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


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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