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The angular correlation of the annihilation radiation from positrons Argyle, Percy Edward


The angular distribution of the annihilation radiation from slow positrons in copper has been determined with a pair of scintillation counters in coincidence. A strong Cu⁶⁴ source from the Chalk River pile provided the positrons. Very small solid angle geometry was used and results of high internal consistency were obtained. The distribution has the form C(λ) = (const) e[superscript -λ/λ₀] where C(λ) is the counting rate for an angular correlation of (180-λ)⁰ between the two quanta. The mean value of λ₀= (3.60±.04)x10[superscript -3]radians. Assuming that this departure from antiparallelism is caused by the motion of the annihilating particles, their mean centre of mass momentum in copper is found to be p̅[subscript Cu] = (0 .985±.01)mc/137. This result lies within the experimental errors of the values 1.2 mc/ 137 and 1.1 mc/137 obtained by De Benedetti for gold and Du Mond for copper, respectively. It also lies within the computational error of the lower limit of 0.8 mc/137 estimated by DeBenedetti. The same equipment was used to measure (μ/ρ) for lead for hν = mc² by a new method-- that of intercepting one of the annihilation quanta in very pure lead and plotting the coincidence rate against lead thickness. An accurately exponential absorption was observed which gave the value (μ/ρ)[subscript Pb]= (0.155± .002) gm/cm². This method avoids almost completely the difficulty of discriminating against scattered radiation inherent in previous absorption measurements.

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