Positron Annihilation Spectroscopy

Positron Annihilation Spectroscopy (PAS) experimental setup
Figure 1. Gamma detectors used for CDBS and PALS techniques

CDBS spectra for Cu implanted polyethylene membranes
Figure 2. CDBS spectra for polyethylene membranes of 50 microns thickness implanted with 90 keV Cu
PALS spectra for Cu implanted polyethylene membranes
Figure 3. PALS spectra for polyethylene membranes of 50 microns thickness implanted with 90 keV Cu
Coincidence Doppler Broadening Spectroscopy (CDBS) and Positron Annihilation Lifetime Spectroscopy (PALS)
Responsabili: Florin Constantin, Marin Focșăneanu, Marta Petruneac

Positron Annihilation Spectroscopy (PAS) is an established analytical technique used to investigate the internal structure of materials, and likewise a nondestructive method and sensitive at the molecular (sub-nanometer) level. Developing high quality positron spectroscopy experiments can aid in the understanding of materials evolution over time in damaging environments, or precisely monitor material behaviour during fabrication and processing.

In order to analyze and study the materials of interest we are able to simultaneously apply two different measuring techniques of positron annihilation spectroscopy, i.e.:
• Positron Annihilation Lifetime Spectroscopy (PALS) which provides information on the electronic density of the annihilation site, because the positron lifetime is sensitive to the size, but also to the concentration of defects in the material. Following the latest methodological developments, the PALS acquisition system is now a digital one, which has obvious advantages, such as online data acquisition, offline use of filters, while maintaining the temporal resolution of the analog system, but with a considerably shorter acquisition time.
As an example (photo on the left), this system was used in the study of a polyethylene membrane superficially implanted with copper ions aiming to understand the influence of the copper ions on the sample’s properties.
• Coincidence Doppler Broadening Spectroscopy (CDBS), a technique that concerns obtaining the impulse distribution of electrons from the studied medium providing information about the chemical vicinity surrounding the annihilation site.

Both ion implantation and positron annihilation spectroscopy measurements were performed using the device shown in Figure 1 in order to ensure that the vacuum is not interrupted and therefore the sample is not contaminated. The device contains in the center a rotating aluminum support with the role of dissipating heat during implantation. The sample is placed on this support which can be rotated 360 degrees as a means to allow the sample exposure to the ion beam and subsequently to the positron source. The positron source, in the form of a small metal disk, is placed at the end of a rod with the role of pressing the source on the implanted face of the sample, in the center of the PAS arrangement.

Positron annihilation spectroscopy data examples obtained on a Nafion membrane implanted with copper ions are shown in the Figure 2 and Figure 3.

For further details you are welcome to take into consideration the references presented below and likewise to contact us for more detailed information.

[1] F. Constantin, M. Focsaneanu, M. Petruneac, Romanian Journal of Physics 65, 901 (2020)
[2] F. Constantin et al., Digest Journal of Nanomaterials and Biostructures Vol. 6, No 2, April - June 2011, p. 543 - 548

Positrons gun
Figure 4. Slow positron electrostatic accelerator (0-50 keV) developed in DFNA
Monoenergetic Positron Beam under Development
Contact persons: Florin Constantin, Liviu Crăciun, Mihai Straticiuc

High quality positron beams with tunable energy are of paramount interest with respect to defects depth profiling. To achieve such an ambitious goal one may consider to utilize a compact cyclotron as a primary source of β+ emitters. ACSI TR19 cyclotron was commissioned at IFIN-HH back in 2012 and it represents a machine mainly dedicated to short-lived radioisotope production with medical applications. We aim to connect the slow positron accelerator (Figure 4) with the TR19 cyclotron in order to validate a concept that will represent a main tool in the characterization of various materials.

For more details please see the papers below or contact us.

[1] M. Straticiuc et al., Optoelectronics and Advanced Materials – Rapid Communications, Vol. 6, No. 9-10, September - October 2012, p. 836 - 839
[2] A. Vasilescu et al., Applied Surface Science 255 (2008) 46–49
[3] F. Constantin et al., AIP Conf. Proc. 1099, 960, 2009

Analytic techniques
Figure 5. A comparison between PAS and other (more) conventional analytical techniques: Scanning tunneling microscopy (STM), Atomic Force Microscopy (AFM), Neutron Scattering (nS), Optic Microscopy (OM), Transmission Electron Microscopy (TEM). Green rectangle encompases the field of semiconductors production in terms of PAS sensitivity and probed depth.