R-95-03, PSI - ETHZ - ZÜRICH
A. Badertscher², M. Daum¹, R. Frosch¹, P.F.A. Goudsmit², W. Hajdas¹, M. Janousch², P.-R. Kettle¹, V.E. Markushin¹, J. Schottmüller¹³, and Z.G. Zhao²
| ¹ | PSI, Paul-Scherrer-Institut, CH-5232 Villigen-PSI, Switzerland |
| ² | IPP, Institut für Teilchenphysik der ETHZ, CH-5232 Villigen-PSI, Switzerland |
| ³ | Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland |
Our new measurement of the Doppler broadening in the neutron time-of-flight (TOF) in the reaction pi¯ p -> pi° n has led to new results [1] concerning the kinetic energy distribution f(Tpi p) of pi¯p-atoms in liquid hydrogen, at the instant of nuclear capture. We confirm the existence of a strong 'high-energy' part (Tpi p >> 1 eV) [2] containing about half of the pi¯p-atoms. Moreover, the data show, that this 'high energy' part is derived from at least four atomic transitions n -> n´ with 6 % of all pi¯p-atoms having kinetic energies close to 200 eV. The model used to explain the data is shown in Fig.1a (54 kB), and the resulting transition energies Tnn´(fit) and yields Ann´(fit) obtained by fitting to the data (Figs. 1b, 1c : 54 kB) are given in the third and fifth columns of Table 1.
| Transition | Tnn´ [eV] | Tnn´(fit) [eV] | Tnn´(fit) - Tnn´ [eV] | Ann´ [%] |
|---|---|---|---|---|
| 6 -> 5 | 18.4 | 5.6 ± 2.8 | -12.8 ± 2.8 | 17.6 ± 2.1 |
| 5 -> 4 | 33.9 | 21.7 ± 5.5 | -12.2 ± 5.5 | 13.0 ± 1.7 |
| 4 -> 3 | 73.2 | 57.2 ± 9.0 | -16.0 ± 9.0 | 18.4 ± 1.5 |
| 3 -> 2 | 209.1 | 207 ± 19 | -2 ± 19 | 5.9 ± 0.7 |
The uniform low-energy component of f(Tpi p) comes to T1 = (1.16 ± 0.14) eV and contains A1 = (45.1 ± 1.9) % of the pi¯p-atoms. The two-component model used previously [2] is found to be inconsistent with the new, improved quality data (see inserts Fig. 1b, 1c : 54 kB) . The shape of the 'high energy' components of the Tpi p distribution is consistent with those expected from Coulomb de-excitation processes. However, the energies Tnn´ obtained by the fit (for n = 5, 6) are significantly lower. It should be noted, that the theoretical predictions of the Coulomb de-excitation rates [3, 4, 5, 6] disagree seriously amongst each other and those models predicting high rates are favoured by our data.
NTOF LH2Target (112 kB)
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