Method and device for the measurement of high dose rates of ionizing radiation

Abstract

A method is provided for determining the dose rate {dot over (H)} of nuclear radiation field, namely a gamma radiation field, with a radiation detection system (RDS), comprising a scintillator, a photodetector, an amplifier and a pulse measurement electronics. The pulse measurement electronics includes a sampling analog to digital converter, where the nuclear radiation deposes at least some of its energy in the scintillator, thereby producing excited states in the scintillation material, with the excited states decaying thereafter under emission of photons with a decay time τ. Photons are absorbed by the photodetector under emission of electrons, those electrons forming a current pulse, said current pulse being amplified so that the resulting current signal can be processed further in order to determine the charge of the pulse measured.

Claims

1. A method for determining an energy compensated dose rate {dot over (H)} of a nuclear radiation field comprising a gamma radiation field, in pulse pileup scenarios with a radiation detection system (RDS), comprising a scintillator, a photodetector, an amplifier and pulse measurement electronics, said pulse measurement electronics including an analog to digital converter, where the nuclear radiation deposits at least some of its energy in the scintillator, thereby producing excited states in the scintillation material, said excited states decaying thereafter under emission of photons with a decay time τ, said photons being absorbed by the photodetector under emission of electrons, those electrons forming a current pulse, said current pulse being amplified by the amplifier so that the resulting current signal can be processed further in order to determine the charge of the pulse measured, this charge of the pulse being proportional to the energy deposited in the scintillator by the nuclear radiation, whereas the electrical signal is coupled to the pulse measurement electronics, whereas the RDS has a defined maximum permissible mean current for spectroscopic use, above which the voltage of the amplifier, defining the amplification of said amplifier, is reduced in order to prevent saturation and/or harming the amplifier through high currents, wherein the method comprises the following steps: when the maximum permissible mean current is exceeded, reducing the bias voltage applied to the amplifier, so that a current that is less than the maximum permissible mean current flows during the measurement; digitizing and differentiating the analog detector output current signal i.sub.v measured by the pulse measurement electronics using a sampling period Δ, producing the current samples i.sub.Δ; determining the variance of the sampled current signal Var(i.sub.Δ); determining the mean square difference of the sampled current signal, Msd(i.sub.Δ), being a measure of the average current; determining the mean Energy E.sub.γ by using the equation $E_{\gamma }=\frac{\mathrm{Var}\left(i_{\Delta }\right)}{Msd\left(i_{\Delta }\right)}$ applying a non-linear correction function Z(E.sub.γ) to said mean Energy E.sub.γ, said correction function rectifying the efficiency between the scintillator and a tissue equivalent ideal scintillator by accounting for a different energy deposition within the scintillator compared to human tissue, determining the energy compensated dose rate H from current samples i.sub.Δ by using the equation {dot over (H)}=Z(E.sub.γ) Msd(i.sub.Δ).

2. The method of claim 1, the scintillator used comprising an inorganic scintillation material.

3. The method of claim 1, the amplifier used being selected from a group of Photomultiplier Tube (PMT) and Electron Multiplier.

4. The method of claim 1 using a RDS where the photodetector and the amplifier are combined in one device, selected from a group of Avalanche Photo Diode and Silicon Photomultiplier.

5. The method of claim 1, whereby the pulse measurement electronics is coupled to the amplifier output via AC-coupling.

6. The method of claim 1, whereby the pulse measurement electronics is coupled to the amplifier output via DC-coupling.

7. A radiation detection system (RDS), comprising a scintillator, a photodetector, an amplifier and a pulse measurement electronics, said pulse measurement electronics including an analog to digital converter, where the nuclear radiation deposits at least some of its energy in the scintillator, thereby producing excited states in the scintillation material, said excited states decaying thereafter under emission of photons with a decay time τ, said photons being absorbed by the photodetector under emission of electrons, those electrons forming a current pulse, said current pulse being amplified so that the resulting current signal can be processed further in order to determine the charge of the pulse measured, this charge of the pulse being proportional to the energy deposited in the scintillator by the nuclear radiation, whereas the electrical signal is coupled to the pulse measurement electronics, whereas the RDS has a defined maximum permissible mean current for spectroscopic use, above which the voltage of the amplifier, defining the amplification of said amplifier, is reduced in order to prevent saturation and/or harming the amplifier through high currents, wherein the RDS is capable of performing a method according to claim 1.

8. The radiation detection system (RDS) of claim 7 wherein the scintillator comprises an inorganic scintillation material.

9. The radiation detection system (RDS) of claim 7, wherein the amplifier is selected from a group of Photomultiplier Tube (PMT) and Electron Multiplier.

10. The radiation detection system (RDS) of claim 7, wherein the photodetector and the amplifier are combined in one device, selected from a group of Avalanche Photo Diode and Silicon Photomultiplier.

11. The radiation detection system (RDS) of claim 7, wherein the pulse measurement electronics is coupled to the amplifier output via AC-coupling.

12. The radiation detection system (RDS) of claim 7, wherein the pulse measurement electronics is coupled to the amplifier output via DC-coupling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a general setup of such an RDS 1. Gamma radiation is absorbed in the scintillator 2 under emissions of photons. Those are absorbed by the photodetector 3 under emission of electrons 4, which are then amplified by a PMT 5. The output signal from the anode 6 of the PMT is digitized with a high sampling rate and further evaluated by the attached measurement electronics 7.

(2) FIG. 2 shows a simulation of the mean current measured at the anode of a RDS with a PMT as amplifier, which is proportional to the count rate. In this setup, the measurement electronics is connected to said anode via DC-coupling. It can be seen that the current for a count rate of 1.000 Mcps (million counts per second) is much higher than the one for just 100 Mcps and that with 10 Mcps. The current measured for a count rate of just 1 Mcps can hardly be seen in the figure.

(3) Similarly, FIG. 3 shows an identical setup but with the measurement electronics coupled to the anode of the PMT via AC-coupling. When using AC coupling, the mean current is obviously zero. Nevertheless, the current fluctuation differs quite substantially, which can be seen again for count rates of 1.000, 100, 10 and 1 Mcps. Here, the dependence is not a linear one as the current fluctuation varies with the square root of the mean DC current.

DETAILED DESCRIPTION

(4) When determining the dose rate {dot over (H)} it is often sufficient to apply a non-linear correction function that depends on the mean measured energy, Z(η), η being a measure for the mean energy of each pulse. This function is sufficient to compensate the dose rate error of the mean current and should rectify the efficiency difference between a real detector and a tissue equivalent detector. As a consequence, the dose rate can be calculated as follows, λ denoting the count rate:
{dot over (H)}=Z(η)λη  (Eq. 1)

(5) As the function Z(η) is known or can at least be easily determined for each scintillator used in the RDS, two parameters need to be determined for the determination of the dose rate value: the mean energy of the radiation source and the mean current which can be calculated from the count rate and the mean energy of the pulses.

(6) Further calculations show, that the mean square difference Msd of the sampled current signals i.sub.Δ is a measure of the mean current:
Msd(i.sub.Δ)=λE(i.sub.v)=λη  (Eq. 2)

(7) The mean energy E.sub.γ is retrieved by dividing the current variance Var(i.sub.Δ) by its mean square difference Msd(i.sub.Δ):

(8) $\begin{array}{cc}\eta \approx E_{\gamma }=\frac{\mathrm{Var}\left(i_{\Delta }\right)}{Msd\left(i_{\Delta }\right)}=\frac{{E\left(i_v\right)}^2+\mathrm{Var}\left(i_v\right)}{E\left(i_v\right)}& \left(\mathrm{Eq}.3\right)\end{array}$

(9) It follows, that the actual energy compensated dose rate can be determined from the measured and sampled current data i.sub.Δ as follows:

(10) $\begin{array}{cc}\stackrel{.}{H}=Z\left(\frac{\mathrm{Var}\left(i_{\Delta }\right)}{Msd\left(i_{\Delta }\right)}\right)Msd\left(i_{\Delta }\right)& \left(\mathrm{Eq}.4\right)\end{array}$

(11) This method works with any kind of scintillator material, including inorganic scintillator crystals like NaI(Tl), CsI(Tl), CsI(Na), LaBr.sub.3, CLYC, CLLB, CLLBC, CdWO.sub.3, LYSO, NaI(Tl,.sup.6Li), Gd.sub.3Al.sub.2Ga.sub.3O.sub.12, which are frequently used in spectrometers for identification of radionuclides. As a direct measurement of the pulse energies is not required, the method can also be used in all pulse pileup scenarios and with a reduced amplification of the photodetector—amplifier combination. It is therefore possible to determine high dose rates, that is dose rates which would lead to a shutdown of the amplifier of a RDS when operated according to methods known in the prior art due to high count rates.

(12) As the method claimed allows the use of a standard RDS with a scintillator material, making use of high sampling rates, for the measurement of high dose rates, it is no longer necessary to provide an additional detector like a Geiger-Müller counter or an ionization chamber for this measurement. The setup of such detectors is easier therefore, reducing material, complexity and cost. This is of special advantage in portable radionuclide identification devices (RID), which have to be as simple and lightweight as possible in order to increase robustness and reliability apart from the reduction of costs.