K. Rytsölä

Digital measurement of positron lifetime

A prototype of a digital positron lifetime apparatus is presented. We demonstrate that direct digitizing of detector pulses and subsequent simple analysis with software, can be used to replace the conventional analog electronics chain (constant-fraction discriminators, time-to-amplitude converter and multichannel analyzer). In this work, we use a fast digital oscilloscope. The quality of the lifetime data is shown to be as good as with a usual apparatus. For a pulsed positron lifetime beam the digital system is particularly suitable as no coincidence detection is needed and only one analog pulse has to be analyzed.

High-intensity variable-energy positron beam for surface and near-surface studies

Abstract We report on the construction and performance of a magnetically confined slow positron beam. The beam is designed for surface physics and near-surface disorder studies and it operates in ultrahigh vacuum with a base pressure of 3 nPa. The beam uses a Co-58 source with a backscattering W(110) single crystal moderator. We get 4.7 × 106 slow positrons per second striking the sample using a 300 mCi source corresponding to a fast-to-slow positron conversion efficiency 0.28 ± 0.02%. The energy of the incident positrons can be varied from 1 eV to 35 keV. Some applications concerning positron re-emission and diffusion as well as positronium desorption from a surface state are shown.

An experimental study of positronium bubbles in helium fluids

The bubble formation around a positronium atom has been studied in both 3He and 4He by lifetime measurements at temperatures 1.4-40K and at pressures up to 70 bar. Bubble formation is observed at all these temperatures and in all phases. The bubble is found to be almost as large in the solid as in the liquid. No qualitative difference is found between the two isotopes. The lifetime data are analysed using the square-well model, which is found to be valid in a well defined density region. The values obtained for the well depth correspond to the Ps-He scattering length as=0.75 AA. The bubble radii are about 15 AA at low pressures, decreasing to close to 7 AA at the highest pressures. The transition from free to bubble state with increasing density is found to occur without any abrupt change in the annihilation rate. The transition is reasonably well defined below 10K but becomes diffuse with increasing temperature.

Positron annihilation in xenon

Positron lifetime measurements in the temperature range from 150 to 340 K and pressures up to 120 bar show localisation of positronium in all the three phases of xenon. The density functional calculations at 300 K combined with the density fluctuations qualitatively agree with the experimental results. Clustering of Xe atoms around positrons is not quite evident in the experiment results but it is supported by the theory. Incomplete thermalisation of the positron seems to show in the experimental data. A new explanation is suggested for the low positronium fraction extracted from the lifetime spectra at low densities, namely that it may be associated with positronium annihilating during slowing down in analogy with positrons annihilating prior to thermalisation.

Localized state of positron in argon

Abstract Positron lifetime measurements over the temperature range 100 K to room temperature and at pressures up to 80 bar suggest that below room temperature clustering of argon atoms occurs around the positron. The experimental results are well reproduced by density-functional calculations with the value (-3.0±0.2) a 0 as the e + -Ar scattering length.

Successful implementation of fast preamplifiers in a positron lifetime spectrometer

A method to improve the long-term stability of fast coincidence apparatuses, like a positron lifetime spectrometer, is reported. Ageing of the photomultiplier tubes (PMT), i.e., nonreversible degradation of the gain, can be slowed down by lowering the supply voltages over the PMTs and by compensating the lower gain with fast preamplifiers set at the anodes. The timing characteristics of the PMTs can be preserved by using voltage dividers with which the voltage in the input optics of the PMTs remains high enough for good photoelectron collection efficiency (for XP2020 above 300 V). With this setup, the anode current and the rate of gain degradation can be reduced at least by a factor of 20 with no loss in the time resolution of the spectrometer.

Positronium bubble in liquid helium

Accurate positron lifetime measurements have been performed in liquid helium-4 under saturated vapour pressure between 4.2K and 1.5K. The annihilation rate of free positrons is found to be proportional to the density of helium, indicating that no clustering of atoms occurs around the positrons. The lifetime of the orthopositronium varies from 99.1 ns to 9.14 ns. The structure of the positronium bubble is studied by combining the measured pick-off rates of the orthopositronium with the existing 2 gamma angular correlation data on the parapositronium. Both the bubble radius and the depth of the potential well inside the bubble are determined from experimental results.

Scattering Effects in a Positron Lifetime Beam Line

Electrons can be used as test particles of the pulsed positron lifetime beam. The beam should be designed so that electron scattering effects do not distort the timing spectrum. To study these effects we have developed a simulation program which takes into account the collisions of the particles with beam structures and calculates the final time distribution. This program has been used in the design of an improved chopper for positron lifetime beam.

Positron annihilation and cluster formation in nitrogen fluid

The annihilation of positrons in nitrogen has been studied both experimentally and theoretically. Lifetime measurements over the temperature range 60 to 400K and at pressures up to 80 bar show that below 300K clustering of nitrogen molecules occurs around the positron. Experimental data are compared to those in other gases. Results of density functional calculations are in very good agreement with experiment and indicate the value -2a0 for the e+-N2 scattering length. The effective electron number Zeff for nonlocalised positrons is observed to decrease with density from 30 at low densities to 12 in the liquid. Calculations show that this effect cannot be due to multiple scattering. Instead, the effect is explained to be caused by the energy dependence of Zeff.

Linearity Tests of a Digital Positron Lifetime Spectrometer

We present linearity test results for a completely digital p ositron lifetime spectrometer (DPLS) based on the 1 ch, 2 GS/s Acqiris DP210 digitizer. In genera l the performance of the apparatus is comparable to that of an analog spectrometer. A particula r feature of the linearity of the digital apparatus is that one can observe ”semidifferential” nonli earity arising from the fact that the anode pulses are sampled with finite number of samples with discrete a mplitudes and thus the number of different digitized anode pulses is finite. This means that a lso the possible time intervals measured by the apparatus are discretized. Our measurements show that in positron lifetime measurement setup this semidifferential nonlinearity is smoothed out even at 2 GS/s.