Migration to new ampoule types for the NPL secondary standard ionisation chambers

As the pre-calibrated sample containers used for activity assay in the two NPL secondary standards ionisation chambers are being phased out, suitable replacements have been identified. Characterisation checks have been carried out on the new ISO ampoules and a long-term recalibration schedule has been devised. Around 40 calibration factors have been determined so far and comparison of ion chamber responses for the two ampoule types showed variations of up to 7% for low energy photon emitting radionuclides. - Highlights: • Present sample containers for NPL secondary standard ionisation chambers phased out. • Suitable replacement ampoules identified and characterisation checks performed. • Long-term calibration schedule devised and calibration factors determined. • Effect of new measurement matrix significant for low energy emitting radionuclides. • Recalibration enables maintenance of NPL's secondary standards of activity