lmsobtained as described above. For this purpose the"lms were loaded in a stainless-steel press of appro-priate inner size, e.g. 20]20]20 mm3, and pressedby means of a stamp and a set of screws. Theassembly was then placed in an oven at a temper-ature ¹

1chosen so that the scintillator was plastic

but not melting, for a time t1, which has been

optimized by trial and error, and then taken out ofthe oven and allowed to cool down naturally toroom temperature. ¹

1and t

1have been varied for

a great number of samples, typically between 1000Cand 1300C and 1 and 4 h, respectively, until theconditions to produce well polarizable blocks withat "rst sight acceptable scintillation and opticalcharacteristics could be determined.

3. Polarization results

A few chunks split from the oversized blocksproduced were placed in a 18]18]5 mm3 brasscavity around the NMR coil, a straight piece ofbare copper wire grounded to the bottom. Thecavity was then placed in the mixing chamber ofour standard test cryostat [11] and the samplepolarized in a "eld of 2.5 T with a homogeneity ofabout 2]10~4 over the sample volume.

Gratifying degrees of polarization (60}80%)could be obtained in samples made out of eitherpolyvinyltoluene- or polystyrene-based scintillatormaterial. Samples of BC-400 and BC-408 dopedwith TEMPO were transparent, homogeneousblocks of yellow colour, in which protons could bepolarized as well as and behaved very much like inthe samples of polystyrene-based material, butshowed a signi"cantly worse scintillation e$ciency.We present in Table 1 the di!erence in optimumpolarization frequencies for positive and negativepolarization f`!f~, the obtained maximum pro-ton polarization and the proton spin-lattice relax-ation time ¹

1,1for a few samples of BC-400 and

BC-408.Table 2 shows the maximum proton polarization

and the proton relaxation time measured in sam-ples made out of polystyrene-based scintillator withdi!erent nominal concentrations of TEMPO. Allsamples have been pressed at the same temperature

B. van den Brandt et al. / Nuclear Instruments and Methods in Physics Research A 446 (2000) 592}599594

Table 1Polyvinyltoluene-based scintillators doped with TEMPO

Material Spin conc. ¹1

t1

f`!f~ p-pol. ¹1,1

!

(p.c./g) (0C) (min) (MHz) (%) (s)

BC-400 2.3]1019 120 120 #75 200 (¹"1.1 K)BC-400 1.1]1019 100 120 225 #67/!71 204 (¹"1.2 K)BC-408 2.0]1019 120 120 210 #69/!69 210 (¹"1.2 K)

!In a magnetic "eld of 2.5 T.

Table 2Polystyrene-based scintillator with di!erent concentrations of TEMPO

Sample Spin conc. ¹1

t1

f`!f~ p-pol. ¹1,1

!

(p.c./g) (0C) (min) (MHz) (%) (s)

a 0.67]1019 120 120 205 #67.5/!71.6 334 (¹"1.2 K)b 1.00]1019 120 120 210 #74.7/!72.4 255 (¹"1.2 K)c 1.45]1019 120 120 210 #76/!67.5 273 (¹"1.1 K)d 2.00]1019 120 120 220 #83/!84 235 (¹"1.1 K)


Fig. 1. Magnetic "eld dependence of the proton relaxation time¹

1,1in samples with di!erent TEMPO concentrations for

di!erent temperatures.

¹1

for the same time t1, and all had acceptable

scintillating and optical characteristics.The degree of polarization obtained was con-

siderably higher than in samples that had beendoped by di!usion. This con"rms that solution intoluene followed by evaporation of the solvent pro-duces a more homogeneous distribution of the freeradical in the scintillator, and indicates that warmpressing under the described conditions does notspoil the polarizability of the TEMPO-doped scin-tillator. It should be remarked that the optimumpolarization frequencies for positive and negativepolarizations are 200}220 MHz apart, very close tothe value dictated by the solid-state e!ect [12], andthat frequency modulation has been employed toattain the polarizations reported. A nominal con-centration of 2]1019 p.c/g was found to give highdegrees of polarization in a reasonable time. Forthe sample in which 80% and more polarizationcould be obtained 7 h were needed to go from!80% to #80%, but 85 min were enough toreach #60% and 135 min to reach #70%.

We measured the proton relaxation time ¹1,1

ofthese samples at low temperatures in magnetic"elds of 0.4, 0.8, 1.2 and 1.6 T to determine whether

they could be used for a frozen spin target (seeFig. 1). The sample with the lowest TEMPOconcentration and the longest relaxation time hadat 90 mK a proton relaxation time of 70 h in 0.4T and of 240 h in 0.8 T, while the time necessary toreach at 2.5 T the full polarization of ca. 70% wasaround 24 h. We think that operation in frozen spin


Table 3Polystyrene-based scintillator with di!erent concentrations of TEMPO


t1

f`!f~ p-pol. ¹1,1

!

(p.c./g) (0C) (min) (MHz) (%) (s)

e 0.83]1019 130 120 225 #57/!61 180 (¹"1.22 K)f block 1.54]1019 130 120 285 #82.1/!80.0 206 (¹"1.20 K)g block 2.00]1019 130 120 * #69.3/!71.3 272 (¹"1.24 K)


Fig. 2. Proton relaxation time ¹1,1

versus temperature for dif-ferent TEMPO concentrations in 2.5 T.

mode with these polarized scintillators should bechosen only if absolutely necessary.

A few samples were prepared using a somewhathigher value of ¹

1. All of them could be reasonably

or well polarized (see Table 3), all had again accept-able scintillation and optical properties [13]. Wehave measured the proton relaxation time at lowtemperatures in di!erent magnetic "elds also in twoof these samples (see Figs. 1 and 2). A few sampleswith a nominal concentration of 2]1019 p.c./ghave been prepared by varying in a reasonablerange temperature ¹

1and duration t

1of the ther-

mal treatment (see Table 4). No really signi"cantdi!erence in polarization behaviour could be foundin the samples so prepared.

4. Deuterated scintillator

After obtaining proton polarizations in excess of80% in sizeable blocks of TEMPO-doped polysty-rene-based scintillator which could be used as `po-larized detectorsa [14], one could expect ca. 20%deuteron polarization in the correspondingdeuterated material in case of validity of the EqualSpin Temperature (EST) hypothesis. We could ob-tain a small quantity of scintillator based on 98.7%deuterated polystyrene but prepared with the stan-dard, non deuterated additives exactly in the sameway as the hydrogenated one [15], and proceededto dope it with di!erent concentrations of TEMPO.Processing it as speci"ed in Section 2, with¹

1"1200C and t

1"120 min, transparent, light

pink blocks could be produced which, when testedon a pion beam, showed the same scintillationcharacteristics as the hydrogenated counterparts.In Table 5 are reported the results of a "rst

investigation of these deuterated samples in ourlaboratory DNP set-up, where deuteron and pro-ton polarization were measured synchronously andindependently by means of two Q-meter systems,one working at 16 MHz, the other one at 106 MHzon the same NMR coil, consisting of 8 windings ofbare 0.3 mm copper wire.

The time required to go from maximum positiveto maximum negative deuteron polarization insample l was 6 h, in sample m 2.5 h were enough toreach from maximum positive polarization 70% of


Table 4Polystyrene-based scintillator subjected to di!erent thermal treatments


t1

f`!f~ p-pol. ¹1,1

!

(p.c./g) (0C) (min) (MHz) (%) (s)

h 2]1019 110 150 #84 187 (¹"1.2 K)i 2]1019 120 105 #74.4 207 (¹"1.2 K)j 2]1019 130 80 #72 194 (¹"1.2 K)k 2]1019 130 120 245 #78.3/!80.7 172 (¹"1.2 K)


Table 5Deuterated polystyrene-based scintillator with di!erent concentrations of TEMPO

Spin conc. f`!f~ p-pol. d-pol. ¹1,$

! ¹1,1

! ¹1,$

" ¹1,1

"

(p.c./g) (MHz) (%) (%) (s) (s) (h) (h)

l 1.0]1019 215 #79/!77 #18/!19 1448 * 50 53m 1.5]1019 * !/! #22/!25 845 * 32 27n 2.0]1019 290 #70/! #21/!22 566 168 26 20

!In a magnetic "eld of 2.5 T at 1.15 K."In a magnetic "eld of 0.8 T at 95 m K.

the "nal negative polarization, while in samplen 6 h were needed to achieve maximum negativepolarization. The deuteron spin-lattice relaxationtime ¹

1,$is at 1.15 K in 2.5 T more than 3] longer

than the proton spin-lattice relaxation time ¹1,1

,while at ca. 100 mK in 0.8 T it is only 1.5 timeslonger. It should be remarked that the proton- andthe deuteron-spin system are not always at equaltemperature, either during the DNP process or atsaturation, which recommends to measure thedeuteron polarization directly and not via the pro-ton system when performing a scattering experi-ment.

5. A special dilution refrigerator

The scintillation light produced in the polarized18]18]5 mm3 block placed in the mixingchamber of the polarized target dilution refriger-ator has to be brought to a photomultiplier atroom temperature, in a place where the stray "eldof the polarized target magnet is low enough not toperturb its operation. To this purpose a lightguidehas been incorporated in the so-called targetholder,

an element of our standard polarized target system[11], avoiding major modi"cations to the dilutionrefrigerator. A lightguide, a 12 mm o.d. rod ofBicron BC-800, has been placed in a tight-"tting 12mm i.d. stainless-steel tube running parallel to themicrowave guide, see Fig. 3, and sealed at the roomtemperature end against atmospheric pressure bya sliding viton o-ring. The performance of the dilu-tion refrigerator with this special target holder wastested as far as the cooling power and the polariza-tion capabilities are concerned. With the room tem-perature end of the lightguide covered with blacktape a temperature of 60 mK could be reached,indicating that most infrared radiation was ab-sorbed at higher temperatures in the lightguide,and the same degree of polarization could beattained as in a standard sampleholder withoutlightguide.

6. Scintillation test

The scintillating properties of many samples fab-ricated have been characterized against a block ofuntreated scintillator of the same size, using always


Fig. 3. Dilution refrigerator: details of the sampleholder withlightguide and of the scintillating target itself.

the same light readout set-up, by means of a 90Srsource, the proton beam of a tandem accelerator,and in the actual scattering experiments with inci-dent neutrons or pions of medium energy. The lightoutput of the polarizable samples now reachesroutinely between 20% and 25%, in some caseseven more than 30%, of the light output of theuntreated scintillator. Protons of energy down to1.5 MeV can be detected, with an energy resolutiondegraded to about 25% of that of the untreatedmaterial and an almost unchanged timing resolu-tion [16]. The e$ciency of a polarized scintillatortarget is not only determined by its mere scintillat-ing characteristics but also by the properties of thelight readout system. The in#uence of geometricalparameters, material and surface qualities has beeninvestigated in extensive simulations and then ex-

perimentally veri"ed [17]. The most crucial para-meter turned out to be the diameter of the lightgu-ide, which was then chosen to be 12 mm, themaximum size compatible with the existing designof our cryogenic system. In the actual con"gurationof the read out system about 12% of the photonsgenerated in the target reach the photomultipliervia a 1150 mm long rod of low attenuation plexi-glass (Bicron BC-800) coupled to the target by anappropriate quartz adapter.

7. Conclusions

A scintillating polarized proton target has beendeveloped. Proton polarizations of more than 80%could be reached in a dilution refrigerator suited formedium-energy particle physics experiments underoptimum conditions in the laboratory, and polar-izations in excess of 60% could be routinelyachieved in operation on a particle beam. Prelimi-nary results of more than 20% polarization in a fullydeuterated scintillator indicate that a scintillatingpolarized deuteron target can be realized. This willfurther enlarge the spectrum of the spin physicsexperiments envisageable with polarized targets.

Acknowledgements

It is a pleasure to thank Prof. N.A. Russakovichfor his continued interest and support andDr. I.B. Nemchonok (JINR, Dubna) for providingus with excellent scintillator material.

References

[1] At CERN in 1968 (recollection of one of the authors) andat JINR-Dubna in 1972. N.S. Borisov, E.I. Bunyatova,Yu.F. Kiselev, et al., JINR Preprint P6-7408, Dubna, 1973.

[2] E.G. Rozantsev, Free Nitroxyl Radicals, Plenum Press,New York, 1970.

[3] B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter,S. Mango, PSI Ann. Rep. 1 (1994) 12.

[4] E.I. Bunyatova, Nucl. Instr. and Meth. A 356 (1995) 29.[5] B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter,

S. Mango, Nucl. Instr. and Meth. A 356 (1995) 36.[6] B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter,

S. Mango, JINR Comm. E-1-98-211, Dubna, 1998.


[7] L.J. de Bever et al., Measurement of the neutron-protonspin correlation parameter at forward angles, PSI-propo-sal Z-96-02, PSI, CH-5232 Villigen, 1995.

[8] B. van den Brandt, E.I. Bunyatova, P. Hautle, J.A. Konter,S. Mango, in: C.W. de Jager et al. (Eds.), Proceedings ofSPIN96, 12th International Symposium on High-EnergySpin Physics, Sept. 10}14, 1996, Amsterdam, World Sci-enti"c, Singapore, p. 238.

[9] This appears to be characteristic of PVT as a solvent forTEMPO, possibly because of super conjugation with theCH

3group. E.G. Rozantsev, private communication.

[10] R. Bilger et al., PSI Ann. Rep. 1 (1997) 22.[11] B. van den Brandt, J.A. Konter, S. Mango, Nucl. Instr. and

Meth. A 289 (1990) 526}531.[12] A. Abragam, Principles of Nuclear Magnetism, Oxford

University Press, Oxford, 1961.

[13] In fact the one with 1.54]1019 p.c./g has been used withsuccess in an experiment aiming at the measurement of theneutron}proton spin correlation parameter at forwardangles. S. Buttazzoni et al., PSI Scienti"c Report 1,1998.

[14] B. van den Brandt, P. Hautle, J.A. Konter, S. Mango, E.I.Bunyatova, H. Denz, R. Meier, J. Jourdan, H. WoK hrle,Proceedings of SPIN98, 13th International SymposiumOn High-Energy Spin Physics, September 1998, Protvino,Russia, World Scienti"c, Singapore, 1999.

[15] Yu.K. Akimov, E.I. Bunyatova, I.B. Nemchonok, A.I.Churin, Plastic scintillators based on deuterated and stan-dard Polystyrene for a polarized active target, RussianJ. Instr. Exp. Tech., submitted for publication.

[16] H. WoK hrle, private communication.[17] H. Denz, Diplomarbeit UniversitaK t TuK bingen, 1998.


polarized scintillator targets

Documents