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Nuclear Instruments and Methods in Physics Research A 526 (2004) 7–11

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doi:10.1016

Thirty years of research with lithium compounds in Saclay

J. Ball

CEA, DAPNIA/SPhN, CE Saclay, Gif-sur-Yvette 91191, France

Abstract

An overview of the research performed in Saclay using lithium products is given starting with the pioneering work of

Abragam’s group and ending with the shut down of the local accelerators. From the very beginning, lithium

compounds have been linked to the development of polarized targets. In 1960, Abragam used 6LiF to validate the

Overhauser effect in metals and to check experimentally Dynamic Nuclear Polarization. In 1980, work on 6LiD showed

promising results. This would lead to the first use of this material in polarized targets during the following decade, until

its present successful behaviour in COMPASS polarized target.

r 2004 Elsevier B.V. All rights reserved.

PACS: 29.25.Pj

Keywords: Polarized target; DNP; Lithium deuteride; Lithium hydride

1. Introduction

In 1984, we did a presentation at the 4thInternational Workshop on Polarized TargetMaterials and Techniques, already held in BadHonnef, about the first polarization results we hadachieved on 7LiH: These results turned out to bevery disappointing [1], as we did not manage to geta Li polarization higher than 3–4%, under a 2:5 Tas well as a 6:5 T magnetic field when 70% wereexpected. The reasons why and how this didhappen would be understood in the followingyears as is developed below.The studies on lithium compounds in Saclay,

related to polarization, evolved in three stepsthrough different teams. Around 1960, the groupof A. Abragam at Saclay’s Service de Physique duSolide et de R!esonance Magn!etique (SPSRM)started a pioneering work through theory and lab

ddress: jball@cea.fr (J. Ball).

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.nima.2004.03.144

experiments and produced results during thefollowing two decades. From 1983, physicists fromthe Division de la Physique des Particules El-!ementaires (DPhPE), lead by J. Der!egel and L. vanRossum investigated the preparation of polariz-able 6LiD through irradiation techniques. Thiswas followed in 1991 by joint studies of teams ofthe D!epartement d’Astrophysique, de Physiquedes particules, de Physique Nucl!eaire et d’Instru-mentation Associ!ee (DAPNIA) and of the Labor-atoire National Saturne (LNS) to prepare sizableamounts of 6LiD to be used in an experiment witha polarized target at LNS.

2. From the Overhauser to the ‘‘solid’’ effect

In 1953, Overhauser presented a proposal at theAmerican Physics Society (APS) meeting in whichhe suggested to polarize nuclei in metals bysaturating the Electron Spin Resonance (ESR)

d.

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J. Ball / Nuclear Instruments and Methods in Physics Research A 526 (2004) 7–118

line of conduction electrons [2]. This proposal wasmet with general disbelief until its experimentalevidence by Carver and Slichter [3]. The Over-hauser effect then became trendy. Eventually, thefull validation was given by two members ofAbragam’s group using lithium hydride irradiatedby pile neutrons [4]. LiH was chosen as specificallysuited due to the narrowness and the symmetricalshape of its ESR line, and also due to the smallnessof Li metallic particles.Enthusiastic followers mistook some enhanced

signals for Overhauser effects evidence even innon-metallic materials. A particular paper inwhich the author had claimed to have reached100% polarization on the arsenic nuclei of a dopedsilicon semiconductor [5] triggered Abragam’sthinking over the problem. The resulting article[6], entitled ‘‘Overhauser Effect in Nonmetals’’, isnow considered as a classic paper, establishing thegrounds for the ‘‘Solid Effect’’ theory. It surveyedthe different ways in which nuclear spins can relaxthrough their interaction with electrons. It wasshown explicitly that under certain conditions theOverhauser effect could be extended to non-metallic substances, for instance to liquids contain-ing paramagnetic impurities in solution. But it didnot apply to diamagnetic solids containinglocalized paramagnetic impurities. Therefore,the results on the arsenic doped silicon samplecould not be interpreted as a hundred percentpolarization.In 1958, Abragam invented the ‘‘Solid Effect’’

which enabled the enhancement of nuclear polar-ization in solids doped with paramagnetic centersby off center hyperfrequency irradiation of theESR line. By means of the microwave field,the entropy of the nuclear spin system, linked tothe polarization, was transferred to the electronspin system from where it was transferred furtherto the lattice via electron spin-lattice relaxation.The ‘‘Solid Effect’’ validation was performed byAbragam and Proctor through a ‘‘prototype’’experiment [7], using another lithium compound,6Li19F: The 19F; with spin 1

2; acted as the

electron part and the 6Li; with spin 1, acted asthe nuclear part because the gyromagnetic factorsratio between the two species, which would givethe magnitude of the nuclear polarization

enhancement, was

gð19FÞ=gð6LiÞ ¼ 6:4: ð1Þ

The microwaves irradiations of the LiF crystalwere done at the single frequency O of 9:4 MHz;while the magnetic field was tuned, respectively, at0.28 and 0:2 T corresponding, respectively to

Oð7Þ ¼ oð19FÞ8oð6LiÞ: ð2Þ

This yielded positive and negative 6Li polariza-tions. The solid effect would also be called‘‘Dynamic Nuclear Polarization’’ (DNP) by otherresearch groups over the world.

3. The early years:1960–1980

3.1. LiH and LiD as polarized target materials

During the 1960s, lanthanum magnesium nitrate(LMN) was the target material for protonpolarized targets with 70% polarization achiev-able. Its main drawback was a poor ratio r; of theorder of 0.06, of the number of polarized nuclei onthe total number of nuclei of the target. In the late1960s organic substances chemically doped withfree radicals provided ratios r around 0.20 andbecame commonly used [8,9]. During the 1970s,physics experiments requested higher beam inten-sities which weakened target polarizations due toradiation damage. A renewed interest was given to‘‘clean’’ target materials where the paramagneticcenters were created by irradiation as they behavedbetter in intense beams. This would lead to a newgeneration of target materials, mainly ammoniaand lithium hydrides.In Saclay, lithium compounds were already used

for studies on antiferromagnetism, and wereprepared to be polarizable, as it was one step ofthe process. In the case of LiH, the paramagneticcenters, known as ‘‘F centers’’, were produced byelectron beam irradiation on the cubic facecentered lattice of the crystal and associated tothe production of interstitial molecular ions H�

2

[10]. The F center is an anion vacancy in which asingle electron is trapped. Its wave functionextends out over the 6 neighboring Liþ; as seenin the hyperfine structure. A high-irradiation dose

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Table 2

Polarization of 6LiD

Field (T) Build-up time (h) Pð6LiÞ ð%Þ

2.5 20 40

4.8 40 64

6.5 40 71

Table 1

Polarization results for Li compounds

Material Build-up time (h) Field (T) Pmax ð%ÞLiX Li X

7LiF 4 5.5 60 807LiH 40 6.5 94 996LiD 40 6.5 70 70

J. Ball / Nuclear Instruments and Methods in Physics Research A 526 (2004) 7–11 9

would lead to aggregation of centers and yield F2or F3 centers which would disturb the building upof the polarization. At the 1978 Argonne Con-ference, Abragam announced [11] that a 10 mm3

7LiH crystal irradiated at liquid nitrogen tempera-ture by a 3 MeV electron beam and thus having2:1019 F centers=cm3 had been tested for DNPunder a 6:5 T field. After 2–3 days of building upunder 185 MHz microwave field at 200 mK; thepolarizations obtained were 95% for H, 80% for7Li and 35% for the few 6Li isotopic impuritiespresent in the sample, in agreement with the SpinTemperature Theory.

6LiD seemed to be a potentially interestingmaterial especially for deuterated targets. TheESR linewidths ratios between 7Li and 6Li wasexpected to be similar to the ratios of the magneticmoments, namely

mð7LiÞ=mð6LiÞ ¼ 4: ð3Þ

With a narrower ESR linewidth, 6LiD could reachhigher polarizations. Another asset of this materialwas that mð6LiÞ was nearly similar to mðDÞ: Bothnuclei had spin 1 and would get the samepolarization when 6LiD was polarized. Further-more 6Li; pictured as an alpha particle plus a quasiD, would provide an r of 0.5 far higher than otheravailable materials. Abragam considered this ashighly speculative but amusing [11]. It was certainlya hard job for his team to work it out.

3.2. Abragam’s group results

At the Abingdon Workshop in 1979, the firstresults on 6Li and 7Li compounds were reported[12]. 7LiH and 7LiF; easily available, were used asprototype samples to optimize the conditionsbefore starting with 6LiD: The irradiation condi-tions were 101721018 e�=cm2 in liquid argonðTC90 KÞ; in a 1–3 MeV electron beam. Theresults displayed in Table 1 are for positivepolarization.The following year, more results were published

[13,14], as seen in Table 2, using 12 mm3 samplesof 6LiD: The high values of polarized deuteronsobtained were attractive enough for physicists tocompensate the long build-up time (BuT).

4. The experimentalists involvement: 1983–1988

A development work was foreseen to scale upsamples from 10 mm3 to amounts suited forpolarized targets. The reproducibility of the rightcreation of F centers was one of the difficultiesalready encountered. The parameters which had tobe adjusted were obviously the irradiation dose ofthe material and its temperature. The latterappearing to be quite sensitive as witnessed bythe Abragam’s group when irradiating the samplesat liquid argon temperature [12]. Two types ofcenters were then produced according to the colorof the samples. Below the liquid argon level,samples turned out red and more difficult topolarize than the blue ones obtained above theliquid level and thus at a slightly higher tempera-ture and identified as F centers.The DPhPE team, involved in this project in

Saclay, tried first to reproduce Bouffard’s results[14], irradiating 7LiH at the Saclay Linear Accel-erator (ALS), a 600 MeV electron machine. Threeirradiations in 1984 at liquid argon temperaturegave poor results [1]. To be able to control andtune the temperature more accurately, a variabletemperature insert was installed in the irradiationcryostat. From the end of 1985 to the end of 1987,6 more irradiations of 10 cm3 samples of 7LiH(grains of 2 mm in sides dimensions crushed from

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Table 3

Der!egel’s team best results

Period 1985–87 1988

Dose ðe�=cm2Þ 1017 1017 2:1017

IT (K) 183 183 183

Field (T) 2.5 5 2.5

BuT (h) 10 24 11

Pð7LiÞ ð%Þ 30 50 47

PðHÞ ð%Þ 45 70 56

Pð6LiÞ ð%Þ 37

PðDÞ ð%Þ 43

J. Ball / Nuclear Instruments and Methods in Physics Research A 526 (2004) 7–1110

polycrystalline blocks) were done at the ALS. Theoptimum dose came out to be 1:1017 e�=cm2 andthe adequate irradiation temperature (IT) 183 K[15]. The results were considered good enoughthen to start irradiations with 6LiD: Table 3summarizes the characteristics of the best samplesobtained during these two periods.The 1988 best 6LiD sample was used at Paul-

Scherrer Institute by S. Mango and his team in anactual physics experiment. After about 3 days ofbuilding up in a 2:5 T field, 53% were reached forD polarization [16].

T(K)

P (

%)

170 180 190 200 210 220 230

40

30

20

10

Fig. 1. Max PðDÞ vs irradiation temperature.

5. Production for LNS polarized target: 1991–1995

5.1. The Nucleon–Nucleon program at LNS

The Nucleon–Nucleon (NN) experiment at LNShad been investigating nucleon–nucleon elasticscattering since 1980. In 1991 started the last partof the program consisting on the studies of thescattering of a polarized proton beam on apolarized deuteron target. The target cell about15 cm3 in volume, was foreseen to be loaded with6LiD: Assuming that 6Li behaved as bound aparticle and D, soon came out the need todiscriminate the polarized neutron in 6Li fromthe polarized neutron in D. Scattering on 6LiHwas an obvious choice as spin effects in pnscattering of a proton beam on 6LiH would becaused by polarized neutrons in 6Li only. A secondtarget cell was therefore implemented below thefirst one in the dilution refrigerator so to have akind of dummy polarized target to tag the right

events [17]. 6LiH and 6LiD were provided bymembers of the NN collaboration from theLaboratory of Nuclear Problems at JINR-Dubna.Production of irradiated 6LiD and 6LiH becamethen the project lead in collaboration by LNSpolarized target group and by members of theDAPNIA cryomagnetism division.As the irradiation facility at the 600 MeV ALS

was no longer available at Saclay, the new teamhad to go back to the 2:3 MeV acceleratorpreviously used by Abragam’s group.

5.2. The production of irradiated lithium

compounds

A new cryostat [18] able to hold nine samplesirradiated at the same time with different doseswas built. It used a forced flow of helium cooleddown by a liquid nitrogen exchanger loop toensure a stable temperature of the samples.Platinum sensors measured the temperature inthe gas flow upstream and downstream the sampleholders. The doses were measured by means ofcalibrated films. The difficulty of measuring theright temperature in a gas flow became clear whenthe optimized IT showed an offset in comparisonwith former values. This can be seen in Fig. 1where the ideal IT seemed to be about 190 K whenprevious studies had pointed out 183 K as theright temperature. A more sophisticated control-ling system was under development when theproject was stopped and never had a chance to betested.

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3

92 9493

50

40

30

20

10

10 155

P (

%)

Irradiation #

Fig. 2. Summary of the D-polarizations obtained from 1991 to

1995.

J. Ball / Nuclear Instruments and Methods in Physics Research A 526 (2004) 7–11 11

Through the years, the reproducibility of goodconditions to get highly polarizable samples ofabout 50 cm3 remained the major difficulty. Ascan be seen in Fig. 2, where the improving goodresults up to July 93 irradiations were followed bytwo less successful attempts with exactly the sameexperimental conditions as the July 93 ones.The best results, obtained in July 1993 [17], were

achieved with a 6LiD sample irradiated with a3:1017 e�=cm2 dose. At 2:5 T; the D-polarizationwas brought up to 42% with a BuT of 7 h: At5:5 T; 69% were reached. The nuclear relaxationtime measured at 0.33 T and 50 mK was 29 days.This sample was used on the NN experiment. The6LiH sample came from a different batch andshowed poor polarization results, PðHÞ being of30% and Pð6LiÞ of 6% at 2:5 T:

6. Conclusion

In 1995, the studies on lithium compounds werestopped at Saclay corresponding with the shutdown of the Saturne accelerator and the disman-tling of the polarized target group. Nevertheless,work on these materials had already resumed inother labs running experiments with polarizedtargets [19]. Eventually, W. Meyer’s group in Bonnand later in Bochum made a major step in thequest for the best deuterated lithium sample. Afterhaving dedicated the needed amount of time tounderstand the solid states mechanisms responsi-

ble for the polarization of lithium compounds anddeveloped it experimentally [20], S. Goertzmanaged to produce large amounts of highquality samples for the COMPASS experiment atCERN [21].

References

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Proceedings of the 4th International Workshop on

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[3] T.R. Carver, C.P. Slichter, Phys. Rev. 92 (1953) 212.

[4] M. Gueron, Ch. Ryter, Phys. Rev. Lett. 3 (1959) 338.

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[8] H. Glattli, in: G. Shapiro (Ed.), Proceedings of the 2nd

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[18] G. Durand, et al., Proceedings of the 10th International

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