Contemporary alpha ray spectrometry suffers from number of shortcomings: e.g. high time consumption, requirement of expensive chemicals and electrodeposition equipment as well as changing electrodeposition yields. Nevertheless, these and other unfavorable aspects have to be accepted to obtain decay informations about the alpha particle emitting radionuclides.
The aim of our current project is to overcome most of these unfavorable aspects. One way to reach that goal is to couple the chemical sample preparation with the detection process itself. This could be realized by modifying the detector surface with an extraction agent. Based on the type of grafted extraction agent, the well-known radiochemical separation techniques [KRA13] could be performed directly on the surface of an alpha detector. In order to test the feasibility of such an approach the well-established simulation programs SRIM (the Stopping and Range of Ions in Matter) [ZIE10] and AASI (Advanced Alpha Spectrometry Simulations) [SII05] were used to predict the fundamental properties of alpha spectra obtained by grafting a specifically bonding layer to a detector surface.
Figure 1 shows an example of such a simulation where 4.2 MeV alpha particles were emitted from a reasonably thick water layer (30 µm) towards a detector with a thin grafted layer (red line). As one would have expected, the spectrum shows a wide range of alpha-particle energies with no structure up to the emission energy of 4.15 MeV. If one simulates, on the other hand side, the decay from the grafted layer a spectrum is obtained with high resolution as indicated by the low full width at half maximum (blue line). In a real system one would expect a superposition of both contributions.
Based on these promising results a practical approach was attempted. As a proof of principal, alpha detectors were modified with sulfonic acid functional groups, a renowned class of strong cation exchangers [INT86]. It was possible to attach [238U, 234U] uranyl cations onto the detector surface immersed in a solution containing these ions while acquiring the spectrum. In fact, the obtained spectra closely resembled the results of the simulation. One observes clearly distinguished peaks over a continuum. Moreover, by applying a hydrochloric acid solution, the uranyl cations could be redissolved. The resulting spectra taken from demineralized water showed neither the continuum nor the typical alpha peaks at 4.18 and 4.75 MeV, respectively.
The ability to graft molecules by covalent bonding to detector surfaces enables the selective extraction and detection of radionuclides in aqueous solution. This innovation could be beneficial to all applications of alpha ray spectrometry, from environmental measurements to the investigation of the actinide and transactinide elements.
[KRA13] Kratz, Jens-Volker; Lieser, Karl Heinrich Nuclear and Radiochemistry: Fundamentals and Applications. 3., rev. ed. 2013. Wiley: Weinheim
[ZIE10] Ziegler, James F.; Ziegler, M. D.; Biersack, J. P. SRIM – The stopping and range of ions in matter (2010). Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 2010, Vol. 268, 1818–1823
[SII05] Siiskonen, T.; Pöllänen, R. Advanced simulation code for alpha spectrometry. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 2005, Vol. 550, 425–434
[INT86] International Atomic Energy Agency Ion Exchange Technology in the nuclear fuel cycle. 1986: Vienna (Austria)