Brueckmann, H.

Lueck, W.

Schmidt, F.K.

English

Schmidt, F. K.

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Märzfür Experimentelle KernphysikKFK iInfluence of the Absorntionof Unl", .. i."",+i..."nLight onHe Targetreew. K. idtNG M. 11. H.Als Manuskript vervielfältigtFür diesen Bericht behalten wir uns alle Rechte vorGESELLSCHAFT FüR KERNFORSCHUNG M.B.H.KARLSRUHEKERNFORSCHUNGSZENTRUM KARLSRUHEMärz 1972 KFK 1538Institut für Experimentelle KernphysikThe influence of the absorption of the pumpinglight on the degree of pol zation of an opti-ll'cally pumped targetH. Brückmann, W. Lück, SchmidtGesellschaft für Kernforschung m.b.H. Karlsruhell'contributed paper submitted to the tlThird Inter-national Symposium on Polarization Phenomene inNuc ar Reactionsl!Madison, Wisconsin 1970AbstractIn order to investigate the optical pumping of helium the meanfree path A of the pumping light in 4He was measured. At apressure of 7 Torr a value A=lo.8±o.4 cm was obtained for a weakdis . An exact calculation for 3He re ted A=13.5 cm.Therefore the approximation of weak abs ion cannot be consideredto be valid.ZusZur Untersuchung s ischen Pumpvorganges in liumgas wurdedie ttlere ie Weglänge Ader Fumplichtphotonen 4He gemessen.Bei e Druck von 7 und s ter, ladungim 4He sich A=10,8±O,4 cm. Durch e exakte Umrechnung auf"2;/He 19te eses A=13,5 cm. Damit ist e Voraussetzung rschwachen Absorption, die zum ichen eines Po satgrades notwendig ist, cht mehr gültig.2 ~The degree 01' polarization observed experimentallY in anoptically pumped 3He target does not agree with the predietionderived by Cole ve et al. 1) for its dependence on theintensity 01' the pumping light. Up to now the question 01' theinfluence 01' the pumping light intensity on the polarization isnot yet answered and only one theoretical approach 1) to thisproblem is published. This approach assumes a weak absorption.A high absorption 01' the pumping light by the metastable atomsmight be able to expl n the discrepancy between the experiments.The dependence 01' the pumping light tensity on the range 01'the light agas discharge was investigated experimentally.Light from a 4He lamp was circularly pol zed and sent alont'; thea s through a cyli c glass vessel. vessel contained4He gas at apressure 01' either 0.05 mm Hg or 7 mm Hg. Thedimensions 01' the vessel were 20 cm in ngth and 7.0 cm indiameter. Metastable He atoms were produced by a weak rf gassch . The quencyand the geomet 01' the electrodes wereadjusted earefully in order to obtain a constant density 01' meta-st 1e atoms along the axis cf the cell~ With exception of there ans elose to t entrance and e windows this eondition hasbeen ful lIed very properly. The intensity 01' the fluorescentlyscattered light was taken to be a measure 01' the loe ation01' the pumping light intensity. The cent light emittedat 90 de s with respect to the axis 01' cell was observed bya movable phototube.The figure shows the result 01' such a measurement. The tensity01' 'ehe fluorescent light is plotted in a log thmic seale versusthe range eoo e scaled from the entrance w dow. Themeasured values 01' each 01' the two experiments shown are very weIlrepresented by st ght lines. The two experiments differed onlyby the density 01' the metastable He atoms. The pressure 01' the4He gas was 7 mm Hg in both cases. The pure exponential decreaseleads to the conclusion that the decay 01' only one 01' the twopumpi.ng light components are denoted by DO and D3. By aseparatemeasurement 01' the pOlarization 01' the fluoreseent light it wasassured that only the D~ eomponent was present. mhe mean free./path A 01' the photons was obtained from the slope 01' the decay- 3:1I1--I Icurve, At le lowest discharge level possible the mean free pathis ~ = 10.8 ± 0.4 cm while at a slightly higher discharge levelit turns out to be ~ = 9.2 ± 0.4 cm. The mean free path in 3He gaslS easily evaluable from the 4He data measured. In this way for-zJHe a value ~ = 13.5 ± 0.5 cm was obtained corresponding to thelowest discharge level.The measurements discussed exhibit such a high photon absorptionthat the approximation of weak absorption cannot e consideredto be valid. The mean e path of the photons is comparable tothe dimensions of the absorption cello Therefore reabsorptionof fluorescently emi tted photons has to be taken into a c c o un t .This effect decreases the degree of pol zation. In addition tothe polarized pumping light a fraction of unpolarized fluorescentlight acts in the pumping process. It can be shown, that this i8equivalent to an addition relaxation mechanism which reducesthe maximum obtainable target polarization.1) F,D. Colegrove, L.D. Schearer, G.K. WaltersPhys.Rev. 132 (1963) 2561

The influence of the absorption of the pumping light on the degree of polarization of an optically pumped He target

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The influence of the absorption of the pumping light on the degree of polarization of an optically pumped He target

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