PACS: 76.75.+i,79.70.-g,36.l0.Dr OCR Output

stufaces and thin films.

which have a wide range of possible applications e.g. as a magnetic microprobe of

result opens the possibility of producing beams of very low energy polarized muons,

that the moderation process is very fast compared to depolarizing mechanisms. This

solid layers. We iind that the muons nearly retain their initial polarization indicating

eV) obtained from the moderation of a 100% polarized surface muon beam in rare gas

We have measured the polarization of very slow positive muons (kinetic energy ~ 10

ABSTRACT

Institute for High Energy Physics, ETH Ziirich CH-5232 \/illigen PSI, Switzerland

Switzerland

Institute for Intermediate Energy Physics, ETH Zurich CH—5232 Villigen PSI,

Germany

Physics Institute, University of Heidelberg Philosophenweg 12, D-69120 Heidelberg,

Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

Th. Wutzkel, U. Zimmermann3

E. Morenzonil, F. Kottmann", D. Madenl, B. Matthias2, M. Meybergl, Th. Prokschaz,

Generation of very slow polarized positive muons

substrate (250 um thick, 99.999% pure and cooled by a Helium cryostat) with a solid OCR Output

pinging on a cold target which acted as a moderator. This moderator consists of an Al

Once in the UHV chamber, they traversed two 30 nm A1 cold shields before im

shown in Fig. 1.

stainless-steel vacuum window, the ;i+ entered the ultra-high vacuum (UHV) chamber

of 200 um thickness which detected the incident ;i+. After passing through a 50 um

aperture 20 mm wide and 12 mm high defined the if beam spot on a plastic scintillator

;

static E" >< Bseparator adjusted to transmit the ;i+. Lead and copper collimators with anp=27.3 MeV/c, Ap/p = 3%). Positron background in the beam was suppressed by a

(PSI). The channel was tuned to deliver ~ 7 - 105 surface ;r+ per second (momentumThis experiment was performed at the 7rE 1 beam line of the Paul—Scherrer-Institute

making use of the moderation technique.

low energy beam of polarized u+ for surface and thin film ;iSR can be realized by

of the incident high energy beam. This experimental finding demonstrates that a very

emitted from solid Ar and Kr. It is observed that the ;r+ retain most of the polarization

In this letter we report a first measurement of the polarization of very slow ,u+;i+ into a very slow one [6].

on a metal substrate, giving an efficiency of 10‘5 for converting an incoming surfaceenergies. So far, the most efficient moderator appears to be a solid layer of Ar deposited

distribution of these if shows a maximum near 10 eV with a tail extending to higher

thin moderator, very slow ;i+ are emitted from its downstream side [5,6]. The energy

It has recently been shown that, if energetic ;r+ are injected into the back of a

spectroscopic method for these fields.

of ,uS R techniques to surface- and thin iilm physics, thus offering a completely new

of about 10 eV to a few tens of keV would extend the present range of application

The availability of a beam of polarized very slow positive muons in the energy range

surface a+ are approximately 150 mg/cmg and several tens of mg/cm2 respectively.to the study of bulk characteristics of matter since the range and range straggling of

with spin opposite to the direction of motion. The ;iSR technique is presently limited

decay of positive pions at rest, are used [4]. These ;i+ are 100% longitudinally polarized

;iSR [3]). For these studies surface p+ (E 1 4.2 MeV) originating from the two body

polarization once they have thermalized in the sample (muon spin rotation technique

[1,2]. Such information is obtained by measuring the time-evolution of the if spin

and hydrogen diffusion mechanisms and chemical reactions in solids, liquids and gases

microscopic probes of magnetic properties of matter as well as isotopic probes of proton

Polarized positive muons (u+) and muonium atoms (;i+ 6*) have proven to be ideal

of the e+ telescopes. From these events we derive histograms of the ILL+ time-of—fiight OCR Output

ns by a pulse in the start scintillator (Sl) and followed within 10 as by a pulse in one

acquisition system is triggered by a triple coincidence of MCP 2 preceded within 800

a pulse in MCP 2 and a signal in one of the four e+ telescopes. A readout of the data

To acquire the characteristic ;r‘*’ spin precession signal, we measure the time between

field B of up to 100 G perpendicular to the expected spin direction of the polarized ;r+a solid angle coverage of 60% of 47r sr. A pair of Helmholtz coils applies a magnetic

decay e+ are observed by four plastic scintillator pairs surrounding MCP 2 providing

While the very slow [fr arriving at MCP 2 produce pulses in this detector, the subsequent

,4/* spin direction with the direction of emission of the positrons from the ;r+ decay.

standard transverse magnetic field uS R technique which relies on the correlation of the

We observe the polarization of the very slow ,u+ stopped in MCP 2 by using the

tuning purposes.

electrostatic mirror and are detected by MCP 1, which is used for monitoring and beam

Muons which are not completely moderated muons are not defiected appreciably by the

to be antiparallel to the incident beam direction, i.e., parallel to the surface of MCP 2.

alter the spin direction of ;.r+ at these energies so one expects any remaining polarization

an electrostatic mirror. Note that the electrostatic fields of the transport system do not

microchannel plate detector (MCP 2 in Fig. 1) with two electrostatic "einzel" lenses and

mm downstream of the first, is grounded. The extracted particles are focussed onto a

between the A1 substrate and first grid is biased at 500 V. The second grid, located 10.5

two stage system formed by the substrate and two wire grids. The 9.5 mm wide gap

extract the very slow muons from the production region. They are accelerated in a

The Al substrate is electrically insulated and held at a positive voltage of 8 kV to

value is obtained with solid Ar.

of the layer (deposition rate, substrate temperature, thickness and age) [7]. The best

is between 10"* and 10*5 depending on the moderator and the preparation parametersthe target i.e. in the solid rare gas layer. The moderation efficiency in our experiment

that the stopping density distribution of the a+ is centered at the downstream surface of

The total thickness traversed by the ;r+ and the incident beam momentum is chosen such

uum is typically 10’9 mbar to minimize surface contamination from the residual gas.

mounted near the moderator target measures the film thickness. The chamber base vac

of 20 K during deposition and subsequent measurements. An oscillating quartz crystal

pressure of typically 10‘° mbar for 120 seconds. The substrate is held at a temperature

nm) is obtained by condensating onto the Al substrate research grade gas at a partial

layer of Ar or Kr deposited on it. The solid gas layer (thickness between 200 and 300

finite solid angle, target size and absorption of low momentum e+ (not all e+ were OCR OutputThe maximum observable asymmetry, including the effects of the counter geometry,

another /.1,+ within ;§;10 us. With this arrangement we performed two measurements.

only those events originating from a /.4+ which was neither preceded nor followed by

positron was ensured by reducing the beam intensity to ~ 2- 104 ;r+/sec and by recordingfrom the start scintillator. Unique correspondence between an incident muon and a decay

For this measurement we started the time differential ;iS R measurement with the signal

signal produced by surface muons stopped in the MCP in a transverse magnetic field.

to the surface ,u+ beam line after the start scintillator and measuring the asymmetry

The uS R specuometer (bottom part of Fig. 1) was calibrated by attaching it directly

account.

the behavior of a polarized ,u+ stopped in the microchannel plate detector are taken into

of the polarization if the maximum observable asymmetry with our ,uS R apparatus and

;r+ spin is observed in any of the moderators. The decay asymmetry is a direct measure

solid layer of N2. These results are also presented in Table 1. No relaxation of the slow

we have measured a comparable yield and asymmetry for very slow ,n+ produced by a

to the polarization measurements of very slow n+ emitted from solid Ar and Kr layers,

four precession spectra, we obtain the asymmetries A§\jj’§’P“ given in Table 1. In additionBottom). The muon lifetime is denoted by 1-,,. By fitting simultaneously Eq.(l) to the

the constant backgrounds C" vary separately for each telescope (i=Left, Top, Right,

four precession spectra, whereas the precession phases QZ, the normalizations Ng, and

spin relaxation rate A, and the decay asymmetry A ,, are fit parameters common to the

In this equation the free muon precession frequency wp = 2vr>< 13.55 kHz/G, the ;r+

N(t) = ]V@e`*/Te [1 + A,,c"\* cos(w,,t + + Ci. (1)

of the four scintillator telescopes:

precession signal in a transverse magnetic Held of 50 G is plotted in Fig. 3. For each

coincidences between S1 and MCP 2. A resulting MSR spectrum showing the ;r+ spin

peak and we correct for the contribution from the flat background produced by random

measurement we select from the TOF spectrum only those events contained in the

eV and with an energy spread of the same order of magnitude. For the polarization

peak corresponds to ;r+ emitted from the moderator with an initial energy of ~ 10

Figure 2 shows the ;r+ time-of-flight (TOF) spectrum between S1 and MCP 2. The

stop and decay coordinates of the ;r+

2 and stopped by the e+ detectors. A position-sensitive readout of MCP 2 provides the

distribution between Sl and MCP 2 and of the four decay time spectra started by MCP

not provide an unambiguous signature to distinguish between these or other emission OCR Output

moderation of if down a kinetic energy of a few eV but the data available to date do

[5] and hot ;i+ emission mechanisms have been proposed [6,11,12] to explain the

Based on the analogy with e+ emission from solid moderators, diffusion controlled

results are summarized in Table 1 and are in agreement with the results for P.

’’1the rare gas layers according to the expression P' = 100% >< (Aj;§§P" /.4]]% F? ). Thefor an alternative determination of the polarization P' of the very slow ;.t+ emitted from

that these keV if are also 100% polarized. We can therefore use the value of P*‘is given in Table 1. Since even thermalized if in Al do not depolarize, one can expect

l`the electrostatic transport system. The asymmetry observed for these muons (Aj§é,é‘)which are emitted from the Al substrate acting as a degrader and which are accepted by

the frozen gas layer. The broad distribution corresponds to muons of several keV energy

fewer very slow muons with E ~ 10 eV are emitted from the pure Al substrate than from

TOF between Sl and MCP 2 shows a less pronounced, broader peak [7]. Considerably

blank Al substrate. Instead of a narrow peak as in Fig. 2, in the case, the spectrum of the

We also determined the polarization of the slow muons which are emitted from the

lead to a reduced transverse polarization, is negligible.

the magnetic field on the ;i+ spin in flight. Rotation out of this plane, which would

the very slow p+ transport from production target to detector including the action ofplane of <I>ff and is in good agreement with the result of a Monte Carlo simulation ofi 4)° and d>ff= (297 t 4)°. This corresponds to an initial rotation within the precessionThe fits to the Ar data gives the phases i>f= (212 ;l; 4 )°, <I>5= (119 d; 5 )°, <I>§= (29.5

The results show that the very slow ;i+ retain practically the full initial polarization.

in Table 1, are obtained from the expression P = 100% >< (A§$§P”/A§ZQ§°8 " ).°’f

The values for the absolute (transverse) polarization P of the ~ 10 eV if', given

known that a significant fraction of u+ stopped in oxide insulators form muonium [8].

target is due to muonium formation. The MCP consists mainly of a glass [10] and it is

of the asymmetry spectra shows that the missing asymmetry observed with the MCP

2 as the target. In this case we obtain A§§‘,Q,’§°€“ = (20.97 ;l; 0.18) %. A Fourier analysis/.0* in the MCP [9] was then investigated by measuring the asymmetry signal with MCP

Aff§"“°" " =(28.06 ;l; 0.17) % for 100% beam polarization. The behavior of polarizedaccount that the E>< E field in the separator rotates the spin by 4° we expect that

,

0.15) %. From a Monte Carlo simulation of the ;iSR spectrometer and taking intospectra at B=25, 50 and 100 G we obtain an asymmetry of A;§"“°° " = (28.22 zka very pure Al target where no depolarization occurs [8]. From fits to the precession

counted with equal efficiency), was determined by measuring the asymmetry signal in

since depolarization via electron scattering and Coulomb scattering is negligible and OCR Output

On the other hand, epithermal ;r+ emission conserves the initial surface ;i+ polarization

malized ;i+ quickly depolarize and only retain-one third of the initial polarization [16].

the very slow ;r+ by a factor of two, at variance with our results. In solid N2 also, ther

exchange oscillation due to the hyperfine interaction would reduce the polarization of

survival for times non-negligible compared to the period of the electron and muon spin

and Kr, therrnalization leads to practically 100% muonium formation [15]. Muonium

(224 ps), can be excluded as a source of the majority of those particles. In solid Ar

from a muonium atom living for a time scale characteristic of the hyperfine frequency

which assume therrnalization at the onset, or delayed processes, where the ti"` originates

tion and is consistent with the hot muon emission mechanism. Diffusion-like processes,

implies an upper limit to the time scale of the mechanism responsible for the modera

The observation that the very slow ;i+ retain almost the full initial polarization

process which significantly increases the escape depth of the ~ 10 eV muons.

especially in Ar is therefore a consequence of an open energy window in the moderation

the order of 5-10 meV [14]. The particularly efficient moderation to eV energies of iffavorable to a low energy loss rate since they have soft phonon spectra with energies of

ing phonon emission, van der Waals solids such as Kr,Ar and N2 are also particularly

remaining energy loss mechanism is via relatively inefficient phonon emission. Regard

even energetically impossible and the energy loss rate becomes low since the the only

levels, the corresponding efhcient energy loss mechanisms are strongly suppressed or

14 eV [14]). Once the if has reached a kinetic energy of the order of these threshold

Ar and N2, these processes have high threshold energies (band gap energy 11, 14 and

an energy dissipating mechanism [13]. In wide band gap insulators such as solid Kr,

changing cycles involving muonium formation and break-up also become important as

ation). When a if has lost most of its energy, at energies below ~ 10 keV, chargeelectrons, ionizing and exciting the target atoms (electron-hole pair and exciton cre

Initially the surface 'l,L+ rapidly lose energy in matter by Coulomb collisions with

which have not thermalized (epithermal pl').

The hot ;i+ emission mechanism suggests that the observed very slow ;.i+ are muons

up and a lf with several eV of kinetic energy to be reemitted.

bound muonium electron and another hole near the surface causes the muonium to break

by catalyzing a first electron-hole recombination. A second recombination between the

which is formed inside the solid. Muonium formed inside the solid can acquire energy

the very slow ,u‘*‘ are produced from the break up near the moderator surface of muonium

mechanisms. According to the recombination—assisted diffusion mechanism of Ref. [5],

and G. zu Putlitz. OCR Output

Techr1ologie. We also gratefully aclmowledge stimulating discussions with K. Jungmann

This work has been partly supported by the Bundesministerium fur Forschung and

,11+ beam.

new spectroscopic studies of the n=2 states, free from background due to the incident

be expected to contain an admixture of the metastable 2s state and may be used for

through a thin foil or a gas target which neutralizes it. Such a source of muonium, may

The if beam can also be converted into a clean muonium beam by passing the [jr

field of possible applications of very slow th is not restricted to solid state physics.

diffusion in bulk matter and about the behavior of implanted hydrogen isotopes. The

pS R studies, which have provided valuable information about magnetism, defects and

films, vacuum-solid interfaces and solid-solid interfaces would become accessible to

The corresponding estimated range straggling varies between 80% and 20%. Thus thin

tion depths between typically fractions of a nm and a few hundred nm ca.n be achieved.

By varying the energy of the /.4+ between ~ 10 eV and a few tens of keV, implanta

104 very slow polarized ;r+ per second.

designed to deliver more than 108 ;r+/second [18], can provide a source flux of about

up to 10'4 [7], combined with the new surface ;r+ beam line rrE5 at PSI, which is

energies between ~ 10 eV and a few tens of keV. Solid Ar, with conversion efficiencies

a surface tf beam can be used as a source of a tunable beam of polarized [N with

use in a number of new applications [17]. A solid Ar moderator in conjunction with

The discovery that the very slow p‘*’ are polarized is an essential step towards their

excluded.

electrons or ions, which are produced in the wake of the slowing down ;r+, can be

also show that other depolarizing processes such as spin exchange interactions with free

muonium formation does not give rise to a sizable depolarization. Our measurements

the overall time for slowing down to 10 eV is very short (~ 10 ps). As a result even

Eds., Springer Verlag 1992 (Suppl. to Z. f. Physik C 56, S289 (1992)) OCR Output

[18] H.K. Walter, in Future of Muon Physics, K. Jungmann, V. Hughes, G. zu Putlitz

zu Putlitz Eds., Springer Verlag 1992 (Suppl. to Z. f. Physik C 56, S243 (1992)).

[17] E. Morenzoni, in Future of Muon Physics, K. Jungmann, V. W. Hughes and G.

Interactions 65, 819 (1990).

G. Grebennik, S. Kapusta, A. B. Lazarev, S. N. Shilov, V.A. Zhukov, Hyperiine

[16] B.F. Kirillov, B. A. Nikolsky, A. V. Pirogov, V. G. Storchak, V. N. Duginov, V.

Phys. 74, 308 (1981).

[15] R.F. Kiefl, I.B. Warren, G.M. Marshall, CJ. Oram, C.W. Clawson, J. Chem.

Gases, Springer Tracts in Modern Physics, Vol. 107 Springer—Verlag Berlin, 1985.

[14] N. Schwentner, E.E. Koch, J. Jorrner, Electronic Excitation in Condensed Rare

M. Senba, J. Phys. B 21, 3093 (1988) and J. Phys. B 22, 2027 (1989).[13]

[12] A. P. Mills Jr. and W. S. Crane, Phys. Rev. Lett. 53, 2165 (1984).

[1 1] K.G. Lynn and B. Nielsen, Phys. Rev. Lett. 58, 81 (1987).

[10] J.L. Wiza, Nucl. Instr. and Methods 162, 587 (1979).

[9] Supplied by Hamamatsu Photonics, Type F-1217/O3.

[8] J.H. Brewer, K.M. Crowe, F.N. Gygax, A. Schenck in Ref. [2], p. 3.

Hawai, 1993 to be published in Hyperfine Interactions and paper in preparation.

mann, B. Matthias, Th. Prokscha, Proceedings of the ,<rSR93 Conference, Maui,

[7] M. Meyberg, E. Morenzoni, Th. Wutzke, F. Kottmann, U. Zimmermann, K. Jung—

Columbia, (1990).

8850 (1987) and G. D. Morris, Master of Science Thesis, University of British

Morris, M. Senba, J.B. Warren, A.S. Rupaal, J.H. Turner, Phys. Rev. B36,

[6] D.R. Harshman, A.P. Mills, J.L. Beveridge, K.R. Kendall, G.D. Morris, M.

(1986).

A.P. Mills, W.S. Crane, A.S. Rupaal, J.H. Turner, Phys. Rev. Lett. 56, 2850

[5] D.R. Harshman, J.B. Warren, J.L. Beveridge, K.R. Kendall, R.F. Kieil, CJ. Oram,

[4] A.E. Pifer, T. Bowen, K.R. Kendall, Nucl. Instr. and Methods, 135, 39 (1976).

[3] A. Schenck, Muon Spin Rotation Spectroscopy, Adam Hilger Ltd., 1985.

1975.

[2] Muon Physics, Vol. III, V. W. Hughes, C.S. Wu Eds., Academic Press, N.Y.

[1] See for instance the Proceedings of the [15 R93 Conference, Maui, Hawai, 1993.

REFERENCES

Normalized to asymmetry of keV muons emitted from the Al moderator. OCR Output

Normalized to asymmetry of surface muons in MCP.

Muons with keV Energy.

(E ~ 10 CV).Very slow muons

1.50 94.8 i 7.2AP 19.89 je

154 1 28N; 30.71 ;k 5.02 146 i 24

Kr“ 16.81 :1; 85 j; 234.30 80 :1: 20

91.7 :1; 7.5Ar° 18.24 zi; 0.56 87.0 ;l; 2.8

Moderator Asymmetry(%) Polarization P(%)C Polarization P’(%)

Table 1: Results of the polarization measurements

10 OCR Output

text.

Figure 1: UHV apparatus with cryostat used for the moderation studies. For details see

PAIR

SCINTILLATOR

MCP 2

COILHELMHOLTZ

LENSMODERATOR LENS*·EINZEL··"EINZEL"

scm~mLLAToR LN2START

LN 2

yf BEAM

MCP 1

MIRRORLN 2 - SHIELD12LEcTR0sTAT1cHe - SHIELD

11 OCR Output

given by the very slow ,41+ emitted from the Ar moderator.

Figure 2: Time-of—fiight distribution between start scintillator and MCP 2. The peak is

Time (ns)

410 370 330 290 250

f[yU1,..j* f»,;1.L[¤UIn]¤_J_j1_¤/—0H_.\y~*f

400

6.5 Q_ 800

-5 1200

1600

12

from a solid Ar layer and precessing in a 50 G transverse magnetic field.

Figure 3: Decay time distribution (left scintillator telescope) of the very slow ;i+ emitted

Time ( tts)

1 2 3 4 5 6 7 8

T * ' iii J, T · L4 . + j`+ +7++ *

M U l

100

§ 200

CU

400