Neutron-activation revisited: the depletion and depletion-activation models

Characterization of Gafchromic™ film response against radionuclide activity

PURPOSE

In this study, we used Gafchromic™ film XR-QA2 and RT-QA2 to characterize the film energy response against various radionuclides. We introduce a neutron depletion theoretical model that can describe film response as a function of cumulated activity. The film response was investigated with respect to different backscatter media such as polystyrene, perspex, lead and corrugated fibreboard carton (CFC). The sensitivity of the two types of film to different energies was also studied. Lastly, a film stack method was tested to allow the user to obtain sequential, cumulative doses at different time points.

METHODS

Pieces of Gafchromic™ film XR-QA2 and RT-QA2 were exposed to Am-241, Cs-137, Tc-99m, and I-131 to obtain various cumulative activities. After 24 h, each film piece was digitized by scanning it with an Epson Perfection V330 flatbed scanner to obtain 48-bit RGB TIFF images. Afterwards, each image was processed with the Image J software package. The film response was fitted to a theoretically derived function based on the neutron depletion model and the Beer-Lambert Law and compared with an existing fitting function. Layers of the film were also placed together and irradiated with the above-mentioned radionuclides to investigate the possibility of increasing the sensitivity of the film as a dosimeter. The energy response of the two types of film was investigated by irradiating pieces of film with different photon energies.

RESULTS

The theoretical response model fits OD vs cumulative activity accurately. XR-QA2 film shows good energy film response by using CFC as a backscatter material when using radionuclides. From the results, it is also evident that XR-QA2 is more sensitive to low energy gamma rays than RT-QA2. Its OD sensitivity can be increased by 2 ± 0.2 when using a double layer film and by 2.8 ± 0.3 when using a triple-layer film. By using a film stack, the experimental time can be decreased by using the second-order polynomial relationship obtained to relate the stacked film data to the single film data.

CONCLUSIONS

The neutron depletion theoretical model is accurate and contains less free parameters than higher-order polynomial fits. The Gafchromic™ XR-QA2 film is also better to use in nuclear medicine because of its higher sensitivity. The sensitivity of the film as a dosimeter can also be increased by using multiple layers of film. Experiment times can also be decreased by using the film stack method.

Efficient 169 Yb high-dose-rate brachytherapy source production using reactivation

PURPOSE

To present and quantify the effectiveness of a method for the efficient production of ¹⁶⁹ Yb high-dose-rate brachytherapy sources with 27 Ci activity upon clinical delivery, which have about the same dose rate in water at 1 cm from the source center as 10 Ci ¹⁹² Ir sources.

MATERIALS

A theoretical framework for ¹⁶⁹ Yb source activation and reactivation using thermal neutrons in a research reactor and ¹⁶⁸ Yb-Yb₂ O₃ precursor is derived and benchmarked against published data. The model is dependent primarily on precursor ¹⁶⁸ Yb enrichment percentage, active source volume of the active element, and average thermal neutron flux within the active source.

RESULTS

Efficiency gains in ¹⁶⁹ Yb source production are achievable through reactivation, and the gains increase with active source volume. For an average thermal neutron flux within the active source of 1 × 10¹⁴  n cm^-2  s^-1 , increasing the active source volume from 1 to 3 mm³ decreased reactor-days needed to generate one clinic-year of ¹⁶⁹ Yb from 256 days yr^-1 to 59 days yr^-1 , and 82%-enriched precursor dropped from 80 mg yr^-1 to 21 mg yr^-1 . A resource reduction of 74%-77% is predicted for an active source volume increase from 1 to 3 mm³ .

CONCLUSIONS

Dramatic cost savings are achievable in ¹⁶⁹ Yb source production costs through reactivation if active sources larger than 1 mm³ are used.