Experimental Measurements

Two-photon capture processes have been measured on nuclear targets. The $\pi ^-$C and $\pi ^-$Be experiments were motivated by the predicted dominance of the $\pi^+\pi^-\rightarrow\gamma \gamma$ annihilation graph in the elementary $\pi ^-p\rightarrow \gamma \gamma n$ reaction, and sensitivity to the pion field in the nuclear medium.

At the CERN synchrocyclotron, $^{12}$C and $^{9}$Be two-photon capture processes were measured (12) using a large solid angle array of lead-glass Cerenkov detectors. The set-up of the experiment is reproduced in Figure 2.5. The detector consisted of 20 lead-glass truncated prisms of 4 $\times$ 10 cm$^2$ front-face and 15 cm depth viewed by photomultiplier tubes. Negative pions from the CERN synchrocyclotron were stopped in beryllium (Be) and graphite (C) targets of $10\times 10 \times 1$ cm$^3$ dimension, located at the center of the lead-glass array.

Figure 2.5: Layout of the experimental set-up for the CERN measurement by Deutsch et al. (12)

A 5-element beam telescope made of plastic scintillators was used in coincidence. Acceptance of the set-up was typically 20%, and the events were triggered by a coincidence between the telescope and at least two lead-glass photon detectors. The photon energy threshold of the measurement was 25 MeV. Branching ratios of (1.4 $\pm$ 0.2)$\times$10$^{-5}$ and (1.0 $\pm$ 0.1)$\times$10$^{-5}$ for $E_{\gamma} \geq 25$ MeV was measured for the $\pi ^-$C and $\pi ^-$Be two-photon processes respectively.

Figure 2.6: Two photon angular distribution y=$\cos\theta$ for double radiative pion capture on $^{12}C$. The plot shows comparison of theoretical results of Christillin and Ericson (solid curve) with experimental measurements of Deutsch et al. (data points with error-bars). The dashed curve shows the contribution of the $\pi\pi$ annihilation graph.

Figure 2.7: Layout of the experimental set-up for the TRIUMF measurement by Mazzucato et al. (13)

At TRIUMF, a $^{12}$C experiment (13) was performed using a pair of large NaI crystals TINA (46 cm diameter $\times$ 51 cm) and MINA (36 cm diameter $\times$ 36 cm) and two lead-glass photon counters C1 (12.7 $\times$ 12.7 $\times$ 25.4 cm$^3$) and C2 (12.7 $\times$ 12.7 $\times$ 20.3 cm$^3$). The set-up of the experiment is reproduced in Figure 2.7. A 20 MeV $\pi ^-$ beam was stopped in a $25\times 20 \times 1.3$ cm$^3$ graphite target. Plastic scintillation counters (S5 to S10) placed in front of the photon counters were used to identify charged particles. Cosmic ray coincidences were vetoed by large plastic scintillators covering the NaI crystals. Steel shielding was placed between the two NaI crystals to reduce neutron background. A total of 2 $\times$10$^{10}$ pions were stopped at an average beam rate of 1 $\times$10$^{6}$ $\pi ^-$/$s$ and about 500 good 2$\gamma$ events at all angles were collected. Photon energy threshold of the measurement was 17 MeV. A two-photon branching ratio of $(1.2 \pm 0.2) \times 10^{-5}$ for pion capture on carbon was obtained for $E_{\gamma} \geq 17$ MeV.

Figure 2.8: Energy distribution of the two photons in the TRIUMF experiment. Coincidence events at $\theta$=120$^\circ$ are shown by solid line histograms. The dashed line histogram shows the summed energy distribution. The solid and dashed lines are the theoretical predictions of Christillin et al. convoluted with the detector resolution function arbitrarily normalized by a factor 0.75.