65 Zn and 133 Ba standardizing by photon-photon coincidence counting

. The LNMRI/Brazil has deployed a system using X-gamma coincidence technique for the standardizing radionuclide, which present simple and complex decay scheme with X-rays of energy below 100 keV. The work was carried on radionuclide metrology laboratory using a sodium iodide detector, for gamma photons, in combination with a high purity germanium detector for X-rays. Samples of 65 Zn and 133 Ba were standardized and the results for both radionuclides


INTRODUCTION
The importance of the development and implementation of an absolute measurement technique for the radionuclide standardizing is highlighted. It is, in short, the state of the art, due to the complexity and specificity of the radionuclide decay-schemes.
The existence of several techniques enriches and makes the metrological chain robust, in order to provide results with traceability and reliability related to the determination of atomic and nuclear data and the activity quantity.
Techniques for absolute radionuclide standardizing are still few and rare, because their recognition and adoption by the international metrology system requires de confirmation of experimentation and theoretical formulation for years, finally enshrined through key-comparisons promoted by Bureau International des Poids et Mesures (BIPM).
The development of solid Germanium and Lithium detectors has given new possibilities to gammaspectrometry technique such as in radionuclide absolute standardizing by the sum-peak [17,18] and photon-photon [19,20] coincidence counting methods.
This work presents the implementation of absolute radionuclide measurement by photon-photon coincidence counting in LNMRI/Brazil, applied to the standardization of 65 Zn and 133 Ba, examples of simple and complex decays, respectively. In the specific case, the uses of X-gamma emissions do not involve corrections due to angular correlation and simplifies the experimentation and theoretical formulation.
The 65 Zn is used in nuclear medicine and serves to calibrate detectors, as a sodium iodide. The 133 Ba is a radionuclide with half-life relatively long and therefore used to calibrate germanium detectors in low energy region, serving as a source for consistency tests in radionuclide calibrator.

EXPERIMENTAL PROCEDURE
The photon-photon coincidence system was implemented in the LNMRI to standardize radioactive solution with nuclear transitions accompanied by X-rays w emissions below 100 keV. The reduction of HPGe detector efficiency was performed moving away the radioactive sample from detector or introducing absorbers between the sample and the detector. In the figures 1 and 2 are displayed a drawing and a block diagram symbolizing the electronics modules used in the system.   The activities were determined by extrapolation of apparent activities curves in function of the relative inefficiency factor [3], reducing HPGe detector efficiency. To decrease X-rays near 8 keV of intensity, was increased the distance between of 65 Zn samples and HPGE detector by steps of 0.5 cm until the maximum of 5 cm. This movement was done keeping unchanged the distance between the sample and NaI detector, while at the same time the distance from both and HPGe detector was being increased. To decrease X-rays near 30 keV of intensity, were inserted between of 133 Ba samples and detector until 9 aluminum discs, each one with 0.5 cm thick.
The activities were determined by extrapolation of apparent activities curves in function of the relative inefficiency factor [3], reducing HPGe detector efficiency. To decrease X-rays near 8 keV of intensity, was increased the distance between of 65 Zn samples and HPGE detector by steps of 0.5 cm until the maximum of 5 cm. This movement was done keeping unchanged the distance between the sample and NaI detector, while at the same time the distance from both and HPGe detector was being increased. To decrease X-rays near 30 keV of intensity, were inserted between of 133 Ba samples and detector until 9 aluminum discs, each one with 0.5 cm thick.
Three 65 Zn samples were prepared by deposition of standard radioactive solution on a plastic tape fixed at the bottom of an acrylic ring, covered by another similar plastic tape. A total of four 133 Ba samples were prepared on a thin plastic film of polyvinyl chloride (VYNS) and measured by photonphoton system. The validation of this system was carried out with the measuring of the same 133 Ba samples by 4-a coincidence counting system. The measurements were done adjusting each Single Channel Analyzer -SCA with pulse height suitable to detect the selected events. For 65 Zn samples were considered gamma photons of 1115 keV (SCA windows from 7.40 V to 10 V) and X-rays from 8 keV to 9 keV (SCA window from 0.15 V to 0.30 V). For 133 Ba samples were considered gamma photons of 356 keV (SCA windows from 2.60 V to 3.32 V) and X-rays from 30 keV to 36 keV (SCA window from 0.60 V to 0.98 V).

RESULTS
The apparent activities were calculated for each detection efficiency variation by software deployed in the LNMRI/IRD, which uses the coincidence method [21,22,23].
As the selected events for counting are mono-energetic gamma photons and X-rays produced by electron capture, originated in the same decay, and considering too that there's no overlapping in the energy range of the two kinds of selected events, each channel of the instrumentation certainly will register only their respective events and no other one. In these conditions, the N of the BAERG equation (1) for ideal detectors can be substituted by NX to determine all apparent activities with good accuracy without the need to introduce any correction factor in the value found after the experimental curve extrapolation: (1)  where, N0 is apparent activity; NX is -ray count rate of HPGe detector; and N is gamma count rate of NaI(Tl) detector; and NC is the X-gamma count rate coincidence. For each experimental curve, the sample activities were determined through linear regression and extrapolation to zero value of the relative inefficiency factor.
The figure 5 displays the apparent activity curve in function of relative inefficiency factor for the 65 Zn sources. The mean activity concentration found in these samples was 646.02 kBq/g ± 0.56 %, with the uncertainty determination showed in Table 1.
The figure 6 displays the apparent activity curve in function of relative inefficiency factor for the 133 Ba sources.The mean activity concentration found in these samples was 504.23 kBq/g ± 0.39 %, with the uncertainty determination showed in Table 2.   Table 3 Comparison 65 Zn standardization (k = 1) Table 4 Comparison 133 Ba standardization (k = 1)

CONCLUSION
The deployment work of this new X-gamma coincidence system was well succeeded, as shown in table 4 by the comparisons of the 133 Ba samples, which produced difference 0.48 % between the results obtained by X-gamma and 4(PC)-(NaI) coincidence counting, showing that the new system has introduced an important improvement in the LNMRI routines for standardization of samples with radionuclides of complex decay scheme. By the other side, the difference of 0.72 % obtained with the 65 Zn samples shows that is the possibility to standardize radionuclide samples with X-rays in order of 8 keV.