Preliminary Results on Low-energy Positron Age-momentum Correlation Measurements Based on a Radioisotope-source Positron Beam

The positronium (Ps) lifetime technique is well documented as a sensitive probing tool for nanometerscaled open-space study of various oxides and polymers [1,2], in order to elucidate their macroscopic properties such as molecular permeability [3] and ion transport [4]. Our group has so far reported on the usefulness of this technique for investigating free-volume holes/nanopores in a variety of functional materials like separation membranes [5,6], hydrophilic polyolefins [7], and silica films [8]. In 2003, our group proved that the positron and Ps momentum distributions for polymers associate with their constituent elements [9], suggesting that Ps may be useful for probing the chemistry of freevolume surfaces. In light of this idea, we applied the positron age-momentum correlation (AMOC) technique to the study of the chemical environment at free volumes in bulk polymers [10, 11]. In this work, in order to extend its applicability to functional thin materials, an energy-variable AMOC measurement system based on a pulsed 22Na-radioisotope based beam at the National Institute of Advanced Industrial Science and Technology (AIST) was developed. Details of the developed system and preliminary results on the data for reference materials, obtained by the developed system, are demonstrated.


Introduction
The positronium (Ps) lifetime technique is well documented as a sensitive probing tool for nanometerscaled open-space study of various oxides and polymers [1,2], in order to elucidate their macroscopic properties such as molecular permeability [3] and ion transport [4].Our group has so far reported on the usefulness of this technique for investigating free-volume holes/nanopores in a variety of functional materials like separation membranes [5,6], hydrophilic polyolefins [7], and silica films [8].In 2003, our group proved that the positron and Ps momentum distributions for polymers associate with their constituent elements [9], suggesting that Ps may be useful for probing the chemistry of freevolume surfaces.In light of this idea, we applied the positron age-momentum correlation (AMOC) technique to the study of the chemical environment at free volumes in bulk polymers [10,11].In this work, in order to extend its applicability to functional thin materials, an energy-variable AMOC measurement system based on a pulsed 22 Na-radioisotope based beam at the National Institute of Advanced Industrial Science and Technology (AIST) was developed.Details of the developed system and preliminary results on the data for reference materials, obtained by the developed system, are demonstrated.

Experiments
An energy-tunable AMOC measurement system based on a pulsed positron beam was developed with a 0.5-GBq 22 Na radioisotope (RI) positron source (Fig. 1).Positron annihilation lifetime (PAL) measurements were performed using a positron pulsing system (FUJI IMVAC Inc., Japan) with a time resolution greater than 250 ps (FWHM), while Doppler broadening of positron annihilation radiation (DBAR) measurements were performed using a high-purity Ge detector with an energy resolution of 1.63 keV at 1.33 MeV and an efficiency of 39 %.In order to obtain AMOC data, coincidence signals from PAL and DBAR were stored in a PC using a digital signal-processing (DSP) module (TechnoAP Co., Ltd., Japan), which allows us to attain highly-stable, precise measurements (Fig. 2(a)).1D PAL measurements were carried out using conventional analog modules as shown in Fig. 2(b).The numbers in the parenthesis for AMOC denote the respective manufacturer part numbers of the DSP modules.The abbreviations, PMT, TAC, DCFD, and MCA for the analog modules of the 1D PAL measurement represent photomultiplier tube, time-to-amplitude converter, differential constant fraction discriminator, and multi-channel analyzer, respectively.To validate the developed measurement system, the PAL data obtained from the projection to the time axis of the 3D plot for fused silica is compared with that obtained using the 1D PAL measurement 3 ■■■ 011302-3 JJAP Conf.Proc., 011302 (2018) system.Figure 5 shows the PAL data for fused silica, obtained by the 1D measurement and the AMOC measurement at E = 5 keV.The time resolution of the data by the AMOC system (∼ 300 ps FWHM) is somewhat lower than that of the 1D measurement system (∼ 250 ps FWHM) due to the larger size of the BaF 2 scintillator (H60 mm × ∅40 mm × ∅46 mm for the AMOC system and H20 mm × ∅50 mm × ∅50 mm for the 1D system, respectively).The analyzed o-Ps lifetime for the data from the 1D system is (1.65±0.003)ns, while that from the AMOC system is (1.60±0.01)ns as mentioned above.Both o-Ps lifetimes for the present fused silica agree well with the certified value of this fused silica CRM, i.e. (1.62 ± 0.05) ns, indicative of the validity of the present PAL measurement system.Figure 6 shows the variations of the S parameter for fused silica and polyethylene at E = 5 keV as a function of positron age.The S parameter was evaluated from the ratio of counts in the central region of the annihilation photo peak (511 keV ± 0.7 keV) to the whole region (511 keV ± 4.1 keV).For both data, S is rapidly decreased as the positron age increases from time T = 0, and at T > 2 ns S for fused silica (S Silica ) and PE (S PE ) reach the respective values of ∼ 0.50 and ∼ 0.56.These values are ascribed to the momentum due to the annihilations of the long-lived o-Ps.The order of the magnitudes of the S values, that is, S PE > S Silica , is consistent with that obtained by means of bulk AMOC measurements reported previously [11].

Summary
An energy-variable AMOC measurement system based on a pulsed RI beam at AIST has been developed for investigating functional thin materials.Preliminary results obtained for fused silica and polyethylene using the developed system were demonstrated.The obtained AMOC data agreed well with the previous data for similar bulk samples.In contrast to AMOC measurements with other positron sources such as an electron linac [14] and a nuclear reactor [15], the developed system with a RI-based source allows us to measure continuously without the need for operating staffs at positron generation.To further improve the measurement efficiency, the sample geometry of the system is being adjusted, and applications to practical thin materials are under implementation.

Fig. 1 Fig. 2
Fig.1Schematic of the developed system with a radioisotope-base pulsed beam for energy-tunable positron age-momentum correlation measurements.(Not to scale) The details of the measurement modules for AMOC and 1D PAL measurements are described in Fig.2.

Figure 3 ( 2 ■■■Fig. 3 Fig. 4
Figure 3(a) and (b) show a 3D plot of AMOC data obtained for fused silica (certified reference material NMIJ CRM 5601-a [12]) and polyethylene (PE), respectively, measured at a positron incident energy E of 5 keV.A total of ∼ 4 M counts, recorded with a counting rate of ∼ 5 cps, were accumulated for each sample.Figure4shows the respective projections to the time axis (a) and the momentum axis (b) for fused silica and polyethylene.A significant difference between both data for the present samples is visible in the lifetime and momentum spectra.To evaluate the long-lived ortho-Ps (o-Ps) lifetime component, each lifetime spectrum was analyzed using the RESOLUTION software code[13] assuming three lifetime components.The obtained lifetimes τ and relative intensities I of o-Ps for fused silica and PE were τ = (1.60 ± 0.01) ns, I = 57 % and τ = (2.40 ± 0.02) ns, I = 31 %, respectively.

Fig. 5
Fig. 5 Positron annihilation lifetime data at E = 5 keV for fused silica, obtained by the 1D and AMOC measurements.

Fig. 6
Fig. 6 Variations of the S parameter for fused silica and polyethylene at E = 5 keV as a function of positron age.