Practicals

Guide to the Lab Practical

1. Introduction

Three techniques will be applied to measure radioisotopes.

Bulk Radioactivity – measures some or all particles generated by the decaying nucleus. Examples include the geiger counter, scintillation detector, film emulsions, and track counters.

Specific Radioactivity – measure the energy or timing of particles emitted from the nucleus. For example the energy of the alpha particle derived from ²³⁸U decay is 4198 keV, slightly lower than the energy of the alpha particle that occurs from ²³⁴U decay, 4774.6 keV. Variations in the energy of emission also occur with gamma-rays associated with radioactive decay. Alpha counting requires prior chemical separation of the isotopes from the matrix, while gamma counting can be conducted on bulk samples - gamma rays pass relatively easily through small amounts of matter.

Mass Spectrometer (MS) – measures the mass of the isotope. This usually requires chemical separation prior to sample introduction into the mass spectrometer (e.g., Becker et al., 1999).

In this lab practical, four kinds of detectors will be introduced to the trainees, alpha spectrometry, beta counters, radium delayed coincidence counter (RaDeCC, a kind of alpha detector), and gamma counting, as well as a demo on mass spectrometry (MS).

Alpha spectrometry is characterized by good isotope separation (i.e., energy and isotope independent), uniform detector efficiency, and very low background [typically on the order of 0.001-0.003 cpm (counts per minute) for the 4-8 MeV energy range of a new silicon surface barrier detector, increasing with time as a result of the accumulation of recoil products from measured samples]. It allows precise measurements at low activities, and easy calibration with yield tracers. Alpha particles have a very short range. They are absorbed or slowed down by interaction with the sample material or by impurities on the source, i.e., the flat planchet with the purified element plated on it. The technique is therefore not practical for non-destructive measurements. Best results in terms of efficiency and energy resolution are obtained with clean sources prepared from well-purified samples (Godoy et al., 1994).

The RaDeCC System: RaDeCC can be used to measure ²²³Ra and ²²⁴Ra and was developed by Moore and Arnold (1996). It is an example of a timing circuit. Briefly, the short -lived Rn daughters of ²²³Ra and ²²⁴Ra are swept into a scintillation detector where alpha decays of Rn and Po occur. Signals from the detector are sent to a delayed coincidence circuit which discriminates decays of the ²²⁴Ra daughters, ²²⁰Rn and ²¹⁶Po, from decays of the ²²³Ra daughters, ²¹⁹Rn and ²¹⁵Po. The method may be extended to measurements of ²²⁷Ac, ²³¹Pa, ²²⁸Th, and ²²⁸Ra.

Beta counting is not isotope specific. It requires relatively simple equipment. A gas-flow anti-coincidence counter (another timing circuit) is often used and coupled with 10 cm lead shielding. This results in backgrounds of 0.15-0.20 cpm for a source of 25 mm diameter (Risø national laboratory, Roskilde, Denmark). It is the preferred method for ²³⁴Th, and can be used with non-destructive techniques and onboard ship (Benitez-Nelson et al., 2011).

²³⁴Th decays with relatively weak betas (maximum beta energy 0.20 MeV) to ^234mPa, which is a high-energy beta emitter (maximum beta energy 2.29 MeV, half-life 1.17 min). Thus, if a filter is dried and carefully folded [possible with polycarbonate (e.g. nuclepore^TM) and GF/F filters, not with membrane filters, which will break] to fit the beta detector and covered with a thin foil, the beta activity measured is due mainly to ^234mPa, with some contribution from ²³⁴Th.

²³⁴Th can be counted by non-destructive beta counting. In the open ocean, ²³⁴Th activity usually overwhelms other isotopes that might contribute to the beta signal from suspended particles, like ⁴⁰K or ²¹⁰Pb. This should be checked occasionally by following the decay over several months. If the activity does not decline with the half-life of ²³⁴Th (24.1 day), this is an indication of radioactive decay contributions from other radionuclides.

Gamma spectrometry allows identification of a wide range of isotopes without chemical purification and is the ideal method for non-destructive techniques, e.g., ²¹⁰Pb in sediments (Cutshall et al., 1983) and dissolved ²²⁶Ra and ²²⁸Ra (Moore, 1984). Inherent (Compton) background and self-absorption by the sample, energy- and geometry-dependent detector efficiencies, coupled with low gamma branching ratios for some elements, reduces the sensitivity and accuracy of the method. The overall efficiency is generally low compared to other techniques.

References (it is highly recommended that all trainees read through these key papers before you come to the workshop):

Becker J. S., Soman R. S., Sutton K. L., Caruso J. A., Dietze H. J. 1999. Determination of long-lived radionuclides by inductively coupled plasma quadrupole mass spectrometry using different nebulizers. Journal of Analytical Atomic Spectrometry 14 (6), 933-937.

Benitez-Nelson C.R., Buesseler K.O., Rutgers van der Loeff M.M., Andrews J.A., Ball L., Crossin G., Charette M.A. 2001. Testing a new small-volume technique for determining thorium-234 in seawater. Journal of Radioanalytical and Nuclear Chemistry 248, 795– 799.

Cutshall N. H., Larsen I. L. and Olsen C. R. 1983. Direct analysis of ²¹⁰Pb in sediment samples: self-absorption corrections. Nuclear Instruments and Methods 206, 309-312.

Giffin C., Kaufman A. and Broecker W. 1963. Delayed coincidence counter for the assay of actinon and thoron. Journal of Geophysical Research 68(6): 1749-1757.

Godoy J. M., Lauria D. C., Godoy M. L. D. P., Cunha R. P. 1994. Development of a sequential method for the determination of ²³⁸U, ²³⁴U, ²³²Th, ²³⁰Th, ²²⁸Ra, ²²⁶Ra, and ²¹⁰Pb in environmental samples. Journal of Radioanalytical and Nuclear Chemistry 182 (1), 165-169.

Moore W. S. 1984. Radium isotope measurements using germanium detectors. Nuclear Instruments and Methods 223, 407-411.

Moore W. S. 2008. Fifteen years experience in measuring ²²⁴Ra and ²²³Ra by delayed-coincidence counting. Marine Chemistry 109(3-4): 188-197.

Moore W.S. and Arnold R. 1996. Measurement of ²²³Ra and ²²⁴Ra in coastal waters using a delayed coincidence counter. Journal of Geophysical Research 101C, 1321–1329.

2. Hands-on

Trainees will be divided into four groups, with 8 people in each group and 2 people in a sub-group to get familiar with the alpha spectrometers, beta counters, RaDeCC and gamma counters, as well as obtain a demo on MC-ICP-MS. Due to time limitations, alpha, beta and gamma spectrometer samples will be prepared/processed beforehand and trainees will be instructed on how to use these instruments. For RaDeCC, trainees will be instructed on how to process and measure the sample. The arrangements of instruments with instructors and labs are as follows:

June 9 2:00-6:00 pm:

Group 1: Alpha & RaDeCC

Group 2: Beta & Gamma

Group 3: RaDeCC & Alpha

Group 4: Gamma & Beta

June 10 2:00-6:00 pm:

Group 1: Beta & Gamma

Group 2: Alpha & RaDeCC

Group 3: Gamma & Beta

Group 4: RaDeCC & Alpha