Introduction to Environmental Measurement Techniques Radioactivity. Dana Pittauer 1of 48

Transcription

1 Introduction to Environmental Measurement Techniques 2016 Radioactivity Dana Pittauer 1of 48

2 Introduction Radioisotopes are of interest in environmental physics for several reasons: environmental monitoring - the emitted radiation is potentially hazardous tracing - detection limits low, radioisotopes can be used successfully as tracers to monitor e.g. transport processes; dating - due to the ubiquity and the wide range of halflife 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 2of 48

3 Radioactivity is the spontaneous and random disintegration (decay) of an unstable nucleus accompanied by the emission of energetic particles or photons The nuclei of some atoms are unstable. The nucleus of an unstable atom will decay to become more stable by emitting radiation in the form of a particle or elmag. radiation Random process means there is no way to tell which nucleus will decay, and cannot predict when it is going to decay A spontaneous process means the process is not triggered by any external factors such as temperature of pressure All chemical elements have radioactive isotopes, most (not all) have one or several stable isotopes. 3of 48

4 A X 137 Z, e.g. 55Cs Nuclide notation X... chemical element symbol Z... atomic number (number of protons) N... neutron number A... mass number - number of nucleons A=Z+N other notation: 137 Cs or Cs-137 4of 48

5 Nuclide chart 5of 48

6 6of 48

7 -decay: A Z A X 4 Y Z 2 Modes of decay a helium nucleus is emitted from the nucleus with a certain, characteristic kinetic energy (occurs mainly in heavy nuclei) A X A Y e - -decay: Z Z 1 an electron and an antineutrino are emitted; the energy is distributed between both particles (occurs mainly in nuclei with neutron excess) A X A Y e + -decay: Z Z 1 a positron and a neutrino are emitted; the energy is distributed between both particles (occurs mainly in nuclei with proton excess) 7of 48

8 Modes of decay EC: (electron capture) the nucleus absorbs an electron from the electron shell,which together with a proton forms a neutron. A neutrino is emitted (occurs also in nuclei with proton excess and is an alternative process to β+decay). γ-decay: 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, - and - decays leave nuclei in excited states and hence γ-emission often occurs in combination with these decays. + -decay additionally leads to γ-radiation of 511 kev due to the annihilation of the positron with an electron. 8of 48

9 Shielding of radiation 9of 48

10 Decay scheme Information: 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). 10 of 48

11 Decay scheme % of the β-decays of 137 Cs lead to an intermediate (excited) nucleus with a subsequent γ- emission 5.64 % decay directly to the stable nucleus in its ground state. Due to internal conversion, not all transitions from excited to the ground state lead to photon emission. For 137 Cs, the emission probability for the 662 kev photon is about %. 11 of 48

12 Activity (A) Number of decay events in time period dn A( t) N dt λ... decay constant [s -1 ] specific for each radionuclide Units of activity: Bequerel [Bq] = [s -1 ]... desintegrations per second Curie [Ci]... 1 Ci = 3, Bq... activity of 1 g of 226 Ra desintegrations per minute [dpm] Activity concentration A a m [Bq/kg], [Bq/m 3 ], of 48

13 Radionuclides decay exponentially with a specific decay constant λ Radioactive decay λ can be expressed as: where T 1/2... half-life [s] ln 2 T 1/ 2 Then: 13 of 48

14 Decay chains In case the daughter nuclide is also radioactive, a decay chain is formed 14 of 48

15 Origin of radionuclides Natural Primordial radionuclides (+ series) left over from when the Universe was created 40 K, series: 238 U, 235 U, 232 Th Cosmogenic cosmic radiation (protons, alphas, electrons, heavy nuclei) interacts with molecules of atmosphere (N 2, O 2 ) 14 C, 3 H, 7 Be, 10 Be, 22 Na Artificial (man-made) nuclear industry, weapons, medicine, research 15 of 48

16 Natural decay series 238 U 235 U 232 Th 16 of 48

17 Rn products 17 of 48

18 Primordial radionuclides Nuclide Half-life Natural activity in soil [Bq/kg] Natural activity in human body [Bq/kg] 235 U 7,04x10 8 yr small small 238 U 4,47x10 9 yr 22 small 232 Th 1,41x10 10 yr 37 small 226 Ra (radium) ( 238 U-series) 222 Rn (radon) ( 238 U-series) 1600 yr 20 0,2 (bone) 3,82 days small small 40 K 1,28x10 9 yr of 48

19 Cosmogenic radionuclides Nuclide Half-life Source Natural activity 14 C 5730 yr Cosmic-ray interactions 14 N(n,p) 14 C 3 H 12,3 yr Cosmic ray interaction with N and O 7 Be 53,28 days Cosmic ray interaction with N and O 0,22 Bq/g in organic material 10-3 Bq/kg 0,01 Bq/kg 19 of 48

20 Man-made radionuclides Nuclide Half-life Source 3 H (tritium) 12,3 yr Nuclear weapon testing and manufacturing, fission reactors 6 Li(n, ) 3 H, reprocessing facilities 131 I 8,04 days Fission product: nuclear weapon testing, fission reactors, used in nuclear medicine 129 I 1,57x10 7 yr Fission product: nuclear weapon testing, fission reactors 137 Cs 30 yr Fission product: nuclear weapon testing, fission reactors 90 Sr 28,78 yr Fission product: nuclear weapon testing, fission reactors 99 Tc 2,11x10 5 yr Decay product of 99 Mo ( 99m Tc) used in medical diagnosis 239 Pu 2,41x10 4 yr Activation of 238 U ( 238 U+n 239 U 239 Np+ 239 Pu+ ) 20 of 48

21 137 Cs fallout map after Chernobyl accident De Cort et al. (1998): Atlas of caesium deposition on Europe after the Chernobyl accident 21 of 48

22 Comparison Tschernobyl / Fukushima ( 137 Cs) > 3,7 MBq/m 2 60 km > 3 MBq/m 2 60 km Sources: EU, MEXT 22 of 48

23 Interaction of radiation with matter The radiation generated by radioactive decay interacts with matter mainly by the processes of scattering and ionization. All kinds of radiation mentioned here (, -, + and γ) 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 - and -particles, after being emitted from a radioactive nucleus, transfer their energy mainly by direct ionization, γ -radiation can interact with matter by three different processes. 23 of 48

24 Interaction of γ-radiation with matter 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). 24 of 48

25 Interaction of γ-radiation with matter 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). 25 of 48

26 Interaction of γ-radiation with matter gamma spectrum 26 of 48

27 Interaction of γ-radiation with matter 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). 27 of 48

28 Interaction of γ-radiation with matter 28 of 48

29 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. Our experiment concentrates on measurement of γ-radiation for the following reasons: γ-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 γ-radiation is very specific for the emitting nucleus. The second fact of this list indicates the possibility to use spectroscopic methods. 29 of 48

30 Gamma-spectroscopy 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 specific for the nucleus that emitted the photon. 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. from: 30 of 48

31 Gamma-spectroscopy from: 31 of 48

32 Gamma-spectroscopy 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) Gamma spectrometer from: 32 of 48

33 Gamma-spectroscopy MCA (Multichannnel analyzer): has a digital memory, which is separated into several thousand channels (bins). 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. from: 33 of 48

34 Gamma-spectra 34 of 48

35 Gamma-spectra 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. 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. 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 from: 35 of 48

36 Natural and Man-Made Radioactivity in Soil Taking the soil sample Experiments Test source with one gamma line effect of geometry Test source with multiple lines energy calibration Efficiency calibration Soil sample measurement Data analysis 36 of 48

37 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. S S S S Marinelli beaker S D D D D a) Point source b) Voluminous source D c) Borehole (well) detector 37 of 48

38 Evaluation of spectra In order to give specific results, following issues have be resolved: energy calibration efficiency calibration calculation of the uncertainties of the results 38 of 48

39 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 gamma-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 E = a + b * Ch with a = offset, b = proportionality factor and Ch = channel number E = a + b *Ch Energy 3 Energy Energy 1 Energy 2 Channel Nu. 39 of 48

40 Efficiency calibration Only some of the emitted photons will reach the detector - 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 - detector properties Both vary with photon energy For quantitative measurements all these factors have to be accounted for Efficiency calibration: a large number of measurements of samples prepared of different materials in different geometries and with known amounts of radioisotopes with different g energies. In our experiment: only a point source geometry for a single energy 40 of 48

41 Efficiency calibration from: 41 of 48

42 Counting statistics Number of impulses (N) of the observed gamma 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 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 ± σ. 42 of 48

43 Counting statistics In presence of background: approximate the Poisson distribution by the Gaussian distribution. This can be done without large errors for numbers of counts (N) greater than about 10 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 from: 43 of 48

44 Natural and Man-Made Radioactivity in Soil 44 of 48

45 Gamma spectrometry of soil 210 Pb 214 Pb 137 C s 40 K 214 Bi Hardware: coaxial HPGe detector Canberra Industries (50% rel. efficiency) housed in a 10 cm Pb shielding with Cu, Cd and plastic lining Source: Fischer of 48

46 Soil spectrum 137 Cs 46 of 48

47 Radionuclide identification, quantification Identification of using specific energy of photo peaks Some hints: presence of other lines of the same nuclide in the ratio of their intensity presence of mothers and daughters, if nuclide stems from a decay chain origin for non-chain radionuclides is the half-live sensible? for specific activity calculation use strong lines without interference Specific activity calculation (for 137 Cs and 2 other radionuclides): 47 of 48

48 Natural and Man-Made Radioactivity in Soil Practical Marievi Souti Manuel Peréz Mayo room: S4250, phone of 48