Apparent Lifetime Research Articles

Excitation Transfer in Helium

The apparent lifetimes of the $n^{1}P$, $3^{1}D$, $4^{1}D$, and $3^{3}D$ levels of helium are studied as a function of pressure. The collisional transfer of excitation from $n^{1}P$ levels to $\mathrm{nF}$ levels is conclusively shown to be an important mechanism in populating $D$ levels. It is found that the Wigner spin rule inhibits collisional transfer between $4^{1}P$ and $4^{3}F$ levels. For $n>4$, the $n^{1}P$ transfer to $n^{3}F$ levels is about three times greater than to the $n^{1}F$ levels. For $n>4$, the cross sections for collisional transfer from $\mathrm{nF}$ to $n^{1}P$ levels are found to be about 10% of those for the reverse process. This result is in agreement with the principle of detailed balance. The collisional $n^{1}P\ensuremath{\rightarrow}\mathrm{nF}$ transfer cross sections are measured and appear to increase approximately as ${n}^{4}$. The pressure dependence of the apparent $3D$ cross sections are calculated, using the above measurements in conjunction with the $n^{1}P\ensuremath{\rightarrow}\mathrm{nF}$ transfer model. The results are found to be in excellent agreement with experimental measurements.

The time dependence of the low-energy proton belts

The time dependences of the low-energy proton space distributions and differential energy distributions at 8 energies 0.17–3.4 Mev are presented for the period of August 15, 1964, to February 8, 1965. The data were obtained from the directional proton spectrometer on the satellite 1964-45A, which was launched on August 14, 1964, into a near polar orbit with a 3765-km apogee and a 270-km perigee. The proton flux decay constant can be expressed as (1/τ) (day)−1 = (−0.0007 ± 0.0007) + (0.0033 ± 0.0009) Ine E (Mev). Protons of 0.2 Mev decreased with an apparent lifetime of 150 days, those of 1 Mev were constant, and protons of 2.5 Mev increased with an apparent lifetime of 500 days. The maximum peak heights of the energy distributions decreased by less than a factor of 2, and the energy spectra hardened slightly with time. The positions of the maxima remained almost constant. Therefore, the low-energy proton radiation belts were rather stable from August 15, 1964, to February 8, 1965.

Aerosol Behaviour traced by artificial and natural radioactivity

Simultaneous daily samples of nuclear bomb debris have been statistically analysed based on the assumption that day-to-day variations are due to compression or dilution of a basic population. Monthly normalized standard deviations are presented from October 1956 to December 1964. Sorting of results into dry and rainy weather collections suggests that the “quantums” of radioactivity of individual particles are greatest when the bomb debris is collected in rain, indicating that most of the collection in rain is due to the larger particles acting as condensation nuclei. Natural aerosols have a similar size range. This suggests a variation of life-time with aerosol size. Some aspects are illustrated by computation of apparent life-time of gross aerosols, when two groups are in steady state with respect to radon daughters. Apparent life-time is shorter for the RaB/RaD than for the RaD/RaF ratio. Published measurements are discussed. DOI: 10.1111/j.2153-3490.1966.tb00266.x

Polymerisation et croissance des particules de carbone produites par une decharge electrique

The polymerisation and the growth of carbon particles have been reinvestigated. Hydrocarbon molecules have first been dissociated by heating the gas by a condensed electrical discharge. The absorption spectrum of the decaying plasma has then been measured using time resolving technique. These spectra show the same main features as those found using flash photolysis technique. However, the apparent lifetime of the C 2 molecules in its lower state is found to be much shorter. The broad absorption feature having its maximum at about 3900 Å has also a much shorter lifetime. It is suggested that it might be due not to a molecular electron transition but to the scattering of light by non-absorbing spheres. The data available for the Mie extinction coefficients allow to check this tentative interpretation. These experiments seem to agree with the recent results obtained using the classical pyrolysis technique for heating hydrocarbon molecules, which reveals also the existence of two successive processes in the formation and the growth of carbon particles. In our case, we would first have formation of C 2, C 3, etc....molecules growing rapidly to become small carbon “nuclei”. This rapid process is followed by a very different one in which the growth into larger carbon particles proceeds through the direct catalytic dissociation of the hydrocarbon molecules on the carbon particle surface.

Interaction Effects on Lifetimes of Atomic Excitations

The apparent lifetime of an excited atom in a p state is investigated when another atom, in an s ground state, is present. The dependence of the lifetime on the distance RAB between the two atoms is obtained. It is found that the results for a p0 state differ somewhat from those for the p±1 states. (The axis of quantization is taken as the line joining the two atoms.)

Experimental Determination of the Oscillator Strength of the Violet System of OH

The radiative lifetime of the A 2Σ→X 2Π transition of OH has been measured. The apparent lifetime is dependent upon the pressure of the parent molecule from which OH* is produced by electron bombardment. Extrapolation to zero pressure yields a spontaneous radiative transition probability of 0.99×106 sec—1 corresponding to an oscillator strength f= (7.25±0.5) 10—4. This value assumes that the electronic transition moment is constant over the (0, 0) vibrational transition. Utilizing Learner's values of rotation—vibration interaction an f value for the rotationless molecule and the (0, 0) band is f= (8.0±0.8) 10—4. The collisional quenching cross sections between OH* and water or methanol are 70×10—16 cm2 and 110×10—16 cm2, respectively.

Transmission of Monochromatic Radiation in a Two-Level Material

A theoretical study has been made of the transmission of a monochromatic, well-collimated beam of radiation normally incident on idealized material containing only two energy levels. The nonlinear partial differential equations governing the variation of population and photon density in space and time have been solved exactly for arbitrary initial conditions under the assumption that spontaneous emission and thermal relaxation from the excited state can be neglected. These conditions are satisfied in practice when the radiation is in the form of a pulse whose duration is short compared with the characteristic times of these relaxation processes. Two cases are considered in detail: (a) when all atoms are in the ground state and the material absorbs; and (b) when there is a population inversion and the material amplifies. In case (a), the radiation, with a characteristic velocity, ``bores'' its way through an optically dense substance emerging delayed in time and leaving the material in a perfectly transparent or saturated state. In case (b), an incoming pulse is amplified and sharpened—to a degree determined by the gain of the medium. In addition to these cases, the dependence of the apparent spontaneous emission lifetime on the size and population distribution of the material is mentioned.

Search forH5

Following the suggestion that ${\mathrm{H}}^{5}$ might be stable against nucleon emission, a search was made for delayed neutrons resulting from the reaction ${\mathrm{Li}}^{7}(\ensuremath{\gamma}, 2p){\mathrm{H}}^{5}$. The targets were bombarded with a 340-MeV bremsstrahlung beam and the delayed neutrons detected in a shielded paraffin-moderated array of B${\mathrm{F}}_{3}$ counters. Counts were scaled in 14 delay channels of variable width and variable initial delay. Separated isotopes of ${\mathrm{Li}}^{7}$ and ${\mathrm{Li}}^{6}$ were used as targets. As ${\mathrm{H}}^{5}$ cannot be produced from the bombardment of ${\mathrm{Li}}^{6}$, counts observed during the bombardment of ${\mathrm{Li}}^{6}$ were used to measure the background. After background subtraction, the data fall uniformly about zero and show no apparent lifetime. Assuming a 10-msec half-life, an upper limit for the activation cross section is ${\ensuremath{\sigma}}_{A}\ensuremath{\le}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}32}$ ${\mathrm{cm}}^{2}$. A comparison of this with the yield expected from the extrapolation of the measured yields of the reactions ${\mathrm{B}}^{11}(\ensuremath{\gamma}, 2p){\mathrm{Li}}^{9}$ and ${\mathrm{F}}^{19}(\ensuremath{\gamma}, 2p){\mathrm{N}}^{17}$ indicates that the upper limit for the activation cross section is of the order of 1% of the expected yield. Hence it appears unlikely that ${\mathrm{H}}^{5}$ is stable against particle emission.

Photostriction Effect in Germanium

AbstractA new optico‐acoustic phenomenon in germanium is reported. A change in the lattice constant (crystal dilatation) is observed when excess pairs of current carriers have been introduced into the sample by monochromatic illumination. The method of measurement enables one to detect small vibrations of the sample with amplitudes down to 10−11 cm. The effect has been measured in the range from 80 to 1780 cps, and shows no dependence on frequency. The ratio of relative change of the lattice constant to the carrier pair concentration has been estimated to be about 7 × 10−24 cm3 which is in satisfactory agreement with the value predicted by theory. Some features of spectral response of the effect can be explained in terms of 〈100〉 exciton states with relatively long apparent lifetimes.

Experimental Determination of the Oscillator Strength of the First Negative Bands of N2+

The radiative lifetime of the upper state of the 3914 A transition of N2+ was measured to be (6.58±0.35) 10—8 sec, corresponding to an oscillator strength f of (3.48±0.20) 10—2. The apparent radiative lifetime of the upper state of the 3371 A transition of the second positive system of N2 was found to depend upon the conditions of excitation. Extrapolation to excitation threshold yields a lifetime of (4.45±0.6) 10—8 sec.

Measurements of lifetimes of excited states of helium atoms

A new method of measuring lifetimes of excited states of atoms has been devised and applied to helium. A short pulse (10 -8 s duration and 10 kc/s repetition rate) of low-voltage electrons causes atoms of a gas to be raised to excited states. The subsequent decay to lower states is observed by intercepting the emitted photons with a photomultiplier, those belonging to the desired transition being selected by means of a filter. By recording delayed coincidences between the photomultiplier pulses and the electron pulses the rate of decay of the excited state can be measured. If certain conditions are satisfied, the decay is exponential with a decay constant equal to the reciprocal of the natural lifetime of the excited state. The mean lifetimes of the 4 3 S , 3 3 P , 4 3 P and 3 3 D terms of helium are found to be 6⋅75 ± 0⋅10, 11⋅5 ± 0⋅5, 15⋅3 ± 0⋅2 and 1⋅0 ± 0⋅5 respectively, in units of 10 -8 s. These values agree well with the theoretical calculations of Bates & Damgaard. The apparent lifetime of the 3 1 P level is modified by the imprisonment of resonance radiation in a manner in qualitative agreement with Holstein’s theory. The 2 1 S -3 1 P transition probability is found to be (1⋅35 ± 0⋅02) x 10 7 s -1 .

Characteristics ofK+-Particles

A large nuclear emulsion stack was exposed in the magnetically analyzed beam of ${K}^{+}$ particles, at the Berkeley Bevatron. The secondaries of stopped $K$-mesons have been analyzed, and examples of all the known modes of decay: ${K}_{\ensuremath{\mu}2}$, ${K}_{\ensuremath{\pi}2}$, ${K}_{e3}$, ${K}_{\ensuremath{\mu}3}$, $\ensuremath{\tau}$, and ${\ensuremath{\tau}}^{\ensuremath{'}}$ have been identified. The decay processes of the ${K}_{\ensuremath{\mu}2}$, ${K}_{\ensuremath{\pi}2}$, and ${\ensuremath{\tau}}^{\ensuremath{'}}$ mesons have been confirmed, and a possible decay scheme for the ${K}_{\ensuremath{\mu}3}$ has been considered. The masses of the particles decaying in the different ways have been determined from momentum-range measurements, and for ${K}_{\ensuremath{\mu}2}$ and ${K}_{\ensuremath{\pi}2}$ particles, from the $Q$ of the decay as well. The masses of all the particles are consistent with a value of $965{m}_{e}$, within about $10{m}_{e}$. The relative frequencies of the various modes of decay have been estimated, and by comparison with other data, it is found that the apparent lifetimes of all the particles are \ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ sec.

Measurements of Lifetimes of Excited States of Atoms by the Method of Delayed Coincidences

WE have adapted the method of delayed coincidences1 to the investigation of lifetimes of excited states of atomic helium. Helium gas at a pressure of about 0.01 mm. mercury was excited by a beam of 40 eV. electrons, and the photons emitted in the subsequent de-excitation detected by means of an E.M.I. VX 5031 photo-multiplier, the wave-lengths corresponding to the various transitions being selected by means of Barr and Stroud interference filters. The intensity of the electron beam was modulated by means of a grid which was maintained at a potential sufficiently negative to cut off the beam completely except during the application of a positive voltage pulse of duration 2 × 10−8 sec. and repetition rate 10 kc./s. The helium atoms were therefore excited by short bursts of electrons. The voltage pulses from the photomultiplier, corresponding to the ejection of single electrons from its cathode, were amplified and applied to one side of a coincidence unit of resolving time 2 × 10−8 sec. The pulse used for modulating the electron beam was also applied to the other side of the coincidence unit through a variable length of ‘Uniradio 57’ cable. A coincidence was therefore registered only if the delay in this cable was equal to the interval between the electron burst and the subsequent emission of a photon, and for delays longer than the overall resolving time of the apparatus the variation of coincidence-rate with delay should show an exponential decrease, the decay constant being the reciprocal of the mean life of the excited state involved. We are satisfied that for the transitions referred to below, the conditions of our experiment are such that the effects on the apparent lifetime of recombination of ions and of cascade transitions from higher states are negligible.

Mean Lifetimes ofV-Particles and Heavy Mesons

A maximum-likelihood procedure for determining mean lifetimes of V-particles from cloud-chamber data is applied to samples taken from a group of 134 neutral V-particle decays. For 74 events which were consistent with a decay into a proton and a negative π-meson, a lifetime of (2.5±0.7)×10-10 sec is obtained. Dividing the data into low Q and high Q groups on the basis of the calculated energy release in the decay, a value of τL=(2.9±0.8)×10-10 sec is found for those cases with 0<~Q<~50 Mev and a value of τH=(1.6±0.5)×10-10 sec is found for those cases with 50<Q<~150 Mev. While no significant difference exists between these two values, the difference is greater than for other plausible division schemes which are considered. A qualitative discussion of lifetimes is given for the case of 23 charged V-particle decays. For the charged V-particles these data suggest either a lifetime less than that of the neutral V-particles, provided the sample is homogeneous, or, more likely, an apparent average lifetime less than that of the neutral V-particles, if the sample is a mixture of two or more types of particles. The possibility that κ- and/or χ-mesons make up a part of these decays is considered.

The disintegration of μ mesons in carbon

An experiment is described for the study of some of the properties of μ-mesons stopped in carbon. The μ+ and μ− mesons have been studied separately and we have found very precise apparent mean lifetimes in carbon of τ+=2.22±0.06 μs and τ−=2.18±0.07 μs. The data obtained from these measurements are discussed and compared with those of other experimenters to see if there is a possible difference in the behaviour of the positive and the negative mesons. It is concluded that there are no indications for believing that there is a real difference between the natural mean lives of the μ+ and μ− mesons, although one cannot exclude the eventual possibility of a divergence. it is further demonstrated that there are no indications for believing the energy spectra of the disintegration electrons to be greatly different for the + and − mesons.

The Primary Quantum Yield of Dissociation of Iodine in Hexane Solution

The primary quantum yield for the photo-dissociation of iodine in degassed hexane at 25° has been calculated from measurements of the mean lifetime of the chains involved in the exchange of iodine atoms with trans-diiodoethylene. The primary quantum yield to an accuracy of ±15 percent is 0.59 at 436 mμ and 0.37 at 578 mμ. The deviation of this quantity from unity is affected both by the ``primary recombination'' of atoms which fail to escape from the original ``cage'' of solvent molecules, and by the ``secondary recombination'' of the original atoms by diffusion during a time which is very short compared to that in which either is apt to encounter an atom from another molecule. The specific rate constant for the recombination of iodine atoms in hexane solutions has been determined and has been shown to be about one fifth of the rate constant for collision in the gas phase. The presence of air decreases the rate constant for the photochemical exchange of iodine with diodoethylene by a factor of four and increases the apparent mean lifetime of iodine atoms by a factor of one hundred.