PHOTON SPECTRA IN NPL STANDARD RADIONUCLIDE NEUTRON FIELDS

Area of Science:

• Nuclear Physics
• Radiation Detection and Measurement

Background:

• Radionuclide neutron sources are crucial for various applications, but accurate characterization of their emitted photon spectra is essential for radiation safety and metrology.
• Previous measurements of photon spectra from these sources may lack sufficient detail or temporal analysis, impacting dose assessment.
• High-Purity Germanium (HPGe) detectors offer superior energy resolution for precise photon spectroscopy.

Purpose of the Study:

• To measure and analyze the photon energy spectra emitted by key radionuclide neutron sources at the National Physical Laboratory (NPL).
• To investigate the temporal evolution of photon spectra, specifically for Californium-252 (252Cf) sources.
• To utilize the obtained spectral data to refine photon-to-neutron dose equivalent ratios.

Main Methods:

• Utilized a characterized High-Purity Germanium (HPGe) detector to acquire pulse height spectra from radionuclide neutron sources (252Cf, 241Am-Be, 241Am-Li, 241Am-B).
• Developed a response matrix for the HPGe detector through characterization and modeling.
• Applied spectral unfolding techniques to the measured pulse height data to derive photon fluence spectra.

Main Results:

• Measured and presented photon fluence spectra for several radionuclide neutron sources.
• Observed and documented significant changes in the photon spectrum of a 252Cf source over time, comparing a 2-year-old and a 42-year-old source.
• Demonstrated the transition from a continuous spectrum to distinct peaks attributed to long-lived Californium isotopes in aged 252Cf sources.
• Presented spectra from other radionuclide sources, providing valuable spectral data.

Conclusions:

• The temporal evolution of photon spectra from 252Cf sources is significant and characterized by the emergence of long-lived isotope emissions.
• The newly acquired and analyzed photon spectral data enable more accurate determination of photon-to-neutron dose equivalent ratios.
• This work enhances the metrological capabilities at NPL for radionuclide neutron sources and radiation dosimetry.