Towards the polarization of DT molecules

Towards the polarization of DT molecules

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 577 (2007) 320–323 www.elsevier.com/locate/nima Towards the polarization of D...

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ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 577 (2007) 320–323 www.elsevier.com/locate/nima

Towards the polarization of DT molecules S. Bouchigny, J.-P. Didelez IN2P3, Institut de Physique Nucle´aire, Orsay, France Available online 21 February 2007

Abstract The polarization of D and T nuclei should increase the reactivity of DT molecules in HIIF processes. At IPN Orsay, we have developed the static polarization of HD samples using ‘‘brute force’’ (BF). It has been demonstrated, that the ageing technique or the double distillation allow to get nuclear relaxation times larger than 1 h, even at 1.5 K and 1 T. It is advocated that it is possible to achieve by RF the conventional Dynamic Nuclear Polarization (DNP) of the proton and the deuteron contained in the HD molecule, using HD samples distilled at Orsay and suitably irradiated. The persistence after irradiation of the long nuclear relaxation times resulting from the double distillation has already been demonstrated. If feasible, the DNP of HD would open the possibility to polarize DT, which has the same magnetic structure as HD and is the ideal fuel for HIIF. r 2007 Elsevier B.V. All rights reserved. PACS: 33.15.Kr; 33.25.+k; 39.90.+d Keywords: HD and DT polarization; Static and dynamic; Relaxation; Distillation; Nuclear fusion

1. Introduction For polarized HD targets, all nuclear species in the target are polarizable. The H and D can be polarized independently and their relative orientation can be put either parallel or antiparallel [1]. The status of the Orsay HYDILE project based on the Brute Force (BF or ‘‘static’’) polarization of HD samples has been given in a previous paper [2] . Given the very long nuclear relaxation times which could be achieved with purified and aged1 HD, the Dynamic Nuclear Polarization (DNP) of suitable HD material is under investigation again, taking advantage of the recently controlled properties of HD samples, double distilled at Orsay, with precisely measured H 2 and D2 impurities concentrations and long enough relaxation times T H 1 and TD 1 verified by NMR techniques. If fully successful, the Corresponding author.

E-mail address: [email protected] (J.-P. Didelez). ‘‘Ageing process’’ consists in keeping a solid HD sample at low temperature to let the ortho-H 2 ðo2H 2 Þ and para-D2 impurities decay to their magnetically inactive forms para-H 2 and ortho-D2 , therefore increasing the nuclear relaxation time of both protons and deuterons. 1

0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.02.022

DNP method would allow to produce not only for Nuclear Physics experiments ‘‘dream’’ polarized targets which could be polarized and used at high temperature and low field, typically 0.5 K and 1 T [3],but in addition, the DNP developed for HD could be readily applied to the polarization of DT molecules, with the goal to increase their reactivity when used as fuel material in HIIF [4]. The fusion reaction: D þ T ! a þ neutron þ 17:6 MeV

(1) þ 5 He 32

intergoes mainly through the excitation of an mediate state, resulting from the coupling of the spins 1 and 12 of the D and T nuclei to a total spin S ¼ 32. Without polarization of D and T, the statistical distribution of the six possible states gives four S ¼ 32 and two S ¼ 12 states. Only the 32 states can produce the intermediate 32 resonance. With 100% parallel polarization of D and T, all states would contribute to the fusion, increasing the reactivity by 50%. In addition, the polarization allows to control the direction in which the reaction products are emitted, in particular the neutrons have a sin2 y distribution. This can be very useful to reduce damages or activation of costly equipments.

ARTICLE IN PRESS S. Bouchigny, J.-P. Didelez / Nuclear Instruments and Methods in Physics Research A 577 (2007) 320–323

2. BF polarization BF polarization is achieved by putting a solid HD sample in a strong static magnetic field at the lowest achievable temperature and waiting long enough to reach the equilibrium polarization which depends solely on the nuclear species magnetic moments. With commercially available dilution refrigerators (10 mK) and superconducting magnets (15 T), the BF polarization of H could reach 90% and the D vector polarization exceed 30%. Equipments for producing BF polarized HD targets are existing at the IPN Orsay (France). A detailed description of this material has already been published [5]. This comprises essentially a Dilution Refrigerator: 10 mK–13.5 T, in which HD targets are statically polarized; a Transfer Cryostat: 4 K–0.35 T, allowing to remove under a small holding field and at low temperature, the targets from the Dilution Refrigerator to put them into, for example, a variable temperature Storage Cryostat: 1.5–20 K–2.5 T. However, the preferred geometry for such a system is of cylindrical type, the target being kept polarized within an axial field produced by a superconducting coil. There have been studies for Magnetized Target Fusion in a cylindrical geometry [6], but sizeable target radii of the order of cm are necessary to reach the fusion conditions. Such large targets cannot be kept at the very low temperature of a Dilution Refrigerator (10 mK) where the cooling power is in the range of a few m W (1 cm3 of Tritium produces several mW of heat power by intrinsic radioactivity). Therefore, the maximum possible polarizations reachable by the BF method for DT molecules would be low and the gain not worthed the pain. In fact, the main advantage of the DNP compared to the BF method is the possibility to work at temperature and field conditions close to 1 K and 2.5 T, where the cooling power can be of the order of mWs. 3. Dynamic nuclear polarization In 1973, Solem [7] investigated the DNP in radiationdamaged solid HD, reaching 3.7% proton polarization and 0.3–0.4% for the vector polarization of deuterons. Solem showed that H atoms produced by irradiating the HD became stably trapped in the solid [8]. He used microwaves to induce forbidden transitions between the electron spins of these H atoms and the nuclei of neighboring HD molecules, transferring the high equilibrium polarization of the electrons to the nuclei. To reach a high efficiency for this polarization transfer, short electron spin-lattice relaxaD tion times T e1 and long nuclear relaxation times T H 1 and T 1 are essential, so that one electron can polarize hundreds of nuclei. To shorten the electron relaxation times, Solem doped his HD samples with paramagnetic O2 impurities. But he used commercially available HD with substantial H 2 and D2 impurities, which are known to shorten T H 1 and TD . Our approach at IPN Orsay, has been to produce very 1 pure HD by distillation, in order to increase T H and TD 1 1. 4 Typical concentrations in the range of a few 10 for both

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H 2 and D2 residual impurities have been achieved by D double distillation, allowing T H 1 and T 1 in the range of minutes at DNP conditions (1 K and 2.5 T). Additional D ageing of a few weeks allows to bring T H 1 and T 1 in the range of hours and even days. This was expected to be sufficient to perform significant DNP of HD. 4. Distillation Differences in vapor pressure2 between H 2 , HD and D2 can be used to purify HD through a distillation process in a rectification column. Successive liquefaction and vaporization along the column will increase the concentration of the lightest element at the top of the column allowing, in a first step, the extraction of H 2 from HD and, in a second step, the extraction of pure HD from the remaining HD2D2 mixture. Concentrations are measured online by a commercial ‘‘MKS-Spectra Microvision Plus’’ quadrupole mass spectrometer having a limited mass range (only 1–6 mass unit). We have added an input gas manifold designed to guarantee no mass segregation during gas transport from the working pressure of the distillator (500 mbar) to the working pressure of the spectrometer ð1:106 mbarÞ. D2 concentration, ½D2 , can be measured down to 105 and ½H2  down3 to 2  104 . A detailed description of the apparatus and distillation process can be found in Ref. [9]. We have distilled 6.2 mol of HD gas containing 0.5% H 2 and 0.65% D2 which is typical for commercial ‘‘pure’’ HD. We have three samples at the end of the distillation, specifically produced for dynamic polarization. The best amounted to 350 mmol with ½H2 o0:02%, below the detection limit of our spectrometer, and ½D2  ¼ 0:17%. Recently, we have implanted in a variable temperature cryostat a crossed coil NMR system, allowing to measure D directly T H 1 and T 1 for the distilled HD samples at temperature and field settings close to those of the DNP conditions. This has opened for the first time, the possibility to precisely measure simultaneously ½H2  and ½D2  concentrations (with the mass spectrometer) and T H 1 and T D 1 (with the NMR system). The corresponding set of data should help to constrain theoretical models describing D the dependence of T H 1 and T 1 as a function of ½H2  and D ½D2  [10]. Fig. 1 shows the dependence of T H 1 and T 1 as a function of ½H2  and ½D2  and ageing after completion of the distillation. 5. DNP experiment The DNP processes are being nowadays rather well understood. Among the necessary conditions to achieve efficient DNP, are: (i) enough free electron spins in the 2 At 18 K, vapor pressure for H 2 , HD and D2 are, respectively: 461.2, 235.2 and 116.3 mbar, leading to a vapor pressure ratio of order 2 between HD and its impurities H 2 and D2 . 3 Limitation is due to dissociation of HD molecules leading to a significant background at mass 2 from Dþ ions.

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10000

Relaxation Time (minutes)

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10

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T1H Sample 1 T1D Sample 1 T1H Sample 2 T1D Sample 2

0.1 0

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Aging Time (days) D Fig. 1. Evolution at 0.85 T field and 1.8 K temperature of the relaxation time T H 1 and T 1 , as a function of ageing and H 2 and D2 concentrations for 2 HD 4 3 samples. Sample 1 contamination is 2:0  10 H 2 and 1:7  10 D2 and sample 2 contamination is 6  104 H 2 and 9  104 D2 .

solid; (ii) temperature and field allowing high equilibrium polarization of the electrons and (iii) relaxation times of the nuclear species much longer than the electron ones. At 0.5 K and 1 T, the equilibrium polarization of the electrons would be close to 90%, which is typical for the DNP. From Fig. 1 showing the T H 1 dependence as a function of residual impurities, we see that the best sample has a T H 1 longer than 6 min just after the distillation, which can be extended to 2 h with 8 days of ageing. We initially compared our T H 1 e and T D 1 data with values of the order of seconds for T 1 , as reported in Ref. [11]. However, it turned out that both electron and nuclear spin-lattice relaxation times are linked to the amount of o2H 2 in the crystal [12], so that for e purified or aged HD, T H 1 and T 1 remain comparable. e Therefore, to shorten T 1 , Solem doped his HD samples with paramagnetic O2 impurities [8], and showed that with a concentration of 3  104 of O2 , T e1 dropped below 1.0 msec, while T H 1 remained essentially unchanged [7]. Taking that into account, we are now in a very favorable position to perform DNP of HD, with very short T e1 and D very long T H 1 or T 1 . The experiment has been attempted at the Physics Department of the Ruhr-Universita¨t Bochum, where suitable equipment for DNP is available. The HD material distilled at Orsay has been kept 3 weeks at 14 K before being transported to Germany. During the journey the gas was at room temperature and at a pressure of 334 mbars for 10 h. To generate enough unpaired electrons, the HD material has been irradiated with an external source (b-source of 3.7 GBq [13]). At a temperature of D 1.5 K, a field of 2 T and under radiation, T H 1 and T 1 were measured to be 800 s. After 2.5 days of irradiation, both were still above 650 s. It is the first time that such long relaxation times have been observed just after condensation of the HD, gaining at least three orders of magnitude

compared to previously available samples which contained typically 0.2% of H 2 and D2 . Unfortunately, no measurable DNP could be observed, probably due to the difficulty to condense a gas mixture HD2O2 into an homogeneous solid, due to the difference of HD and O2 gas solidification temperatures.

6. Conclusion We have investigated ways to perform DNP of HD. The necessary conditions to obtain high polarizations rates of D ‘‘H’’ and ‘‘D’’ have been identified: (i) long T H 1 and T 1 e obtained by distillation and ageing; (ii) short T 1 by doping the HD material with paramagnetic O2 impurities and (iii) high density of free radicals obtained by irradiation (some other ways using chemical dopants are also under investigation [14]). However, an experimental protocol remains to be established in order to reach our goals. Going from the HD DNP to the DT DNP becomes then ‘‘very simple’’, if we ignore the difficulty of preparing DT molecules, due essentially to the high radioactivity of Tritium, although the manipulation of Tritium gas is not as hazardous as one might think [15]. One can imagine that prepared DT targets can be aged at low temperature T (typically 4 K), so that T D 1 and T 1 become long enough e compared to the T 1 of radicals. Those radicals would be plentifully provided by the high radioactivity of the fuel material itself: 1 mol of DT produces within 1 h 6:3  1018 b rays, which in turn can generate several DT dissociations. Accordingly, a density of 1018 electron spins (radicals) per ccm, suitable for DNP, is reached within 1 h. Finally, it should be noted that the relaxation times do not have to be as long as for polarized target nuclear physics experiments,

ARTICLE IN PRESS S. Bouchigny, J.-P. Didelez / Nuclear Instruments and Methods in Physics Research A 577 (2007) 320–323

since the polarization must survive only during the short time necessary for injection into the fusion reactor. Acknowledgements Financial support for this work is provided by the European Community in the framework of JRA8 project: Polarized Targets for Europe (I3HP Network of the 6th European Framework Program). We are grateful to our Bochum colleagues for their dedication in performing the DNP polarization. References [1] A. Honig, Phys. Rev. Lett. 19 (1967) 1009. [2] M. Bassan, et al., Nucl. Instr. and Meth. 526 (2004) 163.

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[3] S. Bouchigny, et al., Nucl. Instr. and Meth. A 544 (2005) 417. [4] R.M. Kulsrud, et al., Phys. Rev. Lett. 49 (1982) 1248. [5] S. Bouchigny, et al., in: M. Fujiwara, T. Shima, (Ed.), Proceedings of the EMI2001 International Symposium ‘‘Electromagnetic Interactions in Nuclear and Hadron Physics’’, RCNP Osaka, Japan, World Scientific, December 2001, p. 652. [6] M.M. Basko, A.J. Kempt, J. Meyer-ter-Vehn, Nucl. Fusion 40 (2000) 59. [7] J.C. Solem, Nucl. Instr. and Meth. 117 (1974) 477. [8] J.C. Solem, G.A. Rebka, Phys. Rev. Lett. 21 (1968) 19. [9] S. Bouchigny, et al., Proceedings of the PST05 meeting ‘‘Distillation and Polarization of HD’’, Tokyo, World Scientific, to appear. [10] T. Moriya, K. Motizuki, Prog. Theor. Phys. 18 (2) (1957) 183. [11] G.W. Collins, et al., Phys. Rev. B 48 (1993) 12620. [12] T. Kumada, Prog. J. Chem. Phys. 116 (3) (2002) 1109. [13] E. Radtke, et al., Nucl. Instr. and Meth. A 526 (2004) 168. [14] T. Kumada, Private communication. [15] J.-P. Didelez, et al., Il Nuovo Cimento, Ser. X 67 A (1970) 388.