Logo of the Physikalisch-Technische Bundesanstalt

Activity determination and decay data of Cd-113m


113mCd is produced above all by neutron activation in control rods and the screening of nuclear reactors. Due to the half-life of approx. 14 years, 113mCd must be taken into account in the handling and the possible disposal of nuclear waste. The isotope has also been detected in environmental samples in which traces of it are found in the atmosphere as a result of earlier nuclear weapon tests.

Reliable activity measurements of samples with 113mCd have, so far, hardly been possible. As only a few photons are emitted per 113mCd decay, measurements with the aid of gamma spectrometry require detectors with a low background and long measuring times. Up to now, the procedure has only allowed activity measurements to be performed with large uncertainties, as the required photon emission probability was based on only one single measurement which has to be classified as a rough estimation.

Within the scope of a cooperation with the National Oceanography Centre in Southampton (Great Britain), a 113mCd solution with a high radio-chemical purity was made available [1]. The specific activity of the solution was determined by liquid scintillation counting. In this procedure, weighed quantities of the radioactive solution are filled into a liquid scintillator. The scintillation events generated by the radioactive decay are counted in measuring arrangements with two or three sensitive detectors (photomultiplier tubes). To judge the activity from the measured counting rate, the respective detection probability must be known which has been determined with two methods: the CIEMAT/NIST efficiency tracing method and the triple-to-double coincidence ratio (TDCR) method [2]. Both procedures are based on the same free-parameter model. In the CIEMAT/NIST method, the free parameter - which is at first unknown - is determined by measurements with 3H sources of known activity under identical experimental conditions. In TDCR arrangements with three detectors, the free parameter can, however, be determined by the ratio of triple-to-double coincidences. Reference sources are not required for this procedure.

In addition, the photon emissions are measured by means of a semiconductor spectrometer. The detector system is characterized by a low background and has been calibrated with activity standards of PTB. The measurements allowed the specific photon emission rate to be redetermined and, by a combination with the activity measurements of the liquid scintillation counting, the photon emission probability could finally also be redetermined to be (0.01839±0.00029)%. The new value is - by approximately 25% - smaller than the value from 1969 which has been used so far. For the older value, the authors did not indicate any uncertainties, and from the description of the measuring procedure, large uncertainties have to be expected.

Liquid scintillation counting has been continued for more than 11 months to determine also the half-life from the data. In this evaluation, variations of the detection probability have been taken into account by means of efficiency tracing. The result was a half-life of (13.97±0.13) years. Thus, also for this parameter, the uncertainty has also been clearly reduced.


  1. K. Kossert, O. Nähle, P.E. Warwick, H. Wershofen, W. Croudace:
    Activity determination and nuclear decay data of 113mCd.
    Submitted to the magazine Applied Radiation and Isotopes.
  2. R. Broda, Ph. Cassette, K. Kossert:
    Radionuclide Metrology using Liquid Scintillation Counting.
    Metrologia 44 (2007) pp. 36-S52.