Observation of free fluorine μ-nucleonic atom

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  • IL NUOVO CIMENT0 Vet.. 36 B, N. 2 11 Dicembre 1976

    Observation of Free Fluorine ~-Nueleonie Atom.


    _4cadency o] Sciences o] the USSR P. N. Lebedev Physical Institute - 117924 Moscow

    (ricevuto il 23 0ttobre 1974; manoseritto revisionato ricevuto il 18 Febbraio 1976)

    Summary. ~ The existence of the free wnueleonic atom has been experi- mentally shown by measuring the Larmor precession frequencies of the total moment for an atom in magnetic-field intensities 1.1 and 2.1 G. The precession frequencies have been obtained when negative muons stopped and decayed in Ne gas at a pressure of 42 atm.

    1. - In t roduct ion .

    In terms of the conventional terminology, the system (Z nucleus-~-~--meson) is a muonic atomwith Z atomic number. In this paper, according to (1), we call the system Z nucleus with a negative rouen on the K-shell plus an electron shell with Z - -1 electrons a wnucleonic atom with Z - -1 atomic number.

    The ~-nucleonic-atom formation is usually preceded by a heavy destruction of the electron shell of the parent atom in the process of cascade transition of a negative rouen to the K-shell. A new electron shell is formed through the interaction of a ~-nucleonic atom or ion with surrounding atoms and the prod- ucts of medium radiolysis a~rising from rouen stops and their cascade tran- sitions; and the characteristic time needed for a wnucleonic atom to create a bound state in a normal condensed medium is evaluated to be ~ 10 -1~ s (~).

    A chemically inert medium, e.g. the atmosphere of a noble gas, is considered

    (1) V.N. GORELKIN and V. P. SMILGA-" ~trn. Eksp. Teor. _Fiz., 66, 1201 (1974). (*) A. A. DzHUI~A~.V, V. S. EVSEEV, Yu. V. OBUKHOV and V. S. ROGANOV: ~urn. J~ksp. Teor. Fiz., 62, 2210 (1972).



    to be the most suitable one for a search of a free t~-nucleonie atom, since it mini- mizes or even eliminates the probabi l i ty of the interaction of a chemically active single atom. Thus, when negative mouns come to rest in a gas target filled with noble gases (He~ 57e, Ar, Kr, Xe), ~-nucleonic atoms analogues of hydrogen or of a relevant halogen (F, C1, Br, I) may be formed.

    Given the lack on experimental evidence on the existence of ~-nueleonie atoms, we attempted to observe free fluorine wnueleonic atoms in gaseous neon (3).

    For a full electron shell of fluorine atoms to be produced, the main gas ha~ to contain a chemically inactive admixture with an ionization potential lower than the binding energy of the last electron in a ~-nueleonie atom. For this purpose xenon has been chosen, whose ionization potential is 12.08 eV.

    Charge exchange of ~-nucleonic nucleus [~-,*~Ne] +9 results in the neutral 2O atom shel l , F. The ground state of a ~-nueleonic fluorine electron shell is 3p!

    and the nearest metastable level 2pi is 0.05 eV higher. Precession frequen- cies of the total moment of wnucleenic fluorine in a weak magnetic field are

    s for (~% is the precession fre- expected to be equal to 89 for 2P t and oJ~, ~o~ 2p! quency of triplet muonium in a magnetic field of the same intensity) (see appendix). A related contribution of amplitudes corresponding to different precession frequencies to the t ime distribution of decay electrons is discussed in the appendix.

    The method of total moment precession of an atom in the magnetic field was used to observe free ~t-nueleonic fluorine atoms. The asymmetry parameter of the electron angular distribution due to rouen decay was experimental ly measured.

    2. - Exper imental apparatus.

    A special gas target (*) was used to register rouen stops in a noble gas. I t is shown schematically in fig. 1. The cylindrical shell of the target is made of stainless steel, with the diameter of its central part being 10 em and the thickness of the wall 0.2 era, filled with the investigated gas mixture at about 45 atm. A copper shell, 0.05 cm thick, with a light-reflecting coating of CsI and quaterphenyl l ight-converting layer sputtered on it increased the light output due to scintillations in the gas target. The target volume was viewed by a photomultipl ier. The electrode concentric with the body of the target divides it into the central part , where muon stops are registered, and the gas

    (3) V. G. VARLAMOV, YU. P. DOBR~TSOV, B. A. DOLOOS~N and V. G. KIRILLOV- UORYU~OV: ~rn. Eksp. Teor. Fiz. t~is. Red., 17, 186 (1973). (4) V .G. V~LAMOV, YU. P. DOBRV.TSOV, B. A. DOLC, OSHm~r and A. It. ROGOZmN: Proceedings o] the I r~ternatio~al Conlerence on Apparatus in High-Energy Physics (Dubna, 1970), p. 800.


    layer peripheral to the target wall. The gap between the electrode and the target wall is 1 cm. The electrode is a nonmagnetic nichrome net. The size of a unit cell is 0.3 cm, the wire is 0.02 cm in diameter. A d.c. potential of

    1 kV is applied to the electrode to remove free electrons from the gap. A high- voltage pulse with an amplitude ~ 10 kV was applied to the electrode when the particle arrived at the target. The pulse duration at half-pulse height was equal to 100 ns.

    Fig. 1. - The gas target: 1) high-voltage lead-in, 2) shell of the target, 3) electrode, 4) window, 5) photomultiplier.

    In case a particle crossed the gas layer at the target wall, a light signal was registered due to the pulse electroluminesccnce in gas (s). This light signal was synchronous with a high-voltage pulse.

    The absence of the signal G can be interpreted as the particle being stopped either in the gas or in the electrode. The part of the gap where the beam entered the target was filled with an insulator allowing a particle to enter the target without any anticoineidence signal G being registered.

    (5) V.G. VA~A~OV, B. A. DoL6osIrl~I~ and A. M. ROG0ZHI~: Elementarnye chastitsy i kosmic]~skie luchi (The Elementary Particles and Cosmic Rays) (Moscow, 1967). p. 84.

  • 134 v .G . ~rARLA-MOVj B. A. DOLGOSHEIlq~ YU. P. DOBR:ETSOV, ]~TC.

    To determine the exclusion efficiency for the particles crossing the target, it was turned about with respect to the direction of the rouen beam. Therefore~ whenever the particle hit the target, it must be accompanied by an electro- luminescent anticoincidence pulse G. The exclusion efficiency measured for the particle crossing the target layer was not less than 99.6 %.

    The mixture of noble gases in the target continuously circulated through a metallic calcium furnace. The temperature of the furnace was kept at (600 - - 650) ~ C.

    The target was placed in a magnetic field produced by a solenoid, 15 cm in diameter and 30 cm long. The target and the solenoid were shielded by me:~ns of a sandwich magnetic screen (25 cm in diameter~ ]00 cm long) to protect them from the scattered field (.~ 5 G) of the accelerator. In the area occupied by the target the inhomogeneity of the field did not exceed 3 %.

    I t was found that uncontrollable variations of the magnetic-field strength, AH ~ 0.1 G, might occur in the process of measurements due to a variat ion in the residual scattering fields. This fact was taken into account in the data processing.

    The lay-out of the experimental arrangement is given in fig. 2. The 160 MeV/c rouen beam arrived from the meson channel of the synchrocyclotron of the J INR. The polarization of both negative- and positive-rouen beams was within 0.75 0.05.

    Pb 5h ieLd .~

    Fig. 2. - The experimental lay-out and simplified electronic logics: 1), 2), 3), 4), 5) plas- tic scintillation counters; GC gas target; CC1, 2, 3, 4, coincidence circuits; ID integral discriminator.


    A pulse corresponding to a muon being stopped in the gas was formed by coincidence pulses of the scintillation counters 1) ( (20x20 s) and 2) ((10 cm 3) with scintillation pulse G in the target gas. I f the anti- coincidence pulse G from the gas target was absent, the t ime analyser stored the time interval between a muon stop in the target and its decay electron. Elec- trons were registered by coincidence of telescope consisting of three plastic scintillation counters 3), 4), 5) ( (35215 s each).

    Aluminium plates, 6 mm thick, were inserted between the counters to reduce the efficiency of gamma-ray registration. The t ime analyser was pro- vided with (, guard , logics guaranteeing the t ime independence of the random coincidence background within the measuring t ime interval.

    The t ime spectr~ were analysed within a t ime interval of 1 ~s to 5 ~ts after the muon stopped. The 1 ~s shift of the analysed t ime interval excluded decay electrons coming from negative muons at rest in the target electrode and from xenon muonic atoms, since the lifetime of negative muons in both cases is less than 0.2 ~s.

    3. - Tes t exper iments .

    The following test experiments were performed: a) the measurement of the residual polarization of negative muons in a carbon rod placed in the centre gas of the target, b) the observation of the precession of the triplet muonium formed both in molten quartz and in a neon-xenon mixture in a positive- muon beam.

    The results of the above measurements are summarized in table I . I t shows

    TABLE ]. -- The results o] control experiraeats (,).

    Beam Target Mag- Asymmetry Precession Expected value netic parameter f requency of precession field (rad/~s) f requency (G) (rad/~s)

    ~t- C in the gas target 61.2 0 .052 i0 .006 5.32-i-0.08 5.23 (a)

    t~ + SiOz (quarz) 1.1 0.14 9.55:]:0.22 9.7 (*)

    ~+ Si02 (quarz) 2.1 0.12 18.20 18.5 (o)

    i~ + 42atmNe-~latmXe 2.1 0.07 :~0.01 19.5 18.5 -4-0.9 (c)

    (a) Here and in table II the error in the calculated frequency value shows possible variations in the magnetic-field intensity. (b) Corresponds to the frec-muon precession frequency. (r) Corresponds to the triplet-muonium precession frequency.

    the presence of the muon precession in the carbon rod with the precession frequency expected for a free muon. The posit ive-muon precession frequency


    in the muon-xenon mixture can be seen to conform with the value expected for the muonium, and the value of the asymmetry parameter is consistent with a ~ 100 ~ probabi l i ty of muonium formation in the gas mixture (ap- proximately one-ha]f of the muon stops must occur in the electrode of the gas target}.

    4. - Observation of free fluorine ~-nucleonie atom.

    Fluorine F-nucleonic atoms were observed in the neon-xenon mixture (42 atm Ne and 1 arm Xe) by measuring the total moment precession of an atom in magnetic fields of q 1.1 and 2.1 G.

    As has been mentioned above, the F-nucleonic fluorine total moment pre- cession should be characterized by three frequencies (see appendix, formula (A.3)). In fig. 3 curve A) represents the results of the harmonic analysis of the

    Loo ~- ~ ~ A)

    0.50 (y

    0 o

    CL - -050

    o 50 E c~

    o L



    -0 ,50

    k J L L [ L 0 2 4 6 8 1[0 lt2 14 116 1L8 20

    process~on frequency ( racL/~s)

    Fig. 3. -The results of the harmonic analysis of the time distribution for decay electrons: A) experimental curve, B) calculated curve.

    decay electron t ime distribution measured in the field of 1.1 G and the curve B) was especially calculated for the experimental conditions when precession takes place at three frequencies with weights corresponding to the stastistical popu-


    lation of multiplets (see appendix). The harmonic analysis was made with the method described in (B). The comparison of the calculated curve with the experimental one shows the presence of the precession frequencies expected for the case of formation of a free ~-nueleonic fluorine atom. Table I I presents the frequencies, the asymmetry parameters and the Z ~ values obtained from the processing of the experimental data by the least-square analysis using the function (A.3).

    TABLE II. - The results o] measurements o/ residual negative-rouen polarization in the ]luartine muonic atom.

    Target Mag- Asymmetry Value of Expected Val- Number netic parameter precession value of ueof of field frequency precession X Z degrees (G) (rad/~ts) frequency of

    (rad/~s) freedom 20 xoNe 1.1 0.115 ~0.031 9.95-4-0.66 9.7 ~0.9 332 335

    42atmNe-~- 2.1 0.128~0.032 19.0 18.5 188 199

    +1 atmXe 61.2 0.007 ~0.004 - - 5.23 ~0.09 85 91

    The most important results presented in table I I and in fig. 3 indicate that the precession frequency values obtained in both cases (1.1 and 2.1 G) are in good agreement with the calculated ones obtained in terms of the already known magnetic-field intensities. The asymmetry factor value is to be regarded as the min imum value of the actual asymmetry factor, since there is a possibility of muon depolarization because of chemical reactions or of electron shell mo- ment relaxation occurring in atom collisions with impurities. The asymmetry coefficient for the negative muons measured with the precession frequency of the free rouen in the same gas mixture in the magnetic field 61.2 G is also given in table I I . As should be expected, the asymmetry coefficient in this case is close to zero.

    5. - Conc lus ion .

    The existence of a free atomic system such as the fluorine tz-nucleonic atom has been shown. The study of negative-rouen precession in paramagnetic ~-nucleonie atoms seems promising for investigating various properties of gases and condensed media (7).

    (6) V .W. HUGH~S, D. W. McCoLN, K. Z~OCK and R. PR~POST: Phys. ~ev. A, 1, 595 (1970). (7) V. G. VARLAMOV, B. k. DOLG-OSHEIN, YU. P. DOBRTSOV, V. G. KIRILOV- UGRYUMOV, P. L. NEVSKY, A. M. ROGOZHI~ and V. P. SMrLGA: Yad. Fiz., 21, 120 (1975).


    With the aid of the ~-nucleonic atoms one can obtain data on chemical reac- tion rates of single atoms (as in the case of muonium) (e). This especially con- cerns halogen atoms formed in noble gases.

    The study of negative-muon precession in [~-nucleonic atoms in high-intensity magnetic fields may prove valuable for investigating the hyperfine structure of ordinary atoms (~).

    Measurements on hyperfine structure splitting in ~-nucleonic hydrogen may be a valuable supplement to the conventional experimental determination of the hyperfine-splitting constant in muonium (9).

    We attempted to observe free hydrogen ~L-nucleonic atoms (3). However, inadequate statistical a~eura~y of the experiment did not allow us to draw any definite conclusion (').

    The authors are very grateful to Prof. S. S. GERSHTEIN for useful discussions which have considerably influenced this investigation; and also to Prof. I . I . GUREVITCH for interest and encouragement. The authors are indebted to Profs. V. P. DZHELEP0V and L. I . LAPIDUS for the possibility of carrying out this ex...


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