Standardisation of 231Pa by defined solid angle and liquid scintillation methods at NPL.

231Pa is the longest-lived decay product in the 235U decay chain and precise standards of this radionuclide are required to enable accurate, traceable measurements in nuclear dating applications such as geochronometry and nuclear forensics. Three independent techniques were used at NPL to determine the activity per unit mass of a freshly separated 231Pa solution as part of an international CCRI(II) comparison: CCRI(II)-K2.Pa-231.The first method was defined solid angle α counting and is the first result reported in a BIPM comparison with this instrument. The second method was 4πα liquid scintillation (LS) counting combined with the CIEMAT/NIST method using commercially available LS counters. The third technique used was 4π(LS)α-γ coincidence counting on an NPL-built system comprising an LS counter with dual photomultiplier tubes (PMTs) coupled to a 70% relative efficiency HPGe γ-spectrometer. Excellent agreement between the three methods was obtained.

The potential radio-immunotherapeutic α-emitter 227Th - part II: Absolute γ-ray emission intensities from the excited levels of 223Ra.

At the time of publication, radiopharmaceuticals labelled with thorium-227 are in clinical trials in Europe for the treatment of various types of cancer. In part I of this two-part series the primary standardisation of an aqueous solution of 227Th was reported. In part II, the activity derived from the recommended absolute γ-ray emission intensities have been compared to that from the primary standardisation techniques. This comparison showed a negative bias of 4% in the determined activity per unit mass with an 11% spread in the activities determined for the eight most intense γ-ray emissions (Iγ > 1%) from the 227Th α decay. Using the standardised 227Th, measurements of the characteristic γ-ray emissions from the 223Ra excited states were made using a calibrated HPGe γ-ray spectrometer. This has enabled the absolute intensities of 70 γ ray emissions from the 227Th α-decay to be experimentally determined. A significant improvement over the precision of the recommended normalisation scaling factor has been made, with a value of 12.470 (35) % determined. Typically, the precision of the intensities has been improved by an order of magnitude or greater than current recommended values. The correlation matrices for pairs of the most intense γ-ray emission intensities are presented.

The potential radio-immunotherapeutic α-emitter 227Th - part I: Standardisation via primary liquid scintillation techniques and decay progeny ingrowth measurements.

Thorium-227 is a potential therapeutic radionuclide for applications in targeted α-radioimmunotherapy for the treatment of various types of cancer. To provide nuclear medicine departments involved in Phase I clinical trials traceability to the SI unit of radioactivity (Bq), a standardisation of a radiochemically pure 227Th aqueous solution has been performed at the National Physical Laboratory. This was achieved via two primary liquid scintillation (LS) techniques -4π(LS)-γ digital coincidence counting (DCC) and 4π LS counting. These absolute techniques were supported by the indirect determination of the 227Th activity via the measurement of the ingrowth and decay rate of the decay progeny by both ionisations chambers and high purity germanium (HPGe) gamma-ray spectrometry. The results of the primary techniques were found to be consistent, both with each other (zeta score = 1.1) and to the decay progeny ingrowth measurements. An activity per unit mass of 20.726 (51) kBq g-1 was determined for the solution. A procedure has been developed that provided an effective separation of the 227Th from its decay progeny, which was shown by the effective time zero of the 227Th-223Ra nuclear chronometer measured by HPGe gamma-ray spectrometry.

Results of an international comparison for the activity measurement of 177Lu.

An international Key Comparison of (177)Lu has recently been carried out. Twelve laboratories performed assays for radioactivity content on aliquots of a common master solution of (177)Lu, leading to eleven results submitted for entry into the Key Comparison Database of the Mutual Recognition Arrangement. A proposed Comparison Reference Value (CRV) was calculated to be 3.288(4)MBq/g using all eleven results. Degrees of equivalence and their uncertainties were calculated for each laboratory based on the CRV. Most of the values reported by the participating laboratories were within 0.6% of the CRV.

Characterization of the NIST seaweed Standard Reference Material.

The National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) for seaweed was developed through an interlaboratory comparison with 24 participants from 16 countries. After evaluating different techniques to calculate certified values for the radionuclides, the median method was found to be the most representative technique. The certified values were provided for 13 radionuclides and information values were given for 15 more radionuclides. Results for the natural decay series showed disequilibrium in both the uranium and thorium series.