- Email: [email protected]
The detection of muonium in water
CHEMICAL
Volumk 39, number 2
PHYSICS LETTERS
15 April -1976
THE DETECTION OF MUONiUM -IN WATER Pr?ul W. PERCIVAL, Harms FISCHER Pf,_~.sikali~~l,-C~,emis~~,esInstitut der Univer&ijr Ziirich. CH-8OOI Zur&
Swifzerland
Mario CAMANI, Fredy N. GYGAX, Walter RUEGG, Alexander SCHENCK, Hugo SCHILLING Laboratorium fiir Hochenergiephysik CH-5234 Viilbzgen. Switzerland
ETH-Ziin-cJt. c/o Schweizerisches
Institut fiir Nuklearforschung,
.
and Heinz GRAF Physik Imtitut
der Utliveisitiit Ziirich, CH-8001 Zurich Switzerland
Received 22 January 1976
We report the first direct observation of ffle rnuonium atom (fl+e_) in a !iquid samplu. technique muonium spin precession signals were detected in
The atom formed by the combination of a positive muon and an electron is called muonium (Mu E p+e-) [ 1,2] _ From a chemical point of view it can be considered as a light radioactive isotope of hydrogen its mass is l/9 of H, its Bohr radius in 0.532 A, and it has a first ionisation potentia! of 13.539 eV, and a lifetime of 2.2 ps. The pSR technique involves stopping a beam of spin polarized muons in a target and counting decay positrons from the spontaneous ptdecay in a given direction as a function of the time spent by the muons in that medium. By analysis of PSR-histograms muonium has previously been identified in various solids [3-71 and in inert gases [S-l 2] but despite much accumulated evidence consistent with its existence [ 131 it has eluded detection in the
liquid state. It therefore seemed logical to us to start our programme of liquid phase muonium chemistry [ 141 with a search for muonium itself. There are two main reasons why muonium might be undetectable by @R in a given target: short chemical lifetime and fast spin relaxation. With these considerations in mind we chose water as our target material_ The chemical lifetime of muokum in water may be inferred from known hydrogen atom data (rate
Using tic transverse field PSR water at si.. different fields between 4 and 80 G.
less that IO5 M-is-’ have been determined for the reaction of H with various substrates in aquaous solution [ 151). Hydrogen atoms have been identified in water by their ESR spectrum [16,17] and various estimates of the electron spin-lattice relaxation time suggest that it is longer than 10 ps [ 17--191. Thus spin relaxation in muonium should not be significant during the muon lifetime [ 11. Experiments were performed at the SlN * muon channel with a standard pSR assembly (see for example ref. [ 131). Polarized positive muons from the inflight decay of 200 MeV/c pions (produced by bomconstants
barding a molybdenum or beryllium target with a beam of high energy protons) were stopped in the
target, a thin-walled (20.5 mm) spherical glass bulb of 40 mm diameter containing carefully degassed water at ambient temperature. The target was set in the centre of a pair of Helmholz coils so arranged as to apply a dc magnetic field in a direction perpendicular to the muon belam. With a proton beam intensity of 10 PA it was possibIe to achieve a muon stop rate of up to 10” s-l. Decay positrons were counted both in for?
Swiss Insfitute for Nuclear Research.
333
.,-.. .,.
.,
__
-
_. .. -_
ward and backward directions. After ar$y& i~go~davent rate of typically sulted. With such a rate a good decay f1@$iiJ7 events) G3R be accutiulated real time. The general form of a I&R .
S~~{Ij+expC_-tjrP)[l
further logic 1S&l s-r rehisto@am in one hour histogram is [l]
+AP()]),
(I)
where N and E represent a normalization factor and the background taunt respectively, rP is the muon fifetime (2.2 ps) and the asymmetry A is the product of an instrumental factor and the amplitude of the muon polarization at zero t&e. P(r) is the normalized muon pohuization in a given direction, and is a sum of contributions from all muonic species that form in the target. In water a fraction of the muon ensemble (0.55) is believed to undergo epithermal reaction to give muon substituted water [I]. The remainder of the muonS are assumed to be in the form of thermalized mtionium which is expected to react only slowly with water, as discussed above. The pohtrization of this fraction of the muons is given by p(r) = $-((I + 6 j tcos w1z c -k cos i- (I-5)
(COS WIJ
f t COS
where 6 = x/(1 + xz)llz
u43
ferent magnetic fields between 4 and-80 G. It is not possible to pick out the frequenci& directly be eye from the histograms, despite the high statistics, but Fourier transforms ilearly reveal their presence. Fig. ,la shows unsplit muonium precession in a 10 G experiment, and fig. 1b shows the split signal from an 80 C experiment. The observed frequencies are gat% ered in tabIe I where they are c&pared with values calculated from (3)_ The uncertainties qucted arise
from the resolution of the transforms, which is in turn determined by the length of the time window transformed. it should be mentioned that expressions Q)-(4) appljjr equally well to any muon substituted doublet free radical provided w. is replaced by the appropriate hyperfine frequency. The smaher coupling constants in free radicals, however, would give rise to much larger spfittings than those observed. Thus, we feel certain that the observed frequencies belong to muoniurn.
q
O23 I)),
iz1
and the four frequencies are
w12=w__--~,
W23=fiJ_tfi,
Wt4=f-3_+C2fwg,
W43=W_-sL-~w,.
(3)
The
frequencies u__ and 52 and the dimensionless magnetic field parameter x are defined in terms of the muon-electron hyperfine coupling 00 (4463 Miiz), the electron Larmor frequency o, (2.8 MHz/G) and thL?‘muon Larmor frequency r+ (13.55 kHz/G): a-. = $ (we--o,), x =. (We + WJ/Uo
n=+wo -
[(i +X*)r~* -I], (+I
Since the &R arrangement has a time resolution of ~2 ns, the frequencies ~14 and w43 are too high to be detected. However w12 and ~23 should be obseryhle. Both become equal to cr)_ (1.4 MHz/G) in extremelylow field (X < 1) and split by 25;1in intermediate fields. The frequency w_ is known ;m.the literature as ?muonium pre&sion”. We have observed muonium precession at s&iif:
334
.: -. ..
‘.
_
h
:70
90
110
130
FREQ. MHZ
Fig. 1. Sections of the Fourier transformscomputed from accumulated for a sampIeof degasscdwater in (a) 10 G, (b j 80 G transversefield.The time length taken for the tramfor& was 1 US.The two large peaks in (b) arise from‘an instrumentaI.efffkt.
PSR histograms
-CtiEMICAL PHYSICS LETTERS
Volume 39, number 2
Table 1 . Comparison of the observed-and calculated muoniuti sion frequencies
Iiominal Centre frequency, w_(hlHz) field (G)
calculated
4 10 20 30 50 80
observed
5.6 13.9 27.9 41.8 69.7 111.6
5.4 5 0.1 14.2 _t 0.1 28.1 r 0.1 41.7r0.1”) 69.8 2 0.2 112.5*0.2 --a) Only one statistically significant peak A 10 G experiment
with
preces-
Splitting, 252 (MHz) calculated
observed
0.01 0.09 0.36 0.80 2.22 5.68
0.5 f 012 2.0 * 0.5 6.0 + 0.5
was observed.
an undegassed
(but other-
water target did nor give a muonium precession signal. This supports our suspicions that previous unsuccessful attempts to detect muonium in wise similar)
water
and other solvents
faiIed because
of incomplete
removal of oxygen from the target. Not only do we expect muonium to undergo fast chemical reaction with oxygen (by analogy with the hydrogen atom), but the presence of (paramagnetic) oxygen in solution can lead to rapid spin relaxation via Heisenberg spin exchange _ In future experiments we plan to study the chemistry of muonium
in detail
and in particular
we hope
to produce and detect muonic free radicals 1141. This research was supported in part by the Swiss National Foundation for Scientific Research. One of us (WI’) thanks the Royal Society for the award of a European Programme Fellowship.
References [ 1] J.H. Brewer, K.hK Crowe, F.N. Gygax and A. Schenck, in: Muon physics, eds. V.W. Hughes and C.S. Wu (Academic Press, New York, 1975).
15 April 1976
[2] V.I. Golda&ii and V.C. Firsov, Ann. Rev. Phys. Chcm. 22 (1971) 2528. i3l G.G. Myasishcheva, Yu.V. Obukhov, VS. Rogatiov and V.G. Firsov, th. Eksperim. iTeor. Fii. 53 (1967) 451 [English transl. Soviet Phys. JEEP 26 (1968) 2981. [4] 1-L Gurevich. LG. Ivanter, LA. blakariyna, E.A. Mel’eshko, B.A. Nikol’skii, VS. Roganov, V.I. Selivanov, V.P. Smilga, B.V. Sokolov, V.D. Shestakov and 1-V. Takovleva, Phys. Letters 29B (1969) 387. [S] 1.1. Gurevich, LG. ivanter, E.A. Meleshko, B.A. Nikol’skii, V.S. Roganov, V.I. Selivanov, V.P. Smilga, B.V. Sokolov and V.D. Shcstakov, Zh. Eksperim. i Tear. Fiz. 60 (1971) 47 1 [English transl. Soviet Phys. JETP 33 (197 1) 2531: [6] KM. Crowe, R.F. Johnson, J.i-I. Brewer, F.N. Gygax, D.G. Fleming and A. Schenck,Bull. Am. Phys. Sot. 17 (1972) 594. [7] J-H. Brewer, KM. C~OWC,F.N. Gygau, R.F. Johnsoil, B.D. Patterson, D.G. Fleming and A. Schenck, Phys. Rev. Letters 31 (1973) 143. [S] R.&l. Mobley, J.hl. Bailey, W.E.,Clcland, V.W. Hughes and
J.F. Rothberg, J. Chcm. Phys. 44 (1966) 4354. [9] V.W. Hughes, D.W. BlcColm, K. Ziock and R. Prepost, Phys. Rev. A 1 (1970) 595; A2 (1970) 55 1. (lo] R.D. Stambaugh, D.E. Casperson,T.W. Crane, V.W. Hughes, H.F. Kaspar, P. Scuder, P.A. Thompson, H. Orth, G. zu Putlitz and A.B. Dcnison, Phys. Rev. Letters 33 (1974) 568. [ 111 B.A. Barnett,C.Y. Chang, G-B. Yodh, 5.13.Caroll, M. Eckhause, C.S. Hsieh, J.R. Kane and C.B. Spcnce, Phys. Rev. All (1975) 39. I121J.H. Brewer, D.G. Fleming, D.M. Gamer, A.E. Pifer and T. Bowen, to be published. 1131 J.H. Brewer, KM Crowc, F.N. Gygax, R.F. Johnson, D.G. Fleming and A. Schenck, Phys. Rev. A9 (1974) 495. I141 SIN Research Project RA-74-W IlS] P. Neti, Chem. Rev. 72 (1072) 535. [ 161 I<. Eiben and R.W. Fessenden, J. Phys. Chem. 75 (1971) 1186: [ 171 B. Smaller. E.C. Avery and J.R. Rcmko, J. Chem. Phys. 55 (1971) 2414. 1 IS] P. Neta, R.W. Fessenden and R-H. Schuler, J. Phys. Chem. 7.5 (1971) 1654. [ 191 N.C. Verma and R.W. Fessenden, J. Chem. Phys. 58 (1973) 2501.
335
Volumk 39, number 2
PHYSICS LETTERS
15 April -1976
THE DETECTION OF MUONiUM -IN WATER Pr?ul W. PERCIVAL, Harms FISCHER Pf,_~.sikali~~l,-C~,emis~~,esInstitut der Univer&ijr Ziirich. CH-8OOI Zur&
Swifzerland
Mario CAMANI, Fredy N. GYGAX, Walter RUEGG, Alexander SCHENCK, Hugo SCHILLING Laboratorium fiir Hochenergiephysik CH-5234 Viilbzgen. Switzerland
ETH-Ziin-cJt. c/o Schweizerisches
Institut fiir Nuklearforschung,
.
and Heinz GRAF Physik Imtitut
der Utliveisitiit Ziirich, CH-8001 Zurich Switzerland
Received 22 January 1976
We report the first direct observation of ffle rnuonium atom (fl+e_) in a !iquid samplu. technique muonium spin precession signals were detected in
The atom formed by the combination of a positive muon and an electron is called muonium (Mu E p+e-) [ 1,2] _ From a chemical point of view it can be considered as a light radioactive isotope of hydrogen its mass is l/9 of H, its Bohr radius in 0.532 A, and it has a first ionisation potentia! of 13.539 eV, and a lifetime of 2.2 ps. The pSR technique involves stopping a beam of spin polarized muons in a target and counting decay positrons from the spontaneous ptdecay in a given direction as a function of the time spent by the muons in that medium. By analysis of PSR-histograms muonium has previously been identified in various solids [3-71 and in inert gases [S-l 2] but despite much accumulated evidence consistent with its existence [ 131 it has eluded detection in the
liquid state. It therefore seemed logical to us to start our programme of liquid phase muonium chemistry [ 141 with a search for muonium itself. There are two main reasons why muonium might be undetectable by @R in a given target: short chemical lifetime and fast spin relaxation. With these considerations in mind we chose water as our target material_ The chemical lifetime of muokum in water may be inferred from known hydrogen atom data (rate
Using tic transverse field PSR water at si.. different fields between 4 and 80 G.
less that IO5 M-is-’ have been determined for the reaction of H with various substrates in aquaous solution [ 151). Hydrogen atoms have been identified in water by their ESR spectrum [16,17] and various estimates of the electron spin-lattice relaxation time suggest that it is longer than 10 ps [ 17--191. Thus spin relaxation in muonium should not be significant during the muon lifetime [ 11. Experiments were performed at the SlN * muon channel with a standard pSR assembly (see for example ref. [ 131). Polarized positive muons from the inflight decay of 200 MeV/c pions (produced by bomconstants
barding a molybdenum or beryllium target with a beam of high energy protons) were stopped in the
target, a thin-walled (20.5 mm) spherical glass bulb of 40 mm diameter containing carefully degassed water at ambient temperature. The target was set in the centre of a pair of Helmholz coils so arranged as to apply a dc magnetic field in a direction perpendicular to the muon belam. With a proton beam intensity of 10 PA it was possibIe to achieve a muon stop rate of up to 10” s-l. Decay positrons were counted both in for?
Swiss Insfitute for Nuclear Research.
333
.,-.. .,.
.,
__
-
_. .. -_
ward and backward directions. After ar$y& i~go~davent rate of typically sulted. With such a rate a good decay f1@$iiJ7 events) G3R be accutiulated real time. The general form of a I&R .
S~~{Ij+expC_-tjrP)[l
further logic 1S&l s-r rehisto@am in one hour histogram is [l]
+AP()]),
(I)
where N and E represent a normalization factor and the background taunt respectively, rP is the muon fifetime (2.2 ps) and the asymmetry A is the product of an instrumental factor and the amplitude of the muon polarization at zero t&e. P(r) is the normalized muon pohuization in a given direction, and is a sum of contributions from all muonic species that form in the target. In water a fraction of the muon ensemble (0.55) is believed to undergo epithermal reaction to give muon substituted water [I]. The remainder of the muonS are assumed to be in the form of thermalized mtionium which is expected to react only slowly with water, as discussed above. The pohtrization of this fraction of the muons is given by p(r) = $-((I + 6 j tcos w1z c -k cos i- (I-5)
(COS WIJ
f t COS
where 6 = x/(1 + xz)llz
u43
ferent magnetic fields between 4 and-80 G. It is not possible to pick out the frequenci& directly be eye from the histograms, despite the high statistics, but Fourier transforms ilearly reveal their presence. Fig. ,la shows unsplit muonium precession in a 10 G experiment, and fig. 1b shows the split signal from an 80 C experiment. The observed frequencies are gat% ered in tabIe I where they are c&pared with values calculated from (3)_ The uncertainties qucted arise
from the resolution of the transforms, which is in turn determined by the length of the time window transformed. it should be mentioned that expressions Q)-(4) appljjr equally well to any muon substituted doublet free radical provided w. is replaced by the appropriate hyperfine frequency. The smaher coupling constants in free radicals, however, would give rise to much larger spfittings than those observed. Thus, we feel certain that the observed frequencies belong to muoniurn.
q
O23 I)),
iz1
and the four frequencies are
w12=w__--~,
W23=fiJ_tfi,
Wt4=f-3_+C2fwg,
W43=W_-sL-~w,.
(3)
The
frequencies u__ and 52 and the dimensionless magnetic field parameter x are defined in terms of the muon-electron hyperfine coupling 00 (4463 Miiz), the electron Larmor frequency o, (2.8 MHz/G) and thL?‘muon Larmor frequency r+ (13.55 kHz/G): a-. = $ (we--o,), x =. (We + WJ/Uo
n=+wo -
[(i +X*)r~* -I], (+I
Since the &R arrangement has a time resolution of ~2 ns, the frequencies ~14 and w43 are too high to be detected. However w12 and ~23 should be obseryhle. Both become equal to cr)_ (1.4 MHz/G) in extremelylow field (X < 1) and split by 25;1in intermediate fields. The frequency w_ is known ;m.the literature as ?muonium pre&sion”. We have observed muonium precession at s&iif:
334
.: -. ..
‘.
_
h
:70
90
110
130
FREQ. MHZ
Fig. 1. Sections of the Fourier transformscomputed from accumulated for a sampIeof degasscdwater in (a) 10 G, (b j 80 G transversefield.The time length taken for the tramfor& was 1 US.The two large peaks in (b) arise from‘an instrumentaI.efffkt.
PSR histograms
-CtiEMICAL PHYSICS LETTERS
Volume 39, number 2
Table 1 . Comparison of the observed-and calculated muoniuti sion frequencies
Iiominal Centre frequency, w_(hlHz) field (G)
calculated
4 10 20 30 50 80
observed
5.6 13.9 27.9 41.8 69.7 111.6
5.4 5 0.1 14.2 _t 0.1 28.1 r 0.1 41.7r0.1”) 69.8 2 0.2 112.5*0.2 --a) Only one statistically significant peak A 10 G experiment
with
preces-
Splitting, 252 (MHz) calculated
observed
0.01 0.09 0.36 0.80 2.22 5.68
0.5 f 012 2.0 * 0.5 6.0 + 0.5
was observed.
an undegassed
(but other-
water target did nor give a muonium precession signal. This supports our suspicions that previous unsuccessful attempts to detect muonium in wise similar)
water
and other solvents
faiIed because
of incomplete
removal of oxygen from the target. Not only do we expect muonium to undergo fast chemical reaction with oxygen (by analogy with the hydrogen atom), but the presence of (paramagnetic) oxygen in solution can lead to rapid spin relaxation via Heisenberg spin exchange _ In future experiments we plan to study the chemistry of muonium
in detail
and in particular
we hope
to produce and detect muonic free radicals 1141. This research was supported in part by the Swiss National Foundation for Scientific Research. One of us (WI’) thanks the Royal Society for the award of a European Programme Fellowship.
References [ 1] J.H. Brewer, K.hK Crowe, F.N. Gygax and A. Schenck, in: Muon physics, eds. V.W. Hughes and C.S. Wu (Academic Press, New York, 1975).
15 April 1976
[2] V.I. Golda&ii and V.C. Firsov, Ann. Rev. Phys. Chcm. 22 (1971) 2528. i3l G.G. Myasishcheva, Yu.V. Obukhov, VS. Rogatiov and V.G. Firsov, th. Eksperim. iTeor. Fii. 53 (1967) 451 [English transl. Soviet Phys. JEEP 26 (1968) 2981. [4] 1-L Gurevich. LG. Ivanter, LA. blakariyna, E.A. Mel’eshko, B.A. Nikol’skii, VS. Roganov, V.I. Selivanov, V.P. Smilga, B.V. Sokolov, V.D. Shestakov and 1-V. Takovleva, Phys. Letters 29B (1969) 387. [S] 1.1. Gurevich, LG. ivanter, E.A. Meleshko, B.A. Nikol’skii, V.S. Roganov, V.I. Selivanov, V.P. Smilga, B.V. Sokolov and V.D. Shcstakov, Zh. Eksperim. i Tear. Fiz. 60 (1971) 47 1 [English transl. Soviet Phys. JETP 33 (197 1) 2531: [6] KM. Crowe, R.F. Johnson, J.i-I. Brewer, F.N. Gygax, D.G. Fleming and A. Schenck,Bull. Am. Phys. Sot. 17 (1972) 594. [7] J-H. Brewer, KM. C~OWC,F.N. Gygau, R.F. Johnsoil, B.D. Patterson, D.G. Fleming and A. Schenck, Phys. Rev. Letters 31 (1973) 143. [S] R.&l. Mobley, J.hl. Bailey, W.E.,Clcland, V.W. Hughes and
J.F. Rothberg, J. Chcm. Phys. 44 (1966) 4354. [9] V.W. Hughes, D.W. BlcColm, K. Ziock and R. Prepost, Phys. Rev. A 1 (1970) 595; A2 (1970) 55 1. (lo] R.D. Stambaugh, D.E. Casperson,T.W. Crane, V.W. Hughes, H.F. Kaspar, P. Scuder, P.A. Thompson, H. Orth, G. zu Putlitz and A.B. Dcnison, Phys. Rev. Letters 33 (1974) 568. [ 111 B.A. Barnett,C.Y. Chang, G-B. Yodh, 5.13.Caroll, M. Eckhause, C.S. Hsieh, J.R. Kane and C.B. Spcnce, Phys. Rev. All (1975) 39. I121J.H. Brewer, D.G. Fleming, D.M. Gamer, A.E. Pifer and T. Bowen, to be published. 1131 J.H. Brewer, KM Crowc, F.N. Gygax, R.F. Johnson, D.G. Fleming and A. Schenck, Phys. Rev. A9 (1974) 495. I141 SIN Research Project RA-74-W IlS] P. Neti, Chem. Rev. 72 (1072) 535. [ 161 I<. Eiben and R.W. Fessenden, J. Phys. Chem. 75 (1971) 1186: [ 171 B. Smaller. E.C. Avery and J.R. Rcmko, J. Chem. Phys. 55 (1971) 2414. 1 IS] P. Neta, R.W. Fessenden and R-H. Schuler, J. Phys. Chem. 7.5 (1971) 1654. [ 191 N.C. Verma and R.W. Fessenden, J. Chem. Phys. 58 (1973) 2501.
335