Hyperfine Interactions 84(1994)193-198 193

Measurement of atomic capture probabilities of negative pions in metal hydrides

Tadashi Saito a'b, Taichi Miura c, Atsushi Shinohara d, Junichiro Shintai a, Eugene Taniguchi d, Michiaki Furukawa a, Kazuhiro Takesako a,

Nobutsugu Imanishi e, Hisakazu Muramatsu f, Yoshio Yoshimura c, Hiroshi Baba ~ and Hidekazu Doe g

"Department of Chemistry and Laboratory of Nuclear Studies, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan

bRadioisotope Research Center, Osaka University, Toyonaka, Osaka 560, Japan CNational Laboratory for High Energy Physics (KEK), Tsukuba, lbaraki 305, Japan

dDepartment of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan

eDepartrnent of Nuclear Engineering, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan

@acuity of Education, Shinshu University, Nagano 380, Japan gDepartment of Chemistry, Faculty of Science, Osaka City University,

Sumiyoshi-ku, Osaka 558, Japan

Atomic capture probabilities o f negative pions in some metal hydrides were measured. The capture by a hydrogen atom was detected by means of a pair o f the annihilation 'y rays of rc ~ which had been produced by the charge-exchange reaction of ~- with the capturing hydrogen nucleus (proton). This method ensures a high sensitivity and reliability of the measurements . The probabilities obtained were in agreement with previous measurements except for palladium hydride, which showed a much smaller probability than that given in the literature. The atomic capture of n - is well described in the f ramework of the large mesic molecular model, in which the proportionality constant reflects the chemical states of the capturing atoms and also the neighboring ones.

1. Introduction

Negatively charged panicles such as n- and g- behave as heavy analogues of the electron in a Coulomb field and hence can orbit the positively charged nuclei. When n- or It- is projected into materials, it will be slowed down in a chemical compound and eventually captured by one of the constituent atoms. This atomic Coulomb capture is governed by nuclear charge Z. For bi-elemental compounds ZmZ~, the ratio of the capture of n- or Ix- is given by the simple relation:

W(Z)/W(Z') = mZ/nZ'. (1)

�9 J.C. Bahzer AG, Science Publishers

194 T. Saito et al. / Atomic capture of rr- in metal hydrides

This relation is well known as the Fermi-Tel ler law [1]. The capture is also affected by the molecular or solid-state structure, since the early stages of the capturing process are under the influence of the outmost valence electrons. The re- atomic capture probability hence is related to the chemical state of the capturing atoms. Net atomic charges on the constituent atoms can be deduced from the atomic capture ratio.

The atomic capture of n- by elements with Z > 2 is measured by means of pionic X rays emitted in the cascade transition of orbital n - in pionic atoms. The energies of pionic X rays are higher than those of ordinary electronic X rays roughly by a factor of their mass dif ference,-280. Pionic X rays emitted from lighter elements can be measured with a photon spectrometer. In addition, their self- absorption is tolerable even in the case when thicker targets are used according to the low intensity of the incident beam. On the other hand, the n- capture by hydrogen atoms can be measured with high sensitivity by the detection of a pair of 70 MeV 7 rays emitted in the electromagnetic decay of n ~ which is produced by the charge exchange reaction, namely, n-p ~ n~ n ~ 2T. Here, we report a mesochemical study on some metal hydrides, such as palladium hydride.

2. Experimental

The experiment was carried out at a secondary beam course, the ng channel [2], of the 12 GeV proton synchrotron at the National Laboratory for High Energy Physics (KEK). The momentum, its bite and the intensity of the n- beam used were typically 140 MeV/c, 2.5% and 5 x 104 n-/s, respectively. The beam was defined to be of size 4 cm in width and 4 cm in height, with a collimator of a stack of Pb, Sn and Cu metals. Fast n- was decelerated with a graphite degrader, the thickness of which can be varied continuously by remote control. The degrader, consisting of a pair of wedges, ensures homogeneity at any thickness. The optimum thickness of the degrader giving the maximum stopped events was typically 82 mm under the present experimental conditions.

The measured PdHx sample was a sheet of 50 x 50 cm 2 in area and 1 mm in thickness, which was inclined by 40 ~ with respect to the beam direction. Another thinner plate with a thickness of 0.2 mm was used in a single run. Charging of hydrogen into Pd metal was made by electrolysis. The amount of absorbed hydrogen was determined by weighing the Pd cathode on a microbalance before and after the electrolysis. For blank measurements, the same Pd plates containing no hydrogen were used before and after the true runs. Other samples were commercial chemicals of research grade and were packed in thin A1 containers. Blank samples with no hydrogen content were also treated in the same manner.

Stopped n--associated events were measured with an experimental apparatus similar to that described elsewhere [3]. Samples were placed in a chamber, where air was replaced with He gas in order to remove the humidity, that is, hydrogen atoms in water. Stopped n- events were selected with fast NIM circuitry connected

T. Saito et al. / Atomic capture o f n - in metal hydrides 195

to a counter telescope which consisted of three plastic scintillation (PS) counters and a veto PS counter. Highly energetic ~, rays associated with n- capture by hydrogen were measured with a pair of 30 • 30 • 38 cm 3 lead glass Cherenkov counter arrays [3]. These 18 Cherenkov counters were separately calibrated by firing them with the electron beam tuned at 70 MeV/c. Electromagnetic showers were reconstructed in off-line analysis of the stored CAMAC data, and then the true n o events were carefully selected. The efficiency for n o by this detection system was determined to be 0.11 + 0.02 by the normalization measurement using an LiH sample for which the absolute capture probability is given as WH = (3.5 + 0.4) • 10 -2 [4]. The pionic X rays were also measured with two hyperpure Ge spectrometers in coincidence with the stopped events.

3. Results and discussion

The measured atomic capture probabilities are summarized in table I. Gross n ~ events were selected from the 2~, events detected by the Cherenkov counter system by inspection of their trajectories. Net n o events were obtained by the

Table 1

Surrunary of exper imenta l results and the deduced atomic capture probabil i ty Wit for n - in some metal hydrides.

Net n o Sample Incident n - Stopped n - Gross n o ( • 10 -6) WII(• l0 -4)

Stopped n -

(a) (b)

CaH 2

PdHo.65 t 388638 100822 1474 -I- 38 4.6 + 1.6 5.4 + 0.8

PdHo.669 974742 247764 3884 5:64 4.2 + 1.8 5.5 _+ 0.4

PdHo.675 724117 27502 943 + 31 - 7.1 _+ 1.2

PdH~, 0.40 +_ 0.10

ZrH 2 180763 39590 3589 • 61 76 +__ 3 -

346913 5403 4674 + 69 70 _+ 3 58.5 +_ 3.0 6.6 • 0.3

TiH 2 80868 19793 1666 • 41 67 _+ 3 -

334470 55022 4470 • 68 63 • 4 59.6 • 3.2 5.9 • 0.3

95753 19538 6638 • 86 311:1:6 - 28.0 • 0.9

a)Measured by the degrader method . b)Obtained by subtraction of the result in a separate run with the blank sample which contains no hydrogen.

degrader method and also by comparison measurement using the respective blank sample containing no hydrogen. The degrader method is the one measuring the n o events by varying the thickness of the degrader, and correcting for the charge-

196 T. Saito et al. / Atomic capture of tr- in metal hydrides

exchange contribution by fast rc-'s. The blank method includes a comparison measurement in order to subtract the background from the sources other than hydrogen in the target. In table 1, the results obtained by both methods are given in columns (a) and (b). The atomic capture probabilities W H were calculated by taking into account the detection efficiency for ~o as given in the previous section. The error of the detection efficiency is not included in the quoted errors of WH.

Table 2

Comparison of the obtained atomic capture probabili ty W H and references.

Sample W H ( x 10 -4) Reference

this work literature

PdH~ 0.4 + 0.1 2.8 + 0.1 Kachalkin et al. [5]

ZrH 2 6.6 + 0.3 8.7:1:0.7 Kost et al. [6]

Ti l l z 5.9 + 0.3 8.4 :t: 0.8 Kost et al. [6]

Call2 28.0 _+ 0.9 25 _+ 3 Krumshtein et al. [4]

The obtained atomic capture probabilities are compared with references in table 2. As is shown in table 2, W H for PdHx is nearly one order of magnitude smaller than that obtained by Kachalkin et al. [5]. The measurements were repeated three times and two runs included both experimental methods, the degrader method and the blank one. The reproducibility was satisfactory, as is evident from table 1. The other WH values are in agreement with those given in the literature. Therefore, we believe the present result for PdHx is reliable.

It is well known that the atomic capture probability for r~- is described by a model based on large mesic molecules [7]. A large mesic molecule consists of the atomic nuclei fixed at nuclear positions of the ordinary electronic atoms and of the valence re- which occupies the mesic molecular orbit with an extraordinarily large extension. It is considered that the large mesic molecular orbit plays an important role in the early stages of the atomic capture of re-, and hence in the capture probability. This model [8] gives the atomic capture probability for metal hydrides Z,,,H,, as

n Z - 2 - - , (2) Wtt = a m z + n

where a is the proportionality constant. Kost et al. [6] measured the atomic capture probabilities of ~- by hydrogen in transition metal hydrides and interpreted the results in the framework of the large mesic molecular model. It was found that the constant a in the model shows a simple dependence on Z including non-stoichiometric

T. Saito et al. / Atomic capture of tr- in metal hydrides 197

hydrides. The values of a can be divided into two groups, one corresponding to ionic hydrides and the other to metallic hydrides as aid n = (3.0 + 0 .1 )Z- (48 + 3) and amet = (1.49 + 0 . 0 6 ) Z - ( 2 8 + 2) for 19 < Z < 73. It was also argued that the electron density at the hydrogen atom in ionic hydrides is approximately twice that in metallic hydrides. The above relations and the values of a obtained in the present experiment are shown in fig. 1. The value for PdH~ apparently deviates from the

150

ion

100

a met

5O

0 ' i 0 20 40 60 80

Z

Fig. l. Variation of the proportionality constant a of the large mesic molecular model with atomic number Z of the metal in various hydrides. Circles show the values obtained in the present work. Straight lines indicate the relation obtained by Kost et al. [6]. The line labeled by "ion" corresponds to the relation found for ionic hydrides, whereas "met" shows that for metallic hydrides.

line for metallic hydrides. This indicates that the electronic density on the hydrogen in PdH~ is less than that of normally covalent hydrogen. Our result is consistent with the fact that hydrogen diffuses through Pd metal with high mobility and hence it behaves as an electropositive hydrogen atom. To the contrary, Kachalkin et al. [5] discussed that, from their results, palladium hydride should contain no pronouncedly anomalous P d - H bond as interpreted in the framework of the large mesomolecular model, and then accounted for the high mobility by a consideration that the electronic states differ for residing and moving hydrogen in palladium.

In summary, the r~- capture rates for metal hydrides apparently reflect ordinary chemical properties.

Acknowledgement

This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.

198 T. Saito et al. /Atomic capture of ~- in metal hydrides

References

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Hayano, M. Aoki, H. Outa and T. Yamazaki, Nucl. Instr. Meth. A316(1992)134. [3] N. Imanishi, Y. Takeuchi, K. Toyoda, A. Shinohara and Y. Yoshimura, Nucl. Instr. Meth. A261(1987)465. [4] Z.V. Krumshtein, V.I. Petrukhin, L.I. Ponomarev and Yu.D. Prokoshkin, Sov. Phys. JETP 27(1968)906. [5] A.K. Kachalkin, Z.V. Krumshtein, V.I. Petrukhin, V.M. Suvorov, D. Horvath and I.A. Yutlandov0

Sov. Phys. JETP 46(1977)879. [6] M.E. Kost, Z.V. Krumshtein, V.I. Mikheeva, L.N. Padurets, V.I. Petrukhin, V.M. Suvorov, A.A.

Chertkov and I.A. Yutlandov, Russ. J. Inorg. Chem. 21(1976)789. [7] S.S. Gershtein, V.I. Petrukhin, L.I. Ponamarev and Yu.D. Prokoshkin, Sov. Phys. Usp. 12(1970)1. [8] See, for example, L.I. Ponomarev, Ann. Rev. Nucl. Sci. 23(1973)395.