Determination of the reaction rate π_{at rest}⁻ + p → π^o + n in hydrogenous materials

It has been known for several years that electromagnetic calorimetry with scintillating plastic fibers gives good results at high energy (ref. [1] H. Blumenfeld et al., Nucl. Instr. and Meth. 225 (1984) 518; ref. [2] H. Burmeister et al., Nucl. Instr. and Meth. 225 (1984) 530). A measurement done with 40 MeV electrons [2] convinced us that this technique can be applied for the identification of π⁰ particles produced in the capture of negative pions by protons. A prototype detector has been built, calibrated and used in coincidence with a NaI detector to identify the decay of neutral pions produced by stopping a 200 MeV/cπ⁻ beam in a CH₂ target. Results are discussed and compared with a Monte Carlo simulation. Finally, a full-scale detector has been built and successfully used in a measurement of the X-ray yield in pionic hydrogen.

We have observed X-rays from kaons stopping in targets of Li, LiH and Be. The strong interaction widths, shifts and relative yield of the last observed X-ray have been measured. There is evidence for a difference in the yield of the n = 3 → 2 Li X-rays in Li metal and LiH. There is no evidence in the spectra for X-rays from kaonic hydrogen formed in LiH.

A number of processes leading to the emission of photons and neutrons in π⁻ capture reactions are discussed for potential use in radiation therapy to determine the stopping π⁻ region. It is concluded that the scheme of detecting collimated medium energy γ-rays is the most effective.

The yields of the analogous reactions $\text{μ}{}^{\mathrm{-}}{}_{\mathrm{<mtext>at\; rest</mtext>}}\text{+}{}^{12}\text{C}\text{→ ν +}{}^{12}\text{B}{}_{\mathrm{<mtext>stable</mtext>}}$ and $\text{π}{}_{\mathrm{<mtext>at\; rest</mtext>}}{}^{\mathrm{-}}\text{+}{}^{12}\text{C}\text{→ γ +}{}^{12}\text{B}{}_{\mathrm{<mtext>stable</mtext>}}$ have been determined detecting the ¹²B activity produced in a plastic scintillator by stopping π⁻ and μ⁻. Under certain assumptions about the strength of the π⁻ absorption from the mesic 2P level and about the contribution of the particle-stable excited ¹²B states, the ratio of the 1S capture rates obtained is $\text{R = (}\text{Λ}{}^{\mathrm{\pi }}{}_{\mathrm{\gamma }}\text{Λ}{}^{\mathrm{\mu }}\text{)(}{}^{12}\text{C →}{}^{12}\text{B}{}_{\mathrm{g.s.}}\text{) = (1.99 ± 0.53) × 10}{}^{12}$. The theoretically expected value is R = (1.27 ± 0.17) × 10¹².