Radioisotopes in Soil

Transcription

1 Radioisotopes in Soil Helmut Fischer, June 2003 Introduction Radioisotopes are of interest in environmental physics for several reasons: - environmental protection - the emitted radiation is potentially hazardous and therefore radioactivity levels in environmental media have to be monitored and the consequences of contaminations have to be assessed; - tracing as the detection limits can be very low, radioisotopes can be used successfully as tracers to monitor e.g. transport processes; - dating due to the ubiquity and the wide range of half-life times of naturally occurring radioisotopes, they can be used for age determination. The laboratory of environmental radioactivity at the University of Bremen works in all of these fields. Radioactivity Modes of decay All chemical elements have radioactive isotopes, most (not all) have one or several stable isotopes. Radioactivity means the spontaneous transformation of a nucleus to another nucleus, accompanied by the emission of one or several particles. The principal modes of radioactive decay are: a-decay: b - -decay: b + -decay: g-decay: a Helium nucleus is emitted from the nucleus with a certain, characteristic kinetic energy (occurs mainly in heavy nuclei); an electron and an antineutrino are emitted; the energy is distributed between both particles (occurs mainly in nuclei with neutron excess); a positron and a neutrino are emitted; the energy is distributed between both particles (occurs mainly in nuclei with proton excess). the transition of a nucleus from an excited to an intermediate or the ground state is accompanied by the emission of a photon. In many cases, a- and b-decays leave nuclei in excited states and hence g emission often occurs in combination with these decays. b + -decay additionally leads to g-radiation of 511 kev due to the annihilation of the positron with an electron. The kinetic energy of the emitted particles lies in the range of several kev to several MeV.

2 Radioisotopes are denominated by their chemical element symbol, preceded by the number of nucleons A as superscript and the nuclear charge Z (= number of protons) as subscript, e.g Cs for the Cesium nucleus with 82 neutrons. The charge is often omitted. Sometimes the element symbol is followed by a hyphen and the number of nucleons: Cs-137. All radioactive decays are exponential with a specific decay constant: Eq. 1 A(t) = A 0 e -lt which can also be written as Eq. 2 - t ln(2) T A(t) = A 0 e 1/ 2 with the radioactive half-life T 1/2 as characteristic parameter. A is the activity, measured in the SI unit Bq (Becquerel). 1 Bq stands for 1 disintegration per second, therefore in equations Bq is sometimes replaced by s -1. There exists an older unit for activity, Ci (Curie). 1 Ci = 3,7 * Bq. Radioactive decay data can be displayed in form of a decay scheme; the following graph shows a simple scheme of the b - -decay of 137 Cs, an important isotope in radioecology: Fig. 1: Decay scheme of 137 Cs ( classes/phys342l/compton.pdf) Important physical properties that can be extracted from the decay scheme are half-life T 1/2, decay energy E and branching ratio f (the fraction of nuclei which occupy the different decay branches of the scheme). The graph indicates that 94% of the b-decays of 137 Cs lead to an intermediate (excited) nucleus with a subsequent g-emission, and 6% decay directly to the stable nucleus in its ground state. In case the daughter nuclide is also radioactive, a decay chain is formed.

3 All stable and radioactive isotopes can be displayed together in a table of isotopes, accessible e.g. at Decay data of individual radionuclides can be accessed and searched for at Origin of radioisotopes Radioisotopes in the environment can be either of natural origin or man-made. The main contributions to natural radioactivity are due to the primordial nuclides with half-lives of billions of years: 40 K, 232 Th, 235 U and 238 U and, except for 40 K, their corresponding decay chains. One example of a decay chain is given in Fig. 2. Fig. 2: Decay chain of 238 U ( Other nuclides like 3 H and 14 C are generated in the atmosphere by cosmic radiation. The main man-made sources of radioactivity in the environment are nuclear bombs, nuclear power plants, nuclear research and nuclear medicine. The aim of this practical is to look for residues of the 1986 Chernobyl reactor accident in locally obtained soil samples. Nowadays, in northern Germany the environmental radioactivity levels due to this accident are considered harmless, but in 1986 a lot of foodstuff had to be retired from the market, and recommendations for personal security had been issued.

4 Interaction of radiation with matter The radiation generated by radioactive decay interacts with matter mainly by the processes of scattering and ionization. The important process for the detection of radiation and also for the harmfulness to biological systems is ionization, i.e. interaction with the electron shell of the target atoms. Interaction processes with the nuclei of the target atoms (e.g. induced fission or neutron activation) constitute the principal aim in military and civil use of nuclear energy and many research projects. However, they are of minor importance in environmental radioactivity measurements. All kinds of radiation mentioned here (a, b -, b + and g) create secondary electrons by ionization, which in turn cause ionizations until the kinetic energy of the electrons is sufficiently low for them to be absorbed by the electron shell of an atom. Whilst a- and b-particles, after being emitted from a radioactive nucleus, transfer their energy mainly by direct ionization, g-radiation can interact with matter by three different processes. Photo effect: the incident photon transfers all its energy to one electron; the photon is absorbed and the electron leaves the shell. This is the dominating process for low photon energies (below about 100 kev). Compton effect: the incident photon transfers part of its energy to an electron, which leaves the shell. The photon is scattered and its energy is reduced by the amount that was transferred to the electron. This process dominates at intermediate energies (about 100 kev to several MeV). Pair production: in the electric field of a nucleus, the photon is converted to an electron and a positron. This requires at least 1,02 MeV (the mass of electron + positron). The excess energy is transferred to electron and positron as kinetic energy. This process dominates at high photon energies (above several MeV). Measurement of radiation There exist measurement techniques for all types of radiations generated by radioactive decay. All of them are based on the detection of the ionization products. This experiment concentrates on measurement of g-radiation for the following reasons: - g-radiation has a long range in matter and can therefore be measured in bulk samples, so no additional preparation of the sample is necessary; - the energy of g-radiation is very specific for the emitting nucleus. The second fact of this list indicates the possibility to use spectroscopic methods. In fact, a g- spectrometer will be used, and its main components are shown in Fig. 3.

5 Fig. 3: Schematic drawings of a g-spectrometer (left) and of a semiconductor detector (right) ( In the semiconductor detector, the incident photon ideally performs a photo effect and the emitted electron transfers all its energy to secondary and tertiary electrons, which are collected as a charge pulse at the electrical terminals of the detector. The height of the pulse is proportional to the energy of the photon and is very specific for the nucleus that emitted the photon. Unfortunately, this happens only in a small percentage of interactions. Most incident photons undergo Compton scattering and deposit only part of their energy in the detector. The resulting pulse is unspecific for the emitting nucleus. Fig. 4: Schematic drawing of a multichannel analyzer (MCA) ( The pulses are amplified in the preamplifier and main amplifier and then converted to digital information in the ADC (analog-to-digital converter). The digital information is then transferred to a multichannel analyzer (MCA), see Fig. 4. This instrument has a digital memory, which is separated into several thousand channels. Each channel is assigned a pulse height range, increasing with channel number. Each incoming pulse is analyzed for its amplitude, and the content of the corresponding channel is increased by one count. A twodimensional display shows channel numbers in the x and channel content in the y coordinate; a spectrum is formed. Fig. 5 shows a spectrum from a 137 Cs calibration source.

6 counts per channel Energy [kev] Fig. 5: g spectrum obtained with a 137 Cs calibration source. The peak in the center is generated by photons undergoing photo effect ( photo peak ), the region at the left of the peak is generated by photons undergoing Compton scattering ( Compton background ). The peak is not recorded in one single channel alone. It occupies a range of channels and has a form that can be approximated by a Gaussian curve. The characteristic properties are the energy at the center, the width (expressed e.g. as FWHM, Full Width at Half Maximum) and the total number of counts. Fig. 6 shows the expanded view of a typical photopeak. Fig. 6: Expanded view of a photopeak ( The MCA offers software options e.g. to determine the area under a peak, i.e. to sum all channel contents for the channels covering the peak. In Fig. 6 this would cover the area between the two vertical lines. In most spectra, some or all of the peaks of interest sit on the background of other peaks with higher energy or of cosmic radiation. A background subtraction has then to be performed (see below).

7 Sample geometry Sample geometry is important because it influences the sample volume seen by the detector. For different purposes, different geometries may be optimal. In the case of bulk samples of large volume, often a geometry is chosen which lets the sample surround the detector, e.g. in form of a Marinelli beaker. For very small samples, a detector with a bore is chosen, which surrounds the sample. Evaluation of g spectra In order to give specific results, a g spectrometer has to be calibrated in terms of energy and efficiency, and as we are dealing with a counting experiment, the statistical errors of the results have to be calculated. Energy calibration Assuming linear amplifiers, channel numbers are proportional to pulse height. The process of calibration consists of recording a spectrum of a calibration source with several known g energies, finding the corresponding peaks in the spectrum and entering the energies into the MCA. The MCA then performs a least squares fit to the data and assigns each channel an energy according to Eq. 3: Eq. 3 E = a + b n with a = offset, b = proportionality factor and n = channel number. Efficiency calibration For geometrical reasons and due to self-absorption in the sample, only some of the emitted photons will reach the detector. This fraction will depend on the sample geometry. In the detector, only a part of the incident photons will interact, and only a few of the interactions will be photo effects. This fraction depends on detector properties. Both vary with photon energy. All these factors have to be accounted for if quantitative measurements (e.g. activity concentration in the sample) are required. This is achieved by a very time-consuming procedure, the efficiency calibration. It consists of a large number of measurements of samples prepared of different materials in different geometries and with known amounts of radioisotopes with different g energies. As in all measurements the absolute activity is known, the detection efficiency can be calculated according to Eq. 4: Eq. 4 e(e) = N t f A with e(e) = energy-dependent peak efficiency, N = counts in peak, f = branching ratio of observed g emission and A = activity in the sample. A computer program interpolates between the measured energies and produces efficiency calibration curves that can be used for radioisotopes with other g energies. An efficiency calibration curve for a semiconductor detector is shown in Fig. 7.

8 Fig. 7: Efficiency calibration curve for a semiconductor detector: the dots mark measurements for different g energies obtained with different radionuclides, the line is the interpolation used for other energies. ( Counting statistics As mentioned above, the contents (N) of the observed g lines are mere numbers from a counting experiment, and they are often partially buried in a background. This fact produces an uncertainty of the result, which can be expressed by its standard deviation. For any counting experiment the result of which is governed by a Poisson distribution, the standard deviation is Eq. 5 s = N This expresses the fact that a repetition of a counting experiment would in about 2/3 of the cases give a result in the range of N ± s. Fig. 8: Peak and background areas for background subtraction (

9 For the analysis of a peak without background this is sufficient. If we have to subtract a background like in the peak in Fig. 8, we have to approximate the Poisson distribution by a Gaussian distribution. This can be done without large errors for numbers of counts (N) greater than about 10. In this case, we have Eq. 6 N = N tot - N BG and Eq. 7 s = s 2 2 tot +s BG = N tot + N BG In many cases the background has a slope. Therefore the counts in the background areas have to be determined at both sides of the peak and averaged. By this kind of calculation, the value and the statistical error of the activity concentration of 137 Cs in a soil sample (to be measured in the practical) can be determined. Contributions from various radioisotopes Apart from the expected 137 Cs, a lot of other radioisotopes can be found in soil samples, as can be seen in Fig counts per channel Energy [kev] Fig. 9: g spectrum obtained from a soil sample. Apart from the 137 Cs line at 662 kev (see mark), many other lines are visible. In order to identify other nuclides, various data have to be combined. From a gamma energy table: identification of individual lines; presence of other lines of the same nuclide, in the ratio of their branching factors.

10 Plausibility: presence of mothers and daughters, if nuclide stems from a decay chain origin (fallout, reactor, research, medicine, ) All together this can be quite a puzzle. For the purpose of this practical, identification of some nuclides of the natural decay chains, without activity calculation, is sufficient.