Potential errors in the radioassay of 125I

Area of Science:

• Nuclear medicine
• Radiochemistry
• Analytical chemistry

Background:

• Accurate quantification of Iodine-125 (125I) is crucial for radioassays.
• Well-type scintillation crystals are commonly used for detecting gamma-emitting isotopes like 125I.
• Understanding the impact of sample positioning on counting efficiency is essential for reliable results.

Purpose of the Study:

• To investigate the radioassay of very small 125I samples.
• To evaluate the effect of sample position within conventional and side-hole well crystals on counting accuracy.
• To analyze the contribution of primary and sum-coincidence peaks to the overall count rate.

Main Methods:

• Utilized a multichannel analyzer to count 125I samples at various positions within conventional and side-hole well crystals.
• Simulated single-channel pulse-height analyzer analysis by summing counts over specific energy ranges (24 keV, 30 keV, 56 keV).
• Assessed the independence of the primary peak and the positional dependence of the sum-coincidence peak.

Main Results:

• The primary 125I peak count was independent of sample position in both crystal types over several centimeters.
• The sum-coincidence peak contribution varied significantly with sample position in conventional well crystals.
• The side-hole crystal demonstrated a 2-cm plateau for sum-coincidence peak counts, indicating improved positional stability.

Conclusions:

• Sample positioning has a negligible effect on the primary 125I peak, ensuring reliable measurements.
• Conventional well crystals are susceptible to positional variations affecting sum-coincidence counts.
• Side-hole well crystals offer enhanced positional stability for sum-coincidence measurements, improving assay reliability.