Instrument calibration

The calibration of gamma ray spectrometers is the estimation of those constants that relate instrument count rates to either radionuclide concentration or environmental dose rate. This includes the estimation of background radiation, stripping ratios and sensitivity constants.

Background radiation is due to the internal radioactivity of the instrument, cosmic radiation, and atmospheric radon. The background is estimated by taking measurements from a small boat (preferably fibreglass) over a river or lake, and at least 200 m from the shore. The shoreline should be flat. Background count rates are recorded in all energy channels.

FIG. 4.4. Calibration of a portable spectrometer on a transportable calibration pad.

Portable gamma ray spectrometers used for assaying K, U and Th in rocks are calibrated by means of calibration pads. A calibration pad is a slab of concrete containing known concentrations of the radioelements (Figure 4.4). Ideally, calibration pads should simulate a geological source of radiation. The IAEA (IAEA, 1989) recommended four cylindrical concrete pads with dimensions: diameter - 3 m and thickness - 0.5 m. Each of the pads is enriched in either K, U or Th. Recommended concentrations (IAEA, 1989) are 8% K in the K-pad, 50 ppm U in the U-pad, and 125 ppm Th in the Th-pad. The fourth pad serves as a

background pad. Grasty et al. (1991) showed that smaller transportable pads (1×1×0.3 m) are also suitable for calibrating portable gamma ray spectrometers. Geometrical correction factors must be applied to the instrument sensitivities derived from these calibration experiments.

These factors correct for the density and finite dimensions of the calibration pads.

The K, U and Th window count rates obtained over the pads are linearly related to the K, U and Th concentrations in the pads. Let ni (i = 1, 3) be the count rate in the i-th energy window (either K, U or Th), and let s_ij (i = 1, 3; j = 1, 3) be the sensitivity of the i-th elemental count rate to the concentration of the j-th element (either K, U or Th), then

iBG

sij = sensitivity of the spectrometer for the detection of the j-th element in the i-th energy window, (c/s per unit concentration of the j-th element);

c_j = concentration of the j-th element, (% K, ppm U, ppm Th);

niBG = background count rate in the i-th energy window, (c/s).

Since the estimation of K, U, and Th is based on measurements in 3 energy windows, and the background count rates n_iBG can be subtracted, equation (4.1) may be written in matrix notation as follows:

The sensitivity constants, s_ij, are estimated from measurements on the four calibration pads.

The detector is placed on the centre of a calibration pad and count rates, ni, are measured in three energy windows (i = 1, 2, 3). Backgrounds are removed by subtracting the counts measured on the background pad, n_iBG, and subtracting the concentration of the background pad from the concentrations of the other three pads. Equation (4.1) is thus modified as follows:

where ∆c_j = the difference between the concentrations of the j-th element in a calibration pad and the concentration of the j-th element in the background pad.

Or, in matrix notation Th pads minus the K, U, and Th in the background pad.

The sensitivity matrix may then be estimated as energy window) per unit concentration (1% K, 1 ppm U, 1 ppm Th) of the radioelements.

For the K, U and Th energy windows (i=1, 2 and 3), the “stripping ratios” α,β,γ,a,b and g (see §4.1.4) are defined by the ratios of sensitivities as follows

U

The stripping ratios define the ratios of count rates, caused by a single element in an energy window, to the count rate of the same element in its principal energy window. They are used during data processing to estimate the net count rate of a single element in an energy window.

The stripping ratios α,β,γ, and a also give an indirect measure of the energy resolution of a detector system – the smaller their values, the better the energy resolution of the detector.

IAEA recommended concentrations of K, U, Th in calibration pads enable calibration of a portable gamma ray spectrometer, equipped with a NaI(Tl) 76×76 mm detector, to a relative precision of 1%, in a 10 min sampling time. Due to the finite dimensions of calibration pads, a geometrical correction, G, must be applied to the derived sensitivities. The correction depends on the pad dimensions, pad density, and the height of the centre of the scintillation crystal above the pad surface. The ratio, R, of gamma radiation from a cylindrical pad 0.5 m thick to that from an infinite source, with the detector placed on the centre of the pad, and h/r < 0.2, is

r

R=1−h (4.8)

where h = height of the scintillation crystal centre above the surface of the pad (m);

r = radius of a cylindrical pad (m).

The geometrical correction can be applied by multiplying the derived sensitivities (equation (4.5) by G = 1/R. Note that equation 4.8 is based on the assumption that the pads are infinitely thick. For pads of finite thickness, the correction factor also depends on pad thickness and on the linear attenuation of gamma-radiation in the pads (and hence the density of the pads and the energy of the gamma rays). An example of the sensitivities and stripping ratios estimated for a portable gamma ray spectrometer are given in Table 4.1. Examples of geometric correction factors for cylindrical pads are given in Tables 4.2 and 4.3. These were calculated using the computer program of Lovborg et al., 1972. Examples of geometric correction factors for rectangular transportable pads are given in Table 4.4.

TABLE 4.1. TYPICAL ENERGY WINDOW SENSITIVITIES AND STRIPPING RATIOS FOR A PORTABLE GAMMA RAY SPECTROMETER WITH NaI(TL) 76×76 MM DETECTOR (IAEA, 1989)

Sensitivities K window U window Th window counts/s per 1% K 3.36 0 0 counts/s per 1 ppm eU 0.250 0.325 0.011 counts/s per 1 ppm eTh 0.062 0.075 0.128

Stripping ratios Ratio of sensitivities Stripping ratio

α 0.075/0.128 0.586

β 0.062/0.128 0.484

γ 0.250/0.325 0.769

a 0.011/0.325 0.034

b = g 0.000

TABLE 4.2. GEOMETRIC CORRECTION FACTORS FOR CYLINDRICAL PADS 2m IN DIAMETER AND 0.5m THICK

Cylindrical calibration pad (dimensions 2m diam × 0.5 m thick)

Energy (MeV)

Correction factor (detector height = 0.06 m) K pad (density=2.24 g/cm³) 1.46 1.068 U pad (density=2.24 g/cm³) 1.76 1.070 Th pad (density=2.24 g/cm³) 2.62 1.074

TABLE 4.3. GEOMETRIC CORRECTION FACTORS FOR CYLINDRICAL PADS 3m IN DIAMETER AND 0.5m THICK

Cylindrical calibration pad

(dimensions 3 m diam × 0.5 m thick)

Energy (MeV)

Correction factor (detector height = 0.06 m) K pad (density=2.24 g/cm³) 1.46 1.042 U pad (density=2.24 g/cm³) 1.76 1.043 Th pad (density=2.24 g/cm³) 2.62 1.046

TABLE 4.4. GEOMETRIC CORRECTION FACTORS FOR TRANSPORTABLE CALIBRATION (detector height = 0.06 m) K pad (density=2.23 g/cm³) 1.46 1.156 U pad (density=2.24 g/cm³) 1.76 1.165 Th pad (density=2.28 g/cm³) 2.62 1.188 Note: Detector height is the height of the centre of the scintillation crystal above the pad.

Detailed descriptions of available calibration facilities are given in IAEA (1989). Variable moisture content in calibration pads can lead to a change in radiation output. Pads should therefore be kept dry.

The calibration of Ge semiconductor spectrometers for field use is based on experimentally estimating the instrument detection efficiencies for individual gamma ray energies (peak count rate per unit photon flux), and then converting these to isotope ground concentration. A series of point source standards, emitting various gamma energies, of known activity, should be available for calibration (ICRU, 1994). A simpler approach of calibrating Ge spectrometers by means of large concrete calibration pads for evaluating the specific activity (Bq/kg) of natural radionuclides has been suggested (IAEA, 1989).