Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 1, pp. 33−37, 2000

33

Formation Process of Pionic Atoms and Their Behavior in Material
Atsushi Shinohara*
Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama-cho, Toyonaka, Osaka
560-0043, Japan
Received: December 9, 1999
Exotic atoms are the interesting object as a new probe in material science and a new chemical species in futurechemistry. Our group (πAX) has recognized the mesoatomic and mesomolecular phenomena as a subject ofchemistry, and continued the experimental study for the capture process of negative pions since 1982. In the
recent study, we have investigated the pion capture process mainly in liquid and gas phases, focusing to the behavior of pionic hydrogen atoms. A novel method to measure selectively the pion capture for a particular hydrogen ina molecule revealed the effect of the chemical state of the hydrogen on the transfer process. A proposed model,
which combines a LMM model with a transfer process, was useful to wholly understand the capture process
observed. The outline of the formation process of pionic atoms and its behavior in material is described briefly,
and the main results of our studies are reviewed.

1. Background of the Present Studies
Recent progress of an accelerator has allowed us to apply
various exotic atoms to material science, analytical chemistry,
medical science, etc, other than particle physics. Especially,
the use of muons has been increasing, such as µSR, muonium
chemistry, µCF, and so on, in addition to the use of positrons.
Muonic or pionic atoms, which consist of a negative muon orpion and an ordinary atom (nucleus), have been studied since
the discovery of the atoms, and the outline of the formationand decay process of the atoms has been established. 1 - 5
However, more detailed and fundamental studies for formation
mechanism and behavior of such atoms are required to develop
a new chemistry of the 2nd generation substance (muonic or
pionic atoms and molecules) and new application of the phenomenon.
It is accepted that the formation and decay of a pionic atomtake place according to the following step. A negative pion in
the first stage of the slowing down in matter loses the energy
by ionization, and it is slowed down by collision with electronof comparable velocities until finally it is captured by the
Coulomb field of an atom. The pion captured in an outer Bohrorbit cascades down accompanied by the emission of Auger
electrons and by the pionic X rays. In the capture process,“chemical effects” are observed because electrons are considered to affect the Coulomb field in the capturing stage. In thelower levels, the strong interaction affects the cascade schemeand finally causes the nuclear absorption of the pion.
Complete understanding has not yet been attained for themechanism of molecular and atomic capture of pions.
A large mesomolecular (LMM) model is more useful todescribe the capture process including the chemical effects
among various capture models proposed. This model was proposed by Ponomarev et al.1 to explain the obvious chemical
effect observed in hydrides, and extended to the capture phenomenon in general molecules by Scheuwly et al.3 In the firststep (probability P) of the model, it follows that the pion is
captured on the LMM orbital when the valence electron emission occurs, and on the atomic orbital when the core electronof the corresponding atom is emitted as Auger electron. The
pion on the LMM orbital moves to an atomic state of any atomin the molecule and then the pionic atom forms in the second*Corresponding author. E-mail: shino@chem.sci.osaka-u.ac.jp.
FAX: +81-6-6850-5418.

step (probability Q). If the pion is captured by hydrogen, thecapture probability of hydrogen (WH) is expressed by
WH = P Q R,

(1)where R means the probability that the pion in the pionichydrogen is finally captured by the proton without transfer toother atoms.
Ponomarev well explained the hydrogen effect by assuming
the second step to be radiative process. Schneuwly could takethe chemical effects into account in the LMM model by treating the probability P as a localization factor defined by the ionicity parameter σ introduced by Paurling. In our previous
works,6-9 we proposed a modified LMM model, in which thesecond step proceeds via Auger transition, based on the large
chemical effects observed for the pion capture ratio in a seriesof beryllium borides (BexBy) and in MBH4, MOH, and MF(M=Li, Na, K).
As for the pion transfer phenomenon, many gas phase
experiments performed mainly by Russian group showed thatthe pion transfer from pionic hydrogen atoms to other heavier
atoms occurs in gas phase in spite of its short life time. The
semi-empirical formulae for the transfer rate were also derived
from the systematic measurements. 2,4,10,11 The influence ofchemical environment on the transfer process, however, wasscarcely observed and was not involved in such phenomenological treatment.
2. Theme and Goal of the πAX Project
The goal of the πAX project lies in comprehensively understanding the formation process and behavior of pionic atoms
and molecules in order to apply the phenomenon to materialscience, analytical chemistry, bio and medical science and todevelop a new chemistry of 2nd generation substance (muonicor pionic atoms). Especially, the quantitative treatment for
pionic hydrogen atoms will provide a unique probe for hydrogen in material. Figure 1 shows the capture and decay
processes of negative pions in hydrogen-containing molecule(Z-H ). In the present studies (Visiting Researcher’s Programof KEK-PS: E202, E262, E360), we focused the formation andbehavior of pionic hydrogen atoms, as shown with the boldlines in the figure.
The observable quantities are the pionic X rays and two γrays from π0 decay (indication of the final capture by hydro-© 2000 The Japan Society of Nuclear and Radiochemical Sciences

Shinohara

34 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

Figure 1. Pion capture process in hydrogen-containing molecules (Z-H).

gen). The former includes the information on the internaltransfer, and the change of the latter against the concentrationof Z'-atom reflects the external transfer rate. The selection of
the bond nature and structure of the Z-H molecule is also additional degree of freedom in the research technique from theviewpoint of chemistry.
Many experiments were carried out with a newly developed
apparatus, which allows us to measure simultaneously two γrays and low-energy pionic X rays, to achieve the goal mentioned above. These experiments are classified into the following theme.
(1) Selective measurement for the capture probability on thehydrogen at a particular site in the molecule.14,15
Systems: R(alkanes), ROH/ROD, RCOOH/RCOOD.
(2) Measurement of the external transfer rates by two-component systems in liquid and gas phases.16,17
Systems: C6H12/C6H6+CCl4/CS2, H2/CH4+Z(rare gas).
(3) Measurement of pressure dependence of transfer rate ingas phase (in progress).
Systems: H2/CH4+Ar(chaining pressure).
(4) Transfer and structure effects appearing in the intensity
patterns of pionic X rays.18,19
Systems: Fullerenes, H2/D2/CH4+SF6, & All samples studied.
(5) Measurement of the fundamental data (Pionic X-ray
yield, correction data,...).20
Systems: Various elements, H2O+D2O,...
(6) Application of the LMM model.21
Systems: Mx(acac)y, MHx.
(7) Proposal for the combined LMM model (LMM + transfer).14
Based on (1) and (2).
(8) Proposal for a new model including the dynamical
behavior of pionic hydrogen (in progress).
3. Experimental Method
The experimental setup was installed at the πµ-channel22 of
the 12-GeV proton synchrotron of the High Energy
Accelerator Research Organization (KEK). The channel comprises a D4Q2D2Q lens system and has a length of 11.0 m
from the production target to the sample position. The apparatus comprised an energy degrader, a collimator, a radiation
shield, a defining counter system, and a measuring chamber, as

Figure 2. Schematic view of the experimental setup.

shown in Figure 2. Negative pions were slowed down by thegraphite degrader so as to stop in the target. Incident and stopping pions in the sample were counted using a conventionalcounter telescope comprising four plastic scintillation counters(PS1, PS2, PS3, and PS4 (veto)).
The pionic X rays were measured with two or three Ge−
detectors in coincidence with the stop events (1*2*3*4 ). Twocoincident γ rays from a charge-exchange reaction, p + π− -> n
+ π 0 , π 0 -> 2γ (70 MeV each, 60% branching ratio), weredetected using a pair of Cherenkov-detector arrays. The beam
size at the sample position was defined as 4(H) × 3(V) cm2 by
the collimator and the defining counters (PS2 and PS3 in
Figure 2).
The solid or liquid sample was set in the chamber filled withhelium gas in order to avoid any disturbance from air.14 A rectangular container [5.5(H) × 7.0(V) cm2 in area and 2.5, 5.0, or
7.5 mm in thickness] made of thin beryllium or aluminum foil
was used for liquid and powder samples. The self-supporting
sample was also prepared for some compounds as a pressed
method. A high-pressure gas chamber was specially designed
for measurement of low-energy pionic X rays and π0 decays
from a gas sample.18 The chamber had cylindrical form to bearhigh pressure (50kgf/cm 2 ) and had the sample volume of
125cm3. The 2.5mm thick chamber wall was made of berylli-Formation Process of Pionic Atoms and Their Behavior

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000 35

um to measure low-energy pionic X rays (ex. 18keV for carbon). PS-3 and PS-4 counters were mounted inside the chamber and used as the phoswich type counter system.
4. Main Results of the Present Project
Among the above theme, the results of (1) , (3), and (7)were the noticeable ones in our work. Especially, the selective
measurement for hydrogen was successfully done by using thedeuterated compounds.15 The dependence of the transfer rateon gas pressure was also observed at the first time in the
CH4+Ar system. The former will provide a useful technique toinvestigate individually the behavior of the pionic hydrogen
atom originating from a hydrogen atom at different chemical
states in a molecule or material. These results require a newmodel for the capture and transfer processes, which include thelife time of the pionic hydrogen atom and the chemical effects
in the transfer process. Main results of the project are briefly
described below.
4.1. Measurements for External Transfer in Alkanes anda Combined LMM Model.14 We attempted to reveal the contribution of the external transfer process in the pion capture forhydrogen-containing compounds in the condensed phase. We
then measured precisely the capture process in a series ofhydrocarbon (liquid alkanes), which had been analyzed withno contribution of the external transfer so far.23 To explain the
result obtained, we proposed a new model involving a modified LMM model and an external transfer process as mentioned below.
In the modified LMM model,7,9 we introduced a parameteraγ to express the capture probability on hydrogen, where aγ isdefined as a ratio of the sum of the radiative and the nuclear
absorption rates to the Auger transition rate. The probabilityof pion capture by hydrogen WH is obtained from eq 2:ν(1+σ) a γ
WH =
,
(2)
(N+ν)(1-σ)Zeff
where N is the sum of the core electrons relevant to the capture
process (N=kn2+ln1, n1=0), ν presents the total valence electrons (ν=kν2+l).9 The factor a γ /Zeff is thought to correspond to
the the product of the parameter Q and R from a comparison
with eq 1. In this model, therefore, we approximate the parameter a γ as the product of two factors, a γ' and R', where R' is
the non-transfer probability associated only with the externaltransfer. The capture probability WH is then rewritten asν(1+σ) a γ'
WH =
R'.
(3)
(N+ν)(1-σ)Zeff

given as an average over the chemically different states of theelement in the molecule. Observation for the pion capture
probability on a particular atom in the molecule, if possible,
will give significant information for understanding the pion
capture mechanism. We then tried to extract the capture rate
for a particular hydrogen atom using deuterated compounds.
The capture events of pionic hydrogen undergoing no transfer
can be measured by two-γ-rays of π0 produced by the charge
exchange reaction. On the contrary, the reaction of a pionic
deuterium (π−d) is strongly suppressed (about 10−4).24 The
measurement for deuterated molecules therefore provides thecapture probability for hydrogen except for hydrogen at thedeuterated site. Thus we can obtain the capture rate for a particular hydrogen atom with a comparison between the ordinary
compound and the deuterated one.
We selected a series of alcohols, carboxylic acids, and thecorresponding compounds with a deuterated hydroxyl (-OD) or
carboxyl (-COOD) group to study the behavior of the pionic
atoms originating from hydrogen in different chemical states.
As a result, we found that pionic hydrogen atoms from hydroxyl and carboxyl groups show different behavior as demonstrated
in Figures 3a and 3b. The pionic hydrogen from hydroxyl
group strongly favors the transfer to oxygen and that from carboxyl group shows almost no transfer phenomenon. The transfer rate parameters ΛO of individual pionic hydrogen for oxygen
were determined based on the combined LMM model usingΛC=1.7 mentioned above. The result indicates that the chemical state of the hydrogen in the molecule affects the life or

Figure 3a. Capture ratios per atom of the hydrogen bound to carbon
relative to the carbon, RCH/RC, as a function of the number (n) of carbon atoms for alcohols ( ) and carboxylic acids ( ). The dashedline represents the capture ratio for polyethylene. Solid curves are themodel calculations.

The probability R' can be expressed approximately by
R' = 1/(1 + ΛZCZ),(4)

based on a phenomenological model, where ΛZ is the reduced
pion-transfer rate relative to the charge-exchange rate in thecollision with Z-atoms, and CZ is the ratio of the number of theZ-atom to the hydrogen atom. The a γ' value should be recognized as being the probability of isolation of a π−p from the ZH bond or of absorption by the proton without experiencing aninternal transfer (the latter probability is assumed to be negligibly small in eq 2) and hence is determined as a particular parameter for the kind of Z-H bond.
In conclusion, we found the contribution of the externaltransfer in liquid alkanes and obtained the transfer rate parameter for carbon as ΛC=1.7+0.2 and the parameter for isolation
process from the molecule as a γ'=0.140+0.008 based on the
proposed model. The obtained transfer parameters are summarized in Table 1 together with the semi-empirical estimates.4
4.2. Selective Measurement for the Capture on Hydrogen.15
In the analysis of the pion capture process, the main problem isthe fact that the capture probability for a particular Z atom is
2

Figure 3b. Capture ratios per atom of the hydrogen bound to oxygen
relative to the oxygen, ROH/RO, as a function of the number (n) of carbon atoms for alcohols ( ) and carboxylic acids ( ). Solid curve isthe model calculations with ΛC=1.7 and ΛO=4.1, and the dotted curve
represents those with a γ'=0.14 for alcohols. No external transfer was
found for carboxylic acids.

Shinohara

36 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000
velocity of the isolated pionic hydrogen atom and causes thechemical effect in the transfer process.
4.3. Measurement of the External Transfer by TwoComponent Systems in Liquid and Gas Phases.16,18 The
external transfer process is explicitly observed in two-component system consisting of the hydrogen-containing moleculeand the hydrogen-non-containing one. For the liquid phase,
we selected C6H12 or C6H6 + CCl4 or CS2 systems which don’treact with each other and mix with each other in any concentration. The external transfer rate was derived from the dependence of two-γ-events on the capture fraction of C6H12 or C6H6
molecule, which was obtained from the pionic X-ray measurement. In this study, we revealed the structure effect for thetransfer process by comparing between the transfer rate forcarbon of alkaline (sp3 structure) and that for carbon of benzene (sp2 structure) as shown in Table 1. The transfer rates at
51kgf/cm2 were also measured for the H 2 + Z and CH4 + Z(Z=He, Ne, Ar, Kr, Xe) systems to compare with the systematics for the atomic number Z. The obtained external transfer
rate parameters are listed in Table 1 and plotted as a functionof Z1/3 in Figure 4 together with the predictions by the semiempirical formulae.4,10,11
TABLE 1: Transfer Rate Parameters Obtained in the Present
Study
ΛZ
System (liq.) Z=COCl(CCl4)R(alkanes)
1.7±0.2
−
−
ROH
1.7*
4.1±0.5
−
RCOOH
1.7*
4.5±0.5−
C6H12+CCl4
1.7*−
5.9±0.4
C6H6+CCl4
3.4±0.6−
5.9*
Systematics**
2.5
3.0
4.7
System (gas) Z=
He
Ne
Ar
Kr
Xe
H2+Z
1.39±0.10 4.83±0.22 7.21±0.43 8.35±0.49 9.89±0.95
CH4+Z
1.2 ±0.1 2.64±0.22 3.59±0.17 4.23±0.42−* The parameter was fixed in the analysis procedure.
** Estimated from the semi-empirical formula for CmHn+Z (Ref. 4).

4.4. Pressure-Dependence of the Transfer Rate in the Gas
Phase (in Progress). The dependence of the transfer rate on
the gas pressure has not been observed so far in the muon orpion capture process for gas mixtures. The pressure-dependence may be observed for the pionic atoms from the boundhydrogen, which may have a short life as mentioned above, if
the running path of the pionic hydrogen is short and comparable to the mean free path in the material. We then measured
the change of the transfer rates at the pressure 50kgf/cm 2 ,13.5kgf/cm2, and 6.0kgf/cm2 for the H2 + Ar and CH4 + Ar systems.

Figure 4. Z-dependence of the transfer rate parameters (ΛZ). Solidline is the semi-empirical estimates for H2+Z system,4,10 and dashedline for CnHm+Z system.4,11: H2+Z, : CH4+Z, : CmHn+Z(liq.),: C6H6+Z(liq.), Z=alkanes.

Figure 5 shows the pion capture ratios of hydrogen to carbon against the fraction of Ar for different pressures for the
CH 4 + Ar system. One can see small change but apparentchange of the transfer rate with pressure in the figure. On theother hand, we can observe no change of the transfer rate for
the H2 + Ar system.
We can roughly estimate for the mean free path in Ar to beabout 10−8m in gas phase at the pressure of 50kgf/cm2, assuming that the effective interaction radius is an atomic dimension,and for the traveling length (tv) of the pionic hydrogen atom tobe 6×10−10 to 2×10−8 m, providing that the velocity (v) of it isestimated from the kinetic energy of 1−2 eV.25 Then overlapping between the ranges of the mean free path and the travelinglength suggests the possibility that the transfer rate changes
depending on the gas pressure and between the gas and condensed phases. If the observed result is attributed to the mechanism mentioned above, the fact that the pressure-dependence
was observed only for the CH4 system is consistent with thedifference in the life time (t) between the excited states of thepionic hydrogen atom from CH4 and that from H2.
4.5. Transfer and Structure Effects Appearing in the
Intensity Patterns of Pionic X Rays.13,18 Change of the intensity pattern and yields of pionic X rays will be caused by thecontribution of the pion transferred from a pionic hydrogen as
well as the different angular momentum distribution on the initial stage of the capture process. The relation of the change ofintensity ratio was investigated from the viewpoint of the molecular structure and the number of hydrogen for the pionic X
rays of carbon and oxygen for most samples studied. It was
found that the pion brought by the internal transfer mainly contribute the atomic cascade for the pionic X rays of the atombound to hydrogen. The pure structure effect was observed in
the measurements for carbon allotropes including fullerenes
and for various single substances. We discussed about theatomic levels concerned with the transfer based on the cascadecalculations including the contribution of the transferred pion.
5. Summary
Figure 4 shows the dependence of the transfer rate parameters obtained on the atomic number and a comparison between
the transfer rates in the liquid phase and those in gas phase.
One can see that the present results slightly deviate from theZ1/3-dependence which is assumed in the empirical formulae,
and that the transfer rates in liquid phase are rather faster thanthose in gas phase for alkane systems. The pressure-dependence of the transfer rate was also found for alkane systems. It
is generally accepted that the difference between the H2 system

Figure 5. Pion capture rations of hydrogen to carbon atoms against: 13.5the fraction of Ar at different gas pressure.
: 51kgf/cm2,
kgf/cm2, : 6.0kgf/cm2.

Formation Process of Pionic Atoms and Their Behavior
and CH4 system in gas phase is attributed to the fact that thepionic hydrogen atom from the bound hydrogen (CH4) lies in alower excited state compared with that from H2 molecule.4 If
this means that the life of the pionic hydrogen from alkanes isshorter than that from H2, the results of the pressure dependence is qualitatively consistent with the difference between
the liquid and gas phases. The present result also indicates thata dynamical model, in which the life, velocity, molecular
structure, and so on are taken into consideration, is required forthe pionic hydrogen (with short life) originating from boundhydrogen.
In conclusion, the combined LMM model and the cascadecalculation including transfer process successfully explained
the results of the capture rates, transfer rates, and pionic X-ray
intensity patterns for most sample systems studied. Here wecan conclude that these model calculations are very useful tounderstand the formation and decay process of most pionic
atoms in various materials. After this, quantitative discussion
about the discrepancy between the results of liquid and gas
phases and further accumulating the data of a γ' and ΛZ for various chemical systems are needed to achieve the goal of ourstudy.
Acknowledgements. The πAX project is a collaboration by
Drs. T. Saito, T. Miura, A. Yokoyama, T. Muroyama, K.
Takamiya, Y. Hamajima, H. Muramatsu, S. Kojima, Y.
Yoshimura, N. Imanishi, I. Fujiwara, M. Furukawa, H. Baba,
and A. Shinohara, and many graduate students of Nagoya,
Osaka, Kyoto, and Niigata Universities. The experiments were
performed under the Visiting Researcher’s Program of KEK.
We are grateful to Professors K. Nakai, K. Nakamura, K.
Takamatsu, T. Oshima, J. Chiba for their interest and supports.
We also wish to thank the staff of the KEK-PS and the beam
channel for steady provision of the pion beam.
References(1) L. I. Ponomarev, Ann. Rev. Nucl. Sci. 23, 495 (1993).
(2) V. I. Petrukhin, Yu. D. Prokoshkin, and V. M. Suvorov,Zh. E'ksp. Teor. Fiz. 55, 2173 (1968).
(Sov. Phys. JETP, 28, 1151 (1969)) .
(3) H. Schneuwly, V. I. Pokrovsky, and L. I. Ponomarev,
Nucl. Phys. A312, 419 (1978).
(4) D. Horva'th , Radiochim. Acta 28, 241 (1981).
(5) D. F. Jackson, Phys. Lett. 95A, 487 (1983).
(6) N. Imanishi, T. Furuya, I. Fujiwara, A. Shinohara, H. Kaji,and S. Iwata, Phys. Rev. A 32, 2584 (1985).
(7) N. Imanishi, T. Fukumura, A. Shinohara, and H. Kaji,
Phys. Rev. A 35, 2044 (1987).
(8) N. Imanishi, S. Miyamoto, S. Ohashi, and A. Shinohara,
Phys. Rev. A 35, 5244 (1987).

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000 37(9) N. Imanishi, S. Miyamoto, Y. Takeuchi, A. Shinohara, H.
Kaji, and Y. Yoshimura, Phys. Rev. A 37, 43 (1988).
(10) V. I. Petrukhin and V. M. Suvorov, Zh. E'ksp. Teor. Fiz.
70, 1145 (1976) (Sov. Phys. JETP 43, 595 (1976)).
(11) V. I. Petrukhin, V. E. Risin, I. F. Samenkova, and V. M.
Suvorov, Zh. E'ksp. Teor. Fiz. 69, 1883 (1975) (Sov.
Phys. JETP 42, 955 (1976)).
(12) N. Imanishi, Y. Takeuchi, K. Toyada, A. Shinohara, andY. Yoshimura, Nucl. Instr. and Meth. A 261, 465 (1987).
(13) A. Shinohara, M. Furukawa, T. Saito, H. Baba, T. Miura,and N. Imanishi, Nucl. Instr. and Meth. B 84, 14 (1994).
(14) A. Shinohara, J. Shintai, T. Muroyama, J. Kurachi, M.
Furukawa, T. Miura, Y. Yoshimura, T. Saito, T. Ohdaira,and N. Imanishi, Phys. Rev. A 53, 130 (1996).
(15) A. Shinohara, T. Muroyama, C. Murata, T. Miua, T. Saito,
A. Yokoyama, S. Kojima, and M. Furukawa, Phys. Rev.
Lett. 76, 2460 (1996).
(16) A. Shinohara, T. Muroyama, T. Miura, T. Saito, A.
Yokoyama, and M. Furukawa, Hyp. Int. 106, 301 (1997).
(17) T. Muroyama, A. Shinohara, T. Saito, A. Yokoyama, K.
Takamiya, S. Morimoto, K. Nakanishi, H. Baba, T. Miura,
Y. Hamajima, T. Kaneko, H. Muramatsu, S. Kojima, andM. Furukawa, J. Radioanal. Nucl. Chem. 239, 159 (1999).
(18) T. Muroyama, A. Shinohara, M. Furukawa, T. Miura, T.
Saito, and A. Yokoyama, Radiochim. Acta 80, 31 (1998).
(19) A. Shinohara, T. Muroyama, T. Miura, A. Yokoyama, T.
Takamiya, T. Kaneko, T. Saito, J. Sanada, H. Araki, S.
Kojima, Y. Hamajima, H. Muramatsu, H. Baba, and M.
Furukawa, J. Radioanal. Nucl. Chem. 239, 169 (1999).
(20) A. Shinohara, T. Muroyama, F. Shigekane, M. Furukawa,
T. Saito, A. Yokoyama, S. Watanabe, and T. Miura, Phys.
Rev. A 49, 4221 (1994).
(21) T. Saito, T. Miura, A. Shinohara, J. Shintai, E. Taniguchi,
M. Furukawa, K. Takesako, N. Imanishi, H. Muramatsu,
Y. Yoshimura, H. Baba, and H. Doe, Hyp. Int. 84, 139(1994).
(22) K. H. Tanaka, Y. Kawashima, J. Imazato, M. Takasaki, H.
Tamura, M. Iwasaki, E. Takada, R. S. Hayano, M. Aoki,
H. Outa, and T. Yamazaki, Nucl. Instr. and Meth. A 316,
134 (1992).
(23) M. R, Harston, D. S. Armstrong, D. F. Measday, S.
Stanislaus, P. Weber, and D. Horva'th, Phys. Rev. A 44,
103 (1991).
(24) R. MacDonald, D. S. Beder, D. Berghofer, M. D.
Hasinoff, D. F. Measday, M. Salomon, J. Spuller, T.
Suzuki, J. M. Poutissou, R. Poutissou, P. Depommier, andJ. K. P. Lee, Phys. Rev. Lett. 38, 746 (1977).
(25) E. C. Aschenauer, K. Gabathuler, P. Hauser, J. Missimer,
A. Badertscher, P. F. A. Goudsmit, H. J. Leisi, H.-Ch.
Schroder, D. Sigg, Z. G. Zhao, D. Chattelard, J.-P. Egger,
and V. E. Markushin, Phys. Rev. A 51, 1965 (1995).