Gamma spectroscopy

Gamma spectroscopy is the measurement of the spectrum of the gamma radiation from a radioactive source. Gamma quanta have not any, but certain (discrete ), characteristic for the particular nuclide energies, similar to the optical spectroscopy, the spectral lines for the substances contained in the sample are characteristic. Therefore, the gamma spectroscopy is an important method for the study of radioactive substances, such as radioactive waste, in order to decide about their treatment.

• 3.1 Energy calibration
• 3.2 Intensity Calibration
• 3.3 Spurious spectral

Construction of a gamma spectrometer

Detector

The main part of the measuring apparatus, the gamma spectrometer is a suitable radiation detector. For most gamma emitters with their energies between about 50 keV and a few MeV are best suited semiconductor detectors made ​​of high purity germanium ( High Purity Germanium, short name HPGe ) or less pure, with lithium doped ( " gedriftetem " ) germanium ( short name Ge ( Li) ). For the energy range below 50 keV lithium - drifted silicon detectors ( abbreviation Si ( Li) ) are suitable.

HPGe detectors are in operation to avoid generated by thermal processes " noise " signals cooled with liquid nitrogen. The lithiumgedrifteten detectors require cooling even this constantly, even during storage and transport.

Besides semiconductor detectors also scintillation detectors with single crystals of sodium iodide or bismuth germanate (BGO ) are used. Their advantage is that they can be manufactured with larger dimensions than the semiconductor detectors, so that a higher probability of response of the detector is achieved. This is important when radiation of very low intensity is to be measured, such as the investigation of people on radioactivity in the body. Scintillation detectors do not need refrigeration. Their disadvantage is the much lower energy resolution ( see below).

Recording of the spectrum

The electrical pulses generated by the detector are generally supplied to a multi-channel analyzer to obtain the spectrum of an amplifier. In simple cases, such as for learning purposes in teaching laboratories, a single channel analyzer can be used with a downstream electronic counter instead; Here, the spectrum is successively in time, energy for energy registered. Therefore Einkanalmethode provides an undistorted spectrum only in such nuclides, whose half -life is long compared to the duration of the measurement.

In the representation of the spectrum is usually the energy level (as channel number) and the intensity plotted vertically (as channel content ).

The illustrations show spectra of 137Cs and 60Co.

Quantum energy and pulse height

There are essentially three different processes, it can, by a gamma ray ionization and thus a detector pulse. It has a unified quantum energy result in a characteristic distribution of pulse heights. Only the largest of these pulse heights - the local maximum in the spectrum, which corresponds to the total energy of the quantum, the photopeak or Full Energy Peak ( FEP) - is used for spectroscopy. Those pulses that correspond to less than full power, form the belonging to this FEP Compton continuum.

In the illustrations of this continuous part is clearly visible with further seated thereon peaks. Peaks on the continuum may be achieved by other effects or even be the FEP to other States represented in the spectrum of gamma energies; in this case, brings each of them in turn, "his" Compton continuum. Therefore, the ground rises in the registered spectrum - which must be deducted from the respective peak area - with falling energy more and more.

Measures energy and intensity

Is measured using both the energy of each photon, and the registered intensity of each spectral line. To identify nuclides and for example to be able to determine their activity, the spectrometer must be calibrated with respect to both metrics.

Energy calibration

The energy calibration is performed using the gamma energies of known nuclides of a preparation. You may also suffice known gamma energies of the environment stemming from the radiation " underground " such as the line of the 40K at 1461 keV annihilation line and the positron from the secondary cosmic radiation at 511 keV. The pulse height ( channel number) corresponds mostly as accurate linear photon energy that two gamma lines are adequate as calibration points to obtain the assignment of channel number for the entire energy spectrum.

Intensity calibration

The intensity measure the count rate ( number of pulses per unit time) at a quantum energy (graphic: the area under the respective photo- peak). The size of interest is one of the flux density of photons at the detector, or - generally - the activity of the nuclide in question in the measured sample. If one of these quantities are determined absolutely, the efficiency or probability of response of the detector must be calibrated as a function of gamma energy.

These measurements with calibration standards of known composition AND activity will require that one can refer, for example, by the Physikalisch -Technische Bundesanstalt ( PTB). Such standards emit gamma rays of different energies. The thus measured count rates resulting measurement points from which for the range between the lowest and the highest gamma energy used in the calibration measurement by computational (formerly graphical ) interpolation calibration curve is obtained. The response probability is outside this range so that can not be calibrated because the then required extrapolation would not provide sufficient accuracy. The intensity calibration curve is not linear.

If such an intensity calibration is carried out, an energy calibration ( see above) is necessarily included.

Spurious spectral

In addition to the photo peak corresponding to the energies of the incident gamma rays, can be caused by various unavoidable side effects more maxima in the spectrum that can not be confused with the real spectral gamma (see figures). An example of this escape lines.

Energy resolution

The energy resolution is the smallest distance between two energies in which the two photo peaks can still be analyzed separately. It corresponds approximately to the half width of each peak. Semiconductor detectors achieve a half-value width below 2 keV so that even very densely located gamma or lines can be separated. In a scintillation detector, on the other hand, for example, as one of the pictures shows the 662- keV photopeak of 137Cs around 70 keV wide. Scintillation detectors are therefore particularly suitable where the nature of the nuclide is known and is less about actual spectroscopy to the quantitative determination.

To take advantage of the energy resolution of the detector, must have the digital resolution, i.e. the number of channels for registration of the spectrum can be appropriately selected. For a measuring range from 0 to 2 MeV or 0-4 MeV channels are useful, for example, in a semiconductor detector 4096 or 8192; with a scintillation detector satisfy 512 or 1024 channels.