Brueckmann, H.

Finken, D.

Friedrich, L.

English

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Oktober 1970Institut für Experimentelle KernphysikKFK 1351Polarized Positively Charged Hydrogen Ion Beams from ChargeExchange Collisions between Metastable H(2S) Atoms and HalogensH. Br ückmann, D. Finken, L. FriedrkhNUCLEAR INSTRUMENTS AND METHODS 87 (1970) 155-156; © NORTH-HOLLAND PUBLISHING CO.POLARIZED POSITIVELY CHARGED HYDROGEN ION BEAMSFROM CHARGE EXCHANGE COLLISIONS BETWEEN METASTABLE H(2S) ATOMSAND HALOGENSH. BRÜCKMANN, D. FINKEN and L. FRIEDRICHInstitut für Experimentelle Kernphysik des Kernforschungszentrums und der Universität Karlsruhe, GermanyReceived 6 July 1970Charge exchange coIIisions with halogens can be favourablyused to produce polarized positively charged hydrogen ionbeams in a Lamb shift type ion source. Tensor polarizations upThe method which makes use of the Lamb shift hasproved to be powerful means for the production ofpolarized negatively charged hydrogen ions1,2). Takingadvantage ofthe long lifetime in the H (2S) state a beamof metastable hydrogen atoms is polarized by quench-ing only the atoms from some of the hyperfine structurestates to the ground state. For the ionisat'on a processhas to be used which ionizes the polarized metastableatoms selectively. Well defined beams of polarized H-ions are available due to the characteristics of thecharge exchange collisions and to the geometry of sucharrangements. The phase space density and intensityof such negative ion beams are superior to the beamqualities which have been obtained up to now byother corresponding procedures. In spite of theadvantages of a "Lamb shift source " producingnegative ions, investigations were carried out whichaimed at an ionisation process converting only themetastable H (2S) atoms to positively charged protonsor deuterons selectively. Charge exchange collisionsare expected to be suitable for this purpose. As waspointed out in ref. 3 such charge exchange collisionsHe expected to become increasingly selective if theelectron affinity of the exchange atom or moleculeipproaches the ionisation energy of the metastableiydrogen atom (3.3 eV). The first success in converting) (28) atoms into deuterons by a selective ionisationvas obtained by using hydrogen, deuterium and.elium as charge exchange partners").Encouraged by these results we studied especiallyuch charge exchange reactions which exhibit a low-nergy defect. Charge exchange collisions with halogensre expected to be favourable candidates for selectivemisation because electron affinities range between 2.4ad 3.8 eV 4). Selectivity may be due to either areaction'pe involving pseudo-crossing potentials or to a large'oss section for dissociative attachment of an elec-to P33 = - 0.75 were achieved. lodine exhibits a yield of twotimes the yield known for the production of polarized negativelycharged hydrogen ions from argon,tron"). Both reactions presuppose a low energydefect.By Knutson some of the properties of the iodinecharge exchange reaction H(2S)+I2 -+ H+ +1; havebeen independently investigated"), The measurementsindicate that this specific reaction can be effectivelyused for the production of polarized protons in aLamb shift source. His communication containedresults on intensity measurements which were obtainedat particularly chosen iodine vapor densities. For amore detailed study, information on the pressuredependence is needed and one is interested in adirect measurement of the nucIear polarization.As an extension of the investigations of chargeexchange reactionsv") the reaction D(28) + I2 -+ D + +I; was studied systematically. Beam polarizationso (25) +Jz - d;'+ J2 I I D(25J+A _O-.ATerb.lkeV e.-O~e. __ units lk.V. !/'........ Ii~..II 10 IT"-~~r·kl~I 5I~ ! Iüvo-r--~ ....·....··r.-· / I .--L---r-0 I~~ I0.5 1.0 QO-2Torr-cm] 1.0 2.0 [I0-2Torr·cm](a) (b)Flg. 1. (a) The production of deuterons from metastable andground state deuterium atoms as a function of the iodine vaporpressure times the length of the charge exchange cell (in torr- cm).The electron affinity of 1- is 3.07 eV 4) and the value for 12 is2.4 eV 4). The dissociation energy of 12 is 1.54 eV. (b) The cor-responding measurements as shown in (a) for the production ofnegative deuterium ions in argon taken at the same experi-mental conditions. The arbitrary scale of the intensity I is identi-cal for both measurements (a) and (b).155156 H. BR Ü CKMANN et al.were measured by the asymmetry ofneutrons producedin the T(d, ntHe reaction. The experimental set-updescribed in 2,7) was used to study the production ofdeuterons from a neutral beam containing metastableand ground state hydrogen atoms (intensity In orfrom a pure ground state hydrogen atomic beam(intensity 10). These two intensities were measured as afunction of the mass density in the iodine chargeexchange cell. These systematic measurements werecarried out at five different energies between 0.5 and2.4 keV. Fig. la shows as an example the results atat 1 keV beam energy. The corresponding measurementof the production of negative hydrogen ions fromargon is shown in fig. 1b for comparison. The selec-tivity ofthe iodine reaction is at least equal to the argonreaction, whereas the yield of the iodine reaction isroughly twice as high as the yield of the argon reaction.The nuclear polarization to be expected from theintensity measurements was calculated by using theformula given in ref. 3. The results of the nucJearpolarization measurement are shown •in fig. 2. Thetensor polarization P33 was experimentally determinedas a function ofthe density ofthe iodine vapor (fig. 2a)and as a function of the beam energy (fig. 2b). Themeasured nuc1ear polarizations are in good agreementwith the values predicted from the measurement ofthe intensities IT+ and IJ.The iodine charge exchange reaction can be charac-terized by the following features.First the polarization achieved is almost independentfrom the iodine vapor pressure. This special featurefirst observed with iodine might be of great advantagein the application to polarized ion sources. Due to alarge ratio between the cross section for the productionD(2S)+Jz- d" +Ji ~3 deuteronI keV -0.8polorizotion~'h '- -- -- - +-- -- -! -- - -. -- -.---/ . Fr -0.6-t-'k-~t--0.4I --0.20.5 1.0 [10'2Torr,cm] 1 2 [keV](a) (b)Fig. 2. (a) The tensor polarization P33 as a funetion ofthe iodinetarget thickness measured at a beam energy of 1 keV. (b) Thetensor polarization P33 as a funetion of the beam energy. Ateach energy the iodine density was adjusted to obtain a maxi-mum deuteron intensity.of positive ions to the cross section for the productionof negative ions it is expected that such an independence]is a general feature of all exchange reactions leading]to positive hydrogen ion beams which exhibit a large]selectivity for metastable H(2S) atoms.Second the achievable polarization decreases with]increasing beam energy. This general behaviour was!already found in many other reactions"). It can be]understood qualitatively from the pseudo-crossing]theory of charge exchange collisons,Third the comparison of the argon and iodine]reaction shows that iodine yields an Ip 2 which is ialmost two times greater than the corresponding valuelfor the argon charge exchange collision leading to inegative hydrogen ions (I intensity, P polarizationj.]Ip 2 is a measure of the applicability to polarized ionbeams.The experimental results discussed above are to be icompared with the features of other types of sources 1for polarized ions. Although the corresponding studies Iare still in progress the Lamb shift type ion sourcelmight turn out to be advantageous for the production iof polarized proton and deuteron beams also. Deu-lteron beams with a tensor polarization of P33 = -0.7:and intensities in the microampere range will be iavailable in small phase space volumes in the near:future.:The encouragement and the facilities provided by]the institute directors Prof. A. Citron, Prof. W. Heinzand Prof. H. Schopper are greatfully acknowledged.]The authors wish to thank V. Bechtold and P. Ziegler!for the helpful operation ofthe Lamb shift source.References1) W. E. Lamb and R. C. Retherford, Phys. Rev. 79 (1950) 549,B. L. Donnally and W. Sawyer, Phys, Rev. Letters 15 (1965);439; . 'W. Haeberli, Ann, Rev. Nuel. Sei. 17 (1967) 406.2) H. Brückmann, D. Finken and L. Friedrieh, Z. Physik 2241(1969) 486.3) H. Brückmann, D. Finken and L. Friedrieh, Phys, Letters 29Bi(1969) 223.4) H. O. Pritehard, Chem. Rev. 52 (1953) 529;W. B. Person, J. Chem. Phys, 38 (1963) 109;B. Steiner, M. L. Seman and L. M. Branseomb, J. Chem.1Phys. 37 (1962) 1200.5) J. B. Hasted, Physics 0/ atornic collisions (Butterworths,London, 1964) p, 295.6) L. D. Knutson, private eommunieation. ,7) V. Bechtold, H. Brüekmann, D. Finken and L. Friedrieh, zlPhysik 231 (1970) 98. i8) H. Brüekmann, D. Finken and L. Friedrieh, to be published,

Polarized positively charged hydrogen ion beams from charge exchange collisions between metastable H(2S) atoms and halogenes

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Polarized positively charged hydrogen ion beams from charge exchange collisions between metastable H(2S) atoms and halogenes

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