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Rapid developments in genetic science have been accompanied by confusion regarding the predictive power of DNA-based tests and in the impact of such tests on the insurance industry. The United Kingdom actuarial profession has begun to engage in the associated social policy issues and to try to throw some light on the issues through quantitative research and objective analysis. At the same time, many insurance industry actuaries have been involved in work on behalf of the insurance industry to develop a sound basis for permission to be sought to make use of the results of certain predictive genetic tests. This paper briefly outlines some of the history of the development of the debate in the U.K. and draws together some of the debates and discussions which have taken place within the Genetics Group of the Social Policy Board of the U.K. actuarial profession, as well as providing some pointers to the directions which the debate might take in the future, with some important potential consequences for the insurance industry and for actuaries.

We have made new measurements of the lifetimes at 4.29 K and 2.92 K in He I, and at 2.14 K and 1.67 K in He II, without and with an electric field. At 4.19 K we have a new value of the lifetime, 2.01 +/- 0.002 ns, leading to a Z(eff) of 3.53 at zero field. In contrast to earlier reports, Z(eff) decreases very slightly as the temperature is lowered and the density increases. The orthopositronium lifetime at 4.19 K is 98.1 +/- 0.2 ns, at zero field, independent of field up to 18 kV cm(-1). At 4.19 K the free positron decay rate falls exponentially with increasing field to a value 5% below its zero-field value, the drop-off rate being 0.36 +/- 0.04 cm amagat V-1. From 28 through 50 V cm(-1) amagat(-1) the decay rate of the free component rises slowly as expected because of the increasing positronium fraction over the same range of electric field. In liquid argon there is positronium enhancement beginning at or below 3 V cm(-1) amagat(-1), which contrasts with 160 V cm(-1) amagat(-1) for argon gas. At least two lifetimes make up the long triplet component, suggesting different degrees of thermalization, or different states of positronium within the liquid. The effect of the electric field is to enhance the longer-lived, or more thermal component, in strong contrast with the findings of Charlton et al for the gas. The argon results suggest positron behaviour similar to that of electrons in the liquid phase, namely, such particles move in a conduction band subject to small effective scattering and diffusion cross sections. An analytic model in which the scattering length is an adjustable parameter yields the onset of positronium enhancement just below 3 V cm(-1) amagat(-1) when a(s) = -0.043 a(0), corresponding to an elastic scattering and diffusion cross section of 0.034 pi a(0)(2). Consequences for positron thermalization are discussed. A model of positronium losing energy slowly in the liquid without bubble formation is suggested.

We have made new measurements of the lifetimes at 4.19 and 2.92 K in HeI, and at 2.14 and 1.67 K in HeII, without and with electric field. The ortho-positronium lifetime is independent of field up to 20 kV/cm. At 4.19 K, there is no increase observed in the positronium fraction from zero field to 20 kV/cm, in strong disagreement with Manuzio and Rizzuto. Some additional positronium is, however, formed above 20 kV/cm, with a corresponding decrease in lifetime of the free component. At 4.19 K the free positron mean life rises sharply with increasing electric field to a constant value of about 2.14 ns above 5 kV/cm. The increase in the free positron lifetime with field is less marked at 2.92 K. In liquid argon measurements were made at the normal boiling point only, at fields up to 20 kV/cm, above which we experienced electrical breakdown. The ortho-positronium intensity is 7% at zero field and begins to increase at 2.5 kV/cm, reaching 13% at 20 kV/cm at which field value it was still increasing. The ortho-positronium lifetime displayed an 8% increase over this field range. For liquid nitrogen we repeated some of our earlier measurements at 77.3 K and made some measurements at 64.5 K so as to test models for positronium formation in the spur. A good fit to a simple exponential formula is obtained at both temperatures, yielding a single drop-off rate of 0.21 cm/kV.

1.1 This paper owes its existence on two counts to the late Jim Souness whose term of office as President of the Faculty coincided with the bulk of our research. Firstly, as President, he encouraged Faculty research groups to be active and to produce material worthy of sessional papers. More directly it was the late President who funnelled our general investigations of life office management (which had continued after the Group's 1987 sessional paper) towards the issue of demutualisation.There have been several notable demutualisations of life offices in recent years, yet there is little published research in the UK other than case studies into the actuarial issues which these restructurings raise for the profession. Furthermore much of what has been published pertains to overseas regimes not subject to UK-style regulation. However at the time of writing we are aware that another paper (reference 20) was being written concurrently, and it relates specifically to the UK.

The authors have measured the transmission of a positron beam through helium, neon and argon gases from 4 eV up to the positronium formation thresholds. A variable longitudinal magnetic field affects the transmission and allows some degree of phase shift separation, analogously to the performance of an angular distribution experiment. The results for neon and argon are presented.

The total cross sections for positrons on neon and argon atoms have been measured in the energy ranges 15 eV to 272.5 eV and 25 eV to 300 eV respectively. The cross sections indicate clearly that Born values will not be reached until at least 3 KeV. Interpolating between the measured and the valid Born regions has allowed an application of the sum rule which connects scattering length. Born forward scattering amplitude, and the momentum-integral over the total cross section. This procedure gives scattering lengths as = −0.53 ± 0.15 Bohr radii for neon and as = −2.8 ± 0.7 Bohr radii for argon; the errors include maximum credible uncertainties in the interpolations.

Formulae are derived for the expected shapes of delayed coincidence spectra when an exponential decay is measured using a simple circuit having a time-to-amplitude converter or equivalent digital timing device. In the limiting cases when the start pulse rate is low or when the fraction of start pulses which have a related stop is low, the formulae reduce to that of Coleman and Griffith (1973).

The total cross sections for the scattering of positrons by helium have been measured by the method of transmission in the 16 to 270 eV energy range. The experimental results are higher than those of Canter et al. but are in reasonable agreement with recent results of Griffith et al., and at high energies tend towards Born approximation calculations. The integral of the cross section over positron momentum is smaller than the sum rule estimate made by Bransden et al. A tentative value of (0.034 ± 0.017)πa02 is assigned to the positronium formation cross section at threshold.

The lifetimes of positrons have been measured in pure neon gas with and without the application of an electric field and in neon–nitrogen mixtures. The results for pure neon are complicated by not quite pure exponential decays and internal consistency arguments have been used to set minimum values upon the equilibrium (or thermal) value of Zeff and upon the thermalization time. Annihilation in neon–nitrogen mixtures provides an extrapolated Zeff for thermal positrons of 6.02 ± 0.16. This result is consistent with the results for pure neon and enables the thermalization time to be set at a minimum value of 2700 ns amagat. Comparison with the theory of Montgomery and LaBahn and also with the phase shifts of Jaduszliwer and Paul suggests that the true thermalization time in pure neon may be more than twice this value, the possible discrepancy being explainable on the basis of one to two parts per million polyatomic impurity in the neon gas. The Zeff of Montgomery and LaBahn in the '2p → d and 2s → p norm.' approximation is 20% higher than the experimental value. Results under electric field show no unexpected features up to 31.9 V cm−1 amagat−1.

Measurements of positron annihilation rates at room temperature under the influence of an applied electric field have been made in methane at different pressures. The experiments were all carried out in the linear region in which the annihilation rate is accurately proportional to gas density. We find Zeff = 142.7 ± 2.0 at 21 °C and zero electric field. At 1000 Torr Zeff decreases linearly with the electric field, the gradient being about 2.5 × 10−2 per (V cm−1 atm−1) at that temperature. The results repudiate the idea of a positive energy single level Breit–Wigner resonance as being responsible for the high value of Zeff, and in this sense are in agreement with the findings of McNutt, Summerour, Ray, and Huang.

Electron-electron angular correlations have been measured for the ionization of molecular hydrogen and nitrogen by electron impact at 100 eV incident electron energy and also at 250 eV for hydrogen. A total of 18 cases were studied at various secondary energies and primary scattering angles. Strong binary peaks were observed in all cases resembling what would be expected from the ionization of atoms having similar electronic structures. The recoil peaks which were observed were, however, generally weak, especially with nitrogen. A first attempt at determining part of the six-fold differential cross section for molecular hydrogen was made by varying the energy of the lower energy secondary electron while all other scattering parameters were held constant. The result was obtained in the binary peak and shows the vibrational excitation of the residual ion. The observed structure is in reasonable agreement with a computed curve based on Dunn's calculated Franck-Condon factors (1966) for the ionization of H2.

We have determined the collision cross section of positronium in He, Ne, Ar, Kr, Xe, N2, and isobutane gases at thermal energies by studying the diffusion of positronium to the walls of a specially designed chamber. The time distribution of annihilations is measured as a function of pressure for each gas. The presence of an admixture of 20 Torr of isobutane in the chamber quenches the free positron component so that the orthopositronium component is the only exponential decay in the time spectrum of annihilations. Diffusion theory is used to obtain theoretical values of the orthopositronium decay constant λ2. These values are a function of three parameters, two of which are common to all results; the third one is σc, the collision cross section. Monte Carlo simulations of the isobutane experiments verify the theoretical results and determine the wall annihilation parameter which is common to all experiments. The Monte Carlo calculations also enable the determination of the second parameter common to all the experiments, which occurs in a correction term in the diffusion theory. The entire analysis has been carried out twice, one in each of the following extreme cases: (1) assuming that no triplet to singlet conversion takes place at the walls of the cells, and (2) assuming that the maximum conversion takes place at the walls.The cross sections in case (1) are, in units of πa02: He, 0.0166 ± 0.0017; Ne, 0.042 ± 0.003; Ar, 0.086 ± 0.005; Kr, 0.099 ± 0.006; Xe, 0.128 ± 0.008; N2, 0.238 ± 0.014; and isobutane, 0.534 ± 0.016. The implications for other types of experiment and theory are briefly discussed, in particular the bubble model for positronium in liquids.

The angular correlation of outgoing electrons has been studied with improved apparatus for ionization of neon atoms by electrons of 100 eV and 250 eV. Measurements were made of the 2p electron ionization for 11 different sets of parameters. In particular, the angular correlation curves were studied for each of the incident energies keeping the momentum K0a transferred by the fast electron and the energy Eb of the slower emitted electron unchanged. Results are compared with one of the models of Knapp and Schulz (1974). All angular distributions from K0a=0.5 to K0a=1.2 possess double or split lobes in the binary collisions angular region.

Measurements of positron drift in H2 gas have been made in the pressure range 10-200 Torr between 0 and 4 Td. A shape analysis gives a universal function W/Zeff. Using the annihilation parameter Zeff=13.6, the drift speed W increases linearly up to 2.9*105 cm s-1 at 1 Td. Above 1.4 Td there is evidence for positronium formation. These are the first measurement of positron drift speeds.

The theory of electron drift and longitudinal diffusion in a uniform electric field is extended to positrons. The time- and position-dependent positron current is integrated to give a simple formula for the interpretation of positron drift experiments. The treatment is mathematically exact but neglects the transient attainment of a steady speed distribution of the positrons, and does not attempt to treat the boundary condition rigorously. These approximations are discussed, and justified under certain conditions.

Experimental values are presented for the andnihilation rates in gases of slow positrons that have failed to form positronium. The observed values for argon, methane, ethane, propane, n-butane, isobutane, and carbon tetrachloride are found to exceed the values derived from Dirac theory by factors of 3 to 700. A Na/sup 22/ positron source was used in a cylinder where the sample gases were at up to 10 atm. and a constant (25 plus or minus 5) deg C. The effects of molecular electric polarization are discussed. (R.E.U.)

Non-intercepting monitors for measuring the total current or charge passing down the electron drift tubes have been constructed which are suitable for electron pulses from about 50 nanoseconds to 3.5 microseconds in width. Measurements are derived from output signals which are tuned to frequencies in the range 250 kcs to 45 kcs. Alternatively, for viewing pulse envelopes, ferrite coil outputs are integrated by a 25 ohm termination. The monitors are calibrated using an air cooled Faraday cup which is described.

Positron cross sections on helium have been measured by transmission using a weak beam of energy-analyzed positrons in the range 15 eV to 23 eV. The positrons are transmitted in a magnetic field parallel to the axis of the scattering chamber. Comparisons with theory have been made using several sets of phase shifts, and good agreement is found if the latest p- and d-wave phase shifts of Drachman are used. The experiments fail to show any sharp reduction in the transmission at the positronium threshold, from which we deduce an upper limit to the positronium formation cross section of [Formula: see text]. The agreement between our results and those of Costello et al. in the neighborhood of 17 eV is very good.

The time taken for positrons from a radioactive source to reach a velocity distribution in the near-thermal range has been measured for mixtures of nitrogen gas with argon, and of methane with argon. An analysis of the shoulder breadths for the free positron components leads to slowing down times of 16 ns for nitrogen and 0.25 ns for methane at densities of 1 amagat. The long slowing time for nitrogen must occur during the last eV or so of slowing and the dominant mode of energy loss is attributable to rotational excitation of the molecules. For methane the slowing to the threshold for vibrational excitation must be extremely rapid, but there is no clearly established mechanism which could account for the remaining energy loss taking place so rapidly.

The annihilation of positrons in methane gas at room temperature has been measured at pressures below 2 atm. The annihilation rate for the free positron component is proportional to the gas density and corresponds to an effective number of electrons per molecule taking part in annihilation, Zeff = 139.6 ± 1.0 (standard error). The quenching of orthopositronium corresponds to 1Zeff = 0.57 ± 0.07. Resonance annihilation is discussed in a general way.

The total cross sections for elastic scattering of positrons in the energy range from 4 to 19 eV have been measured by the method of transmission. By varying a magnetic field applied along the axis of the scattering chamber the transmitted fraction of the beam is altered, from which individual phase shifts can be extracted. s-, p-, and d-wave phase shifts are given over the entire energy range. The s-wave phase shifts are in agreement with values published by Drachman in 1968, while the p- and d-wave phase shifts are intermediate between values calculated by the same author in 1966 and 1971. The experimental results agree with those of Costello et al., and marginally with our own 1972 results, but are significantly different from those of Canter et al. We compute that the Ramsauer minimum in the diffusion cross section must be 0.04πa02 at 1.6 eV while the minimum in the total cross section is 0.11πa02 at 2.1 eV. The shoulder breadth observed in annihilation experiments is in nice agreement with what one would predict from our phase shifts.

The design and philosophy of a new current-mode trigger pulse generator are described. The pulse generator uses two transistors in a coupled emitter configuration and only ordinary components. The output impedance at collectors is 93 ohms though this may be reduced where it is desired to match other transmission line impedances. Rise time is less than 3 nanoseconds with jitter in the 10-11 second range. Pulse output is ±2.5 volts and extremely clean and square with little undershoot or overshoot. Pulse length may be chosen, in the normal way, to be from ~10 nsec. up to microseconds. The circuit may be retriggered as soon as the output has recovered to zero. Its layout will be discussed and also its use as a replacement for limiter circuits in fast coincidence measurements.

The diffusion and annihilation of slow positrons have been studied by the method of delayed coincidences for helium at 77 °k and densities of 20-70 amagat. The non-exponential shoulder in the free-positron component is confirmed and has a total breadth of more than 1300 ns amagat, corresponding to a Ramsauer-Townsend-type minimum in the diffusion cross section Q of about 0031 πa02 at an energy of 17 ev. Application of electric fields of up to 33 v cm-1 amagat-1 failed to enhance positronium formation, contrary to the findings of Marder et al., we find Q > 0115 πa02 around 177 ev. Zeff is found to be 3677 ± 0025 at thermal energy, dropping to about 33 near 1 ev, in agreement with recent experiments of Lee and Jones. Drachman's theoretical s-wave phase shifts (quasi-lower bounds) appear to be very reliable and we have set an upper limit on these phase shifts by constructing intermediate values and comparing thermalization times with experiment. Drachman's values of Zeff are a few per cent too low below v = 08 ryd1/2, but probably not anywhere by more than 8%. The variation of the positronium annihilation rate with pressure fits an approximately parabolic form above 40 amagat, resembling similar results of Celitans et al. for oxygen-free argon, but at lower densities in helium this annihilation rate becomes anomalous.

The time spectra of positrons annihilating in pure argon gas are found to be characterized by a pronounced shoulder. The shoulder is associated with a velocity-dependent annihilation rate of positrons which are slowing down within the range 9 to 0 eV. The annihilation rate of positrons has been derived as a function of time, and an attempt has been made to convert the results to a function of velocity by establishing a velocity-time density matrix G(v, t). Corresponding theoretical values of the annihilation rates for argon have been calculated from previously published wave functions over the range 0.68 to 6.8 eV. The theoretical values are three times the experimental values at the higher energies in this range. The need to use polarized atomic wave functions for positron scattering and annihilation calculations is clearly indicated. Below 1 eV there is a marked increase in the observed annihilation rate, which is in no way indicated by the theoretical results; this increase is not currently understood but could arise from the formation of an argon-positron ion at low positron velocities.

New results for positron lifetimes in liquid argon confirm earlier results of Paul and of Liu and Roberts. In particular the orthopositronium intensity is anomalously low in this liquid, only 7%. It is shown by experiment that the high singlet positronium and intensity found by Briscoe, Choi, and Stewart could have been due to oxygen impurity.

Departures from the known linear dependence of annihilation rates on gas pressures are described. The annihilation rate per unit pressure in argon is increased by the addition of methane, a thermalization effect which confirms the strong velocity dependence of annihilation for very slow positions in argon. In nitric oxide the annihilation rate varies as a1p + a11p2 where p is the pressure and a1, a11 are constants.

Positron collision studies at low energies using beams of slow positrons below 1 keV appear to have been limited to the following: an unpublished thesis by Cherry1 on secondary electron emission from surfaces under positron bombardment; and two published papers on the e+ - He scattering cross sections by Costello et al.2 and ourselves3. In a second publication of Costello et al.4 they discuss experimental evidence in favour of postulating a negative work function for slow positrons escaping from a metal surface. In the period 1965 to the present there have been several other attempts by various groups to produce slow positron beams, most of them unsuccessful and unpublished. The more recent work by McGowan and his coworkers and by ourselves has all been directed at obtaining a satisfactory beam of slow positrons for atomic collisions or at studying the characteristics of the beam, partly with a view to obtaining a better beam, and partly so as to understand the slow positron emission process about which we shall make some statements in what follows.

A new type of experiment has been devised in which ortho-positronium diffusion to the metal walls of a series of parallel-sided cells is studied by measuring the annihilation lifetimes. Experimental results are subjected to an analysis in which two parameters are varied to give a best fit, one of the two parameters being the scattering cross section. The method requires a number of systematic corrections which have still to be worked out. Preliminary cross section values for helium, argon and isobutane are 0.017 πa02, 0.09 πa02, and 0.54 πa02, respectively.

IN the past ten years there has been much investigation of the interesting phenomenon of microscopic bubbles in liquids, particularly liquid helium, which are maintained by the presence of a light particle, such as an electron1 or positron2. In both cases, with liquid helium the bubble is believed to arise from an effective repulsive potential between bulk liquid and the light particle. Coopersmith3 derived an expression for this effective potential by calculating the free energy of a light particle moving in a space having randomly distributed hard spheres.

IN the course of some recent investigations in experimental physics we have processed a great deal of data with a digital computer using a variety of weighted least-squares fitting programmes. All these programmes, regardless of the specific problem, have the following feature in common. After the best values of the fitted parameters have been obtained a test of each data point is made. If the data point lies more than a specified interval from the best curve, the experimental and fitted values of the ordinate, and the associated uncertainty in the experimental value are punched out. We would like to emphasize the great value of this feature. As a criterion for punch-out a deviation equal to three standard errors was chosen. This corresponds to the well-known 99.3 per cent confidence limit, and eliminates nearly all statistical fluctuations from being punched. Within two months of adopting this procedure we had already experienced several resulting benefits.