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Positron lifetime spectra have been measured at 77 K for KCl and Ag-doped KCl before and after x-irradiation at 77 K and after annealing at room temperature. Radiation at 77 K reduces the intensity of the intermediate lifetime (τ2) component. Radiation-induced defects were monitored optically and by ESR. The experiment shows that the changes observed in the positron decay are associated with the presence ofV K centers.

Ytterbium oxide enriched to 95.9% in mass number 172 was irradiated with 6-Mev protons, An activity decaying by electron capture with a half-life of (6.70 plus or minus 0.04) days was produced and its assignment to Lu/sup 172/ confirmed by the identification of the ytterbium K x ray and by comparison with the activities produced by similar proton irradiations of ihe other enriched isotopes of ytterbium. The 4.0-hour positron activity previously assigned to Lu/ sup 172/ was not observed and is best attributed to an impurity. The observed activiaty of Lu/sup 172 coi;sists of the L and K x rays of ytterbium and gamma rays with energies of 79, 91, 113, 162,203, 270, kev. Because no positron radiation exists in the activity of Lu/sup 172/, the mode of decay Is solely by electron capture to Yb/sup 172/. Gamma-gamma coincidence measurements, energy considerations, and consideration of ihe relative numbers of the radlations obserwed in the activity of Lu/sup 172/ have led to the asslgnment of energy levels at 530 (6+), 1172 (3+), in Yb/sup 172/ In addition to the prevlously known 78.7 (2+)- and radiations ard some obserwed only in conversion electron measurements are shown in a proposed energy level schememore » for the decay of Lu/sup 172/. Approximate branching ratios for the electron capture disintegrations of Lu/sup 172/ are also shown in the level scheme. Few, if any, electron capture transitions of Lu/sup 172/ occur to the ground and first excited states of Yb/sup 172/. Of the two predicted spins for the ground state of Lu/sup 172/, 4-is more consistent with the proposed energy level scheme. (auth)« less

The 3.3-h positron decay of ${\mathrm{Cu}}^{61}$ has been investigated by use of Ge(Li) detectors and coincidence techniques. A total of 19 $\ensuremath{\gamma}$-ray transitions have been observed ranging in energy from 67 to 2120 keV, of which only eight had been reported previously. The decay scheme based on these data incorporates all of these transitions and requires only those ${\mathrm{Ni}}^{61}$ levels that have been excited in nuclear reactions. The excitation energies of the levels observed to be populated in ${\mathrm{Ni}}^{61}$ from the decay of ${\mathrm{Cu}}^{61}$ are 67.3, 282.8, 655.7, 908.8, 1100.2, 1132.4, 1185.5, 1446.9, 1611.8, 1730.2, and 2119.8 keV.

Physical review letters 19(15), 869 - 872 (1967). doi:10.1103/PhysRevLett.19.869, Published by APS, College Park, Md.

A source of pure 66-min ${\mathrm{In}}^{110}$, particularly free from 4.9-h ${\mathrm{In}}^{110m}$, has been made by bombarding a natural indium foil with about 100-MeV protons, and chemically separating tin (predominantly ${\mathrm{Sn}}^{110}$, which is the parent of only 66-min ${\mathrm{In}}^{110}$) from the indium target. Scintillation spectrometer studies have shown that the well known 280-keV gamma ray is emitted in the decay of 4-h ${\mathrm{Sn}}^{110}$ to 66-min ${\mathrm{In}}^{110}$, and the following gamma rays are emitted in the decay of 66-min ${\mathrm{In}}^{110}$: 656, 820, 1120, 1400, 1630, 1780, 2130, 2300, 2400, 2600, 2750, 3000, 3400, and 3650 KeV. The following levels in ${\mathrm{Cd}}^{110}$, fed from the decay of 66-min ${\mathrm{In}}^{110}$, are postulated: 656, 1474, 1541, 1790, 1910, 2060, 2820, 3110, 3400, and 3650 keV. From the study of gamma rays in simultaneous coincidence with two annihilation quanta, the intensity of the positron branching to the 656-, 1474-, and 1790-keV states are estimated to be 71, 0.2, and 0.1%, respectively. The $log\mathrm{ft}$ to these states are calculated to be 5.5, 7.4, and 7, respectively. It is concluded that the allowed positron decay to the 1474-keV state is retarded by a factor of about 100.

Методом 4πβ+−γ совпадений определена вероятность эмиссии позитрона в 1-запрещённом переходе с основного состояния Co56(4+) на второй возбуждённый уровень Fe56(4+). РезультатP β +=0,899±0,142 находится в хорошем согласии с результатами Ипредыдущих измерений. Среднее взвешенное имеющихся результатов даёт ɛ/β+=0,117±0,089. Эти результаты согласуются с теоретическими расчётами для разрешённого перехода. Эффективность 4π пропорционального счётчика определена из интенсивности аннигиляционного пика в одиночном и полном спектре гамма лучей, и в спектре совпадений. Using the 4πβ+−γ coincidence method the probability of positron emission had been determined in the 1-forbidden transition from the ground state of Co56(4+) to the second excited state of Fe56(4+). The resultP β +=0,899±0,142 is in good agreement with the previous measurements. The weighted average of the capture to positron emission ratio of the given results is ɛ/β+=0,117±0.089. These are in agreement with the theoretical calculations for the allowed transition. The detection efficiency of proportional counter for positrons had been determined from the annihilation peak intensity in the single, coincidence and total spectra of the same source.

The evaluation and correction procedure for the differential internal bremsstrahlung (IB) spectrum (at 90° between the direction of the beta particle and IB gamma quantum) from the superallowed positron emitter11C (21 min) is described. First a short outline of the theory and the details of the actual experimental arrangement are given and also a comparison is drawn between the theory and the experiment.

The internal bremsstrahlung in the superallowed decay of the β + -emitter 11 C has been studied in order to check the fundamental concept of the internal bremsstrahlung process.

The relative abundances of the fragment ions that result, respectively, from the β— decay of CH3Br82 and β+ decay of CH3F18 have been measured with a specially designed mass spectrometer. Following the β— decay of CH3Br82, only about 0.4% of the parent ion, CH3Kr+, remained intact, as was anticipated from the rather large recoil energy. The most abundant ion observed was CH3+, 89%. A variety of other ions were also formed and their production is discussed in terms of excitation resulting from a sudden change in nuclear charge. The investigation of the fragment ions formed in the decay of CH3F18 marks the first time a molecule undergoing positron decay has been so studied. The following negative ions were detected: CH3O—, O—, and H—. A large abundance of positive ions, however, was also observed. The origin of these positive ions is critically discussed; and calculations of the probabilities for electron shakeoff following both positron and negatron decay are given for several atomic systems.

The disintegration scheme of the long-lived groundstate isomer of Al/sup 26/ was determined. The maximum positron energy as determined by absorption measurements is 1.16 sintering time 0.05 Mev; this agrees with the expected 1.17 Mev. The spectrum appears to be simple. The scintillation gamma spectrum shows intense positron annihilation radiation, a strong peak at 1.85 sintering time 0.03 Mev, and weak peaks at 1.12 sintering time 0.03 Mev and 2.96 sintering time 0.05 Mev, corresponding to transitions from the known Mg/sup 26/ states at 1.82 and 2.97 Mev. A peak at 0.68 Mev is from the addition of two annihilation photons, one being backscattered from the source and surroundings; the gamma of ~0.7-Mev energy reported by others is not present. The peak at 2.97 Mev is shown to result from 2.97-Mev photons as well as from addition of 1.82- and 1.15-Mev photons. The relative intensities of the annihilation and gamma radiations indicate that Al/sup 26/ undergoes (84.6 sintering time 1.8)% positron emission to the 1.82-Mev state of Mg/sup 26/; (11.4 sintering time 1.9)% electron capture to the same state; (3.7 sintering time 0.3)% electron captare to the 2.97-Mev state followed by emission of 1.15- and 1.82-Mev gamma- rays; and (0.30more » sintering time 0.03)% electron capture to the same state followed by 2.97-Mev radiation. Other energetically possible transitions are apparently negligible in intensity. Auger electrons and x-rays were observed in a proportional counter spectrometer. Analysis of the spectra yielded K-shell fluorescence yields of 0.008 sintering time 0.003 for magnesium and 0.008 sintering time 0.003 for aluminum. (auth)« less

The existence of zinc radioactivities with half-lives 57 minutes and 13.8 hours is confirmed, by deuteron activation of zinc. In addition, it is found that both these activities are produced by deuteron and by fast neutron irradiation of gallium, so that the two periods must be due to isomers of ${\mathrm{Zn}}^{69}$. Both emit negative electrons and the properties of their radiations are given. It is shown that ${\mathrm{Zn}}^{65}$, produced through deuteron bombardment of zinc, has a half-life of 250 days, agreeing with the period found by others after proton and deuteron bombardment of copper. This isotope appears to decay by $K$ electron capture and also, to a lesser extent, by positron emission. No evidence has been found for radioactive ${\mathrm{Zn}}^{71}$. All previously reported zinc activities are discussed and the following table contains all the known periods that are believed to be genuine.

The ratio of electron capture to negative electron emission for the decay of [Formula: see text] has been found to be (1.64 ± 0.08) × 10−4. The half-life for electron capture, relative to the accepted beta emission half-life of 19.2 hours, is 13.3 ± 0.6 years. The ratio was found by mass-spectrometric stable isotope dilution determinations of the quantities of [Formula: see text] and [Formula: see text] produced by the decay.

A method is described to measure positron-gamma angular correlations. The positrons are detected in a plastic scintillator. The disturbance of the energy selection, caused by Compton scattering of annihilation quanta in the plastic, has been avoided by counting these quanta simultaneously in two NaI(Tl) crystals placed face to face near the plastic. The problem of subtracting random coincidences in the fourfold coincidence system is discussed and the results of test measurements on 86Rb and 22Na are compared with previous results.

The probability of the astrophysically important reaction $\mathrm{H}+\mathrm{H}=\mathrm{D}+{\ensuremath{\epsilon}}^{+}$ is calculated. For the probability of positron emission, Fermi's theory is used. The penetration of the protons through their mutual potential barrier, and the transition probability to the deuteron state, can be calculated exactly, using the known interaction between two protons. The energy evolution due to the reaction is about 2 ergs per gram per second under the conditions prevailing at the center of the sun (density 80, hydrogen content 35 percent by weight, temperature 2\ifmmode\cdot\else\textperiodcentered\fi{}${10}^{7}$ degrees). This is almost but not quite sufficient to explain the observed average energy evolution of the sun (2 ergs/g sec.) because only a small part of the sun has high temperature and density. The reaction rate depends on the temperature approximately as ${T}^{3.5}$ for temperatures around 2\ifmmode\cdot\else\textperiodcentered\fi{}${10}^{7}$ degrees.

The hindrance in some unique first-forbidden beta decays is related to the repulsive T = 1 particle-hole force. Calculations are performed for 17 N, 39 Ar and 41 K. The qualitative trend agrees with experiment, but not the numerical results.

The energy of the proton beam from an air insulated Van de Graaff generator was measured absolutely by an electrostatic analyser, and the resonant energies for proton capture by 24Mg and 25Mg were determined by observing the positron yields. The following resonances were observed: 24Mg (p, γ)25Al 225.5 ± 0.3 and 418.4 ± 0.4 keV of half widths 6.0 ± 1.0 and 2.5 ± 0.5 keV respectively. 25Mg (p, γ)26Al 316.7 ± 0.7, 391.5 ± 0.5, 495.6 ± 0.6, 513.4 ± 0.7 and 530.4 ± 0.7 keV, of half widths 12.0 ± 1.0, 8.0 ± 1.0, 5.0 ± 1.0, 3.0 ± 1.0 and 3.0 ± 1.0 keV respectively. The half lives of the positron decay of 25Al and 26Al were measured as 7.62 ± 0.13 and 6.68 ± 0.11 seconds respectively.

The beta decay of Eu 156 has been studied by means of an intermediate image magnetic spectrometer modified for the beta-gamma coincidence studies. An attempt has been made to detect the possible beta feeding to the lowest energy level of the beta vibrational band which is expected in the deformed Gd 156 nucleus. The present study does not give any definite indication of feeding of such a level in the decay of Eu 156 and if it exists it should be less than 0.1%. The energy and intensity of different beta groups and various gamma-rays in this decay are given.

Positron lifetimes in commercial poly(tetrafluoroethylene) (Teflon) were measured as a function of 60Co gamma irradiation of the polymer in air and in vacuum. The rates of decrease with respect to dose in both τ3 and I3 were observed to be faster when irradiated in air than in vacuum. Conversion of o-Ps occurs due to the free radicals formed under irradiation. The decrease in τ3 is due to quenching of o-Ps by the free radicals formed. The faster decrease in I3 (compared to that in τ3) with dose is ascribed to a combined effect of free-radical quenching and decrease in the amount of o-Ps formation due to increased crystallinity and/or Ps compound formation.

The maximum energies and the half lives of three shortlived beta /sup +/ activities near A = 40 were measured with a scintillation detector. The maximum beta /sup +/ energies of 5.00 plus or minus 0.07 Mev (K/sup 38m/), 5.32 plus or minus 0.10 Mev (Sc/sup 42/), and 5.81 plus or minus 0.15 Mev (Ti/sup 43/) are in agreement with the predictions of a semiempirical formula. (auth)

The directional correlation and the polarization direction correlation of the 722 Kev-556 Kev gamma-gamma cascade in Cd 114 have been measured. It is found that the decay sequence of this cascade agrees well with the 4(E2)2(E2)0 sequence. The ratio of the rate of the positron decay to that of the cascade decay is also obtained as 3×10 -3 .

Positron decay curves in various semiconducting elements and in the semimetals As, Sb, and Bi have been measured with a delayed coincidence system with 0.35 nsec full width at half-maximum prompt time resolution. The annihilation in ideal, pure semiconductors is found to be characterized by a simple exponential decay: The second low-intensity decay component may be entirely due to spurious effects. In nonintrinsic Si and Ge powders, however, a second decay component with considerable abundance is observed whose presence is assigned to unclear extrinsic processes. The intrinsic decay component in diamond, Ge, and Si can be interpreted as due to annihilation with the valence electrons, taking into account the screening effect through an evaluation of the dielectric constant. In S, Se, and Te the effect of the electron charge distribution is dominant over the effect of electron polarizability.

The helicity of the electron and positron from muon decay was measured by determining the sense of circular polarization of their bremsstrahlung by the method of absorption in iron magnetized against or along the direction of motion of the particles. The positron is found to be right-handed and the electron left- handed. The results are consistent with the two-component neutrino theory, which assumes (V,A) interaction, conservation of leptons, and left-handed neutrinos. (auth)

The K/e + ratio of the non-unique first-forbidden (2 − -2 + ) transition to the first excited state of 84 Kr has been measured by means of an internal source scintillation spectrometer in coincidence with a Ge(Li) crystal. A mass separator was used to purify the source material. The result K/e + = 3.96±0.16 is about 20 % lower than earlier results of Welker and Perlman and of Konijn, it agrees with a theoretical prediction of Perlman et al.

Accurate measurements have been made of the threshold for the reaction 35 Cl(p, n) 35 Ar, and of the branching ratios and half-life in the decay 35 Ar(β + ) 35 C1. The threshold, 6941.2 ± 1.7 keV leads to the value 4941.6 ± 1.7 keV for the beta end-point in the ground state decay of 35 Ar, and the corresponding f -value is 3112.3 ± 5.0. The result obtained for the branching ratio for the ground state decay was 98.54 ± 0.05 %. The half-life measurements gave the result t 1 2 = 1.770 ± 0.006 sec . After application of a radiative correction of + 1.45 %, the ft value for the mirror transition was thus determined to be 5671 ± 21 sec, excluding uncertainties in the radiative correction. This value is much more accurate and significantly lower than previous measurements. When combined with the result of Calaprice et al. for the spatial asymmetry of positons from the decay of nuclear spin polarized 35 Ar, the present ft value leads to a value for the weak interaction vector coupling constant in beta decay of G β v = (1.460 ± 0.006) × 10 −49 erg · cm 3 . This value is seriously out of line with results obtained from J = 0 + , T = 1 decays, and from other mirror decays. Further experimental work, particularly on the branching ratios and the asymmetry measurement are indicated, as well as considerations of possible nuclear structure or other effects.

A γ-γ-coincidence detector with good efficiency at moderate costs for cosmogenic positron emitters (like 22Na and 26Al) is described. The influences of different operation modes on sensitivity and background are discussed.

The differential cross section for e + e − → γγ is calculated to order α 3 for the case of e + e − colliding beam experiments. Special attention is given to hard photon corrections. For a specific experimental set-up, detailed numerical results are presented. It is found that the radiative corrections can be quite large in the forward direction.

The single gamma-ray spectrum, electron-gamma and gamma-gamma coincidence spectra have been studied in the 182 m Re → 182 W decay with Ge(Li) and NaI(Tl) detectors as well as with a six-gap beta-ray spectrometer. A total of 28 new gamma-ray transitions have been observed and ten new levels introduced. The Iπ K = 2+ 0 level at 1257.5 keV has been confirmed, and another level at 1538 keV with the same quantum characteristics has been established. The rotational band with Kπ = 1+ based on the pp[402↑–404↓] state at 2057,5 keV has been suggested. The character of the decay of the investigated 182mRe (12.7h) isomer indicates that it most probably is the {p 402↑ – 624↑} 2+ state.

The process of electron positron pair emission in nuclear electromagnetic transitions is investigated in the special case where the angle between the directions of emission of the electron and positron is small. It is shown that experimentally this process can be of value in determining the absolute emission rates of high energy γ-rays.

The disintegration scheme involved in the decay of 16-day ${\mathrm{V}}^{48}$ to ${\mathrm{Ti}}^{48}$ has been established by scintillation studies, and spins and parities have been assigned to the levels. The angular correlation of the well-known cascade gamma-ray pair with energies of 0.99 and 1.32 Mev has been measured. The corresponding levels are found to have spins 0-2-4. The positron spectrum has been measured with a magnetic beta-ray spectrometer and shows evidence for positron emission to a level in ${\mathrm{Ti}}^{48}$, 22.2 Mev above the ground state. The positron group involved in the decay to the 2.31-Mev level of ${\mathrm{Ti}}^{48}$ is found to have an end point of 0.69 Mev.

The general expression for the angular· distribution of the electron and neutrino is given in the case of the allowed beta decay of polarized nuclei under the assumption of the G­ parity nonconservation. Explicit formulas are derived for the neutron beta decay and for the pure Gamow-Teller transition. It is pointed out that the magnitude of the pseudo-ten­ sor coupling constant can, in principle, be obtained by measuring the angular distribution of the beta particles with respect to the initial nuclear polarization axis.

The lifetime of the lowest 6 + state in 54 Fe was measured by the β + -γ delayed coincidence method in the decay of the 1.43 min 54 Co isomer. The half life was determined to be 1.24±0.04 nsec and B (E2: 6 + →4 + )=38.9±1.3 e 2 f m 4 . Assuming pure πf 7/2 -2 configuration both for the 6 + and 4 + states in 54 Fe, the polarization charge of the 0f 7/2 proton (δ e eff ) with 56 Ni as a core is deduced to be 0.9 e . From comparison with theoretical calculations a contribution to δ e eff due to the 2\(\hslash\omega\) excitation of an inert core is found to be nearly the same in 48 Ca and 56 Ni.

The decay of 47 V has been investigated by using a two-directional focusing beta-ray spectrometer and NaI scintillators (2 in. 3 mm thickness). Two weak gamma-rays of 1.54 and 1.87 MeV have been found and their intensities are 1.4 and 0.9 percent of the positron decay, respectively. The upper limit for 0.16 MeV gamm-ray is 0.08 percent of the positron decay. The maximum energy of the positrons from 47 V is 1.90±0.04 MeV.

The Mg/sup 24/(#gg,n)Mg/sup 23/ reaction was studied by irradiating a magnesium sample in the x-ray beam of a 25 Mev betatron and measuring the Mg/sup 23/ positron decay by coincidence detection of the annihilation radiation. The cross section had a small peak at 17.2 Mev with the giant resonance appearing at 19.2 Mev. There was a pronounced high-energy tail. These results are compared with recent investigations of the Na/sup 23/(p, gamma )Mg/sup 24/ inverse photonuclear reaction. Both cross sections showed a small peak below the giant resonance region. An attempt was made to interpret the cross-section shapes in terms of the deformation of the Mg/sup 24/ nucleus, but the results were inconclusive. (auth)

The positron decay of Ca38 has been investigated using the Ar36(He3,n) reaction with an enriched Ar36 gas target. A delayed 1568-keV γ ray with a half-life of 0.47±0.02 sec is attributed to a positron branch to the 1695-keV level of K38. Upper limits of 3% or better were found for transitions to other low-lying levels. Using the measured half-life and the decay energies, the branching ratio to the 1695-keV level is calculated from β-decay systematics to be (21±4)%. The logft for this transition (3.41±0.09) establishes Jπ=1+ for the 1695-keV level. An additional result was the observation of an allowed transition from K38 to the 3936-keV level of Ar38.

A source of ${\mathrm{Cu}}^{62}$ was prepared by means of the ($p, \ensuremath{\gamma}$) reaction on an enriched ${\mathrm{Ni}}^{61}$ target. The gamma rays following positron emission were detected with a scintillation spectrometer, and the following energies were measured: 0.88\ifmmode\pm\else\textpm\fi{}0.01, 1.13\ifmmode\pm\else\textpm\fi{}0.02, and 1.17\ifmmode\pm\else\textpm\fi{}0.01 Mev. The relative intensities with respect to the positrons are 0.3, 0.1, and 0.5% with uncertainties of a factor of two either way. Coincidence techniques and half-life measurements were used to identify the gamma rays with the decay of ${\mathrm{Cu}}^{62}$. The half-life for positron emission was measured to be 9.9\ifmmode\pm\else\textpm\fi{}0.2 min. Branching ratios and $log\mathrm{ft}$ values have been calculated and are listed in tabular form.

Experiments on the recoil of the nucleus in the disintegration of ${\mathrm{Cl}}^{38}$ have been continued. The fact (which has been reported in earlier publications) that momentum is not conserved in the system consisting only of the nucleus and the emitted electron has been confirmed. By measuring the momenta of the electron and nucleus it has been possible to ascertain the direction of emission of the neutrino with respect to the direction of the electron in each case. Theoretical predictions according to both the Fermi and the K-U theory are plotted for comparison. The experimental data agree slightly better with the Fermi than with the K-U theory, but it is not safe to attach much significance to the results obtained on this aspect of the problem, because of the possibilities for rather large experimental errors. An investigation into the manner in which droplets in the cloud chamber are produced by the recoil atom has been carried out. Evidence is offered which indicates that the products resulting from dissociation of the gas molecules act as centers for condensation. This effect, in addition to ionization, accounts for the droplets observed.

A member of the odd–odd Z = N family, Sc42, has been produced by proton bombardment of a calcium target greatly enriched in Ca42. The positron emission, which alone is observed, has a measured half-life of 0.68 second, end point 4.8. Mev., and threshold 6.8 Mev. The superallowed transition is between ground states (J, T) tentatively described as Sc42 (0+, 1) and Ca42 (0+, 1).

The three-quantum decay of positrons annihilating in lithium hydride, alkali halides, and alkaline-earth oxides was investigated using a triple-coincidence method. Relative measurements were made by comparing the three-quantum yield in each substance with that in aluminum, taken as a standard. In some cases the three-quantum yield was measured as a function of temperature and employing multicrystalline specimens or single crystals with and without color centers. The results suggest that the essential features of positron decay in alkali halides are connected with the properties of ideal crystals, while this is not true in alkaline earth oxides.

The $^{8}\mathrm{B}$ yield from the reaction $^{6}\mathrm{Li}(^{3}\mathrm{He},n)^{8}\mathrm{B}$ has been measured from 8.9 to 26.5 MeV (lab) by counting the annihilation $\ensuremath{\gamma}$ rays from the 0.762-sec positron decay of $^{8}\mathrm{B}$. The measurement was motivated by the negative results of the Davis solar neutrino experiment, and the consequent possible usefulness of direct tests of neutrino detection systems with neutrinos from the decay of $^{8}\mathrm{B}$. The present upper limit on the capture of solar neutrinos in $^{37}\mathrm{Cl}$ is 1 SNU, corresponding to a flux of 0.74 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ $^{8}\mathrm{B}$ neutrinos per ${\mathrm{cm}}^{2}$ sec. It was found that a $^{3}\mathrm{He}^{+}$ current of 30 mA at 24 MeV is required to produce this $^{8}\mathrm{B}$ neutrino flux at a distance of 8 m from the target, indicating that the contemplated test is not impossible.

The decay of positrons has been investigated in the halogen derivatives of benzene, both at room temperature and at - 100\ifmmode^\circ\else\textdegree\fi{}C, and in a number of halogen compounds. A lifetime of the order of 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec is found in all the investigated compounds but fluorobenzene, and suggests the presence of a positron-halogen bound state; a search for the excited levels of such a bound state, whose decay features were predicted by Goldansky and co-workers, was inconclusive.

Beta-gamma directional correlations for the 485, 435 and 530 keV beta groups in 58 Co, 141 Ce and 110m Ag, respectively, were measured with a magnetic spectrometer and were found to be isotropic in each case.