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Electric field and temperature dependence of positron annihilation in argon gas Lee, Gregory Frank

Abstract

The annihilation rate of positrons in argon has been measured over the temperature range of 135 K to 573 K with applied d.c. electric fields up to 35 V cm⁻¹ amagat⁻¹. Analysis of this data vas carried out assuming that the annihilation rate and the momentum-transfer rate can be approximated by a function of the form Av[sup B] over this velocity range. Results of this analysis indicate a strong velocity dependence in both the annihilation rate and the momentum-transfer rate. For velocities in atomic 2 —1 units (e²/K) corresponding to positron wave number from .03 to .065 a[sub o]⁻¹ the annihilation rate was found to be adequately represented by V[sub a](v) = (1.195 x 10⁶)v[sup -.480] sec⁻¹ amagat⁻¹. The momentum-transfer rate was similarly determined as V[sub d](v) = (2.11 x 10¹¹)v[sup -.49 sec⁻¹ amagat⁻¹. The errors in these results are velocity dependent. The largest error is 16%. This velocity dependence at low energies is not reproduced satisfactorily by any current theoretical models. A modified effective range parameterization of the momentum-transfer cross—section gives a scattering length of A[sub O] = - 4.4 ± .5 a[sub o] for positron-argon collisions. No evidence of a velocity dependence in the orthopositronium quenching rate at room temperature was detected. The orthopositronium quenching rate vas measured at .255 - .009 µsec⁻¹ amagat⁻¹ in good agreement with previous results.

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