METHOD FOR DETERMINING THE NEUTRON FLUX BY USING A PORTABLE RADIONUCLIDE IDENTIFICATION DEVICE (RID) COMPRISING SCINTILLATION MATERIAL WITH IODINE

Abstract

A method for determining neutron flux by utilizing a portable Radionuclide Identification Device (RID) as it is used in homeland security applications is provided. The RID has an inorganic crystal comprising iodine, a light detector and electronics for the evaluation of the output signals of the light detector. The method includes a step of detecting, with the light detector, light emitted by the crystal following the interaction of nuclear radiation with the crystal. The intensity of the light measured is a function of the energy deposed in the crystal by said nuclear radiation during the interaction with the crystal.

Claims

1. A method for determining neutron flux by utilizing a portable Radionuclide Identification Device (RID) as it is used in homeland security applications, comprising an inorganic crystal comprising iodine, a light detector and electronics for the evaluation of the output signals of the light detector, the method comprising the following steps: detecting light emitted by the crystal, following the interaction of nuclear radiation with the crystal, with the light detector, whereas the intensity of the light measured is a function of the energy deposed in the crystal by said nuclear radiation during the interaction with the crystal, determining the energy deposed in the crystal by an interaction (event) by evaluating the output signal of the light detector, preferably by applying a standard pulse height evaluation, selecting a first group of events by selecting those events, having deposed an energy in the crystal above a first pre-determined threshold E.sub.T1, whereas the first threshold E.sub.T1 is high enough so that the majority of events measured originate from the interaction between high energetic cosmic radiation like muons (μ), selecting a second group of events by selecting those events having deposed an energy in the crystal above a second threshold E.sub.T2 and at the same time below a third threshold E.sub.T3, whereas the second threshold E.sub.T2 is higher than the maximum energy deposed by gamma radiation E(γ).sub.max to be measured and lower than the third threshold E.sub.T3, and whereas the third threshold E.sub.T3 is lower than or equal to the first threshold E.sub.T1 so that E.sub.T2 and E.sub.T3 define a window W(n,μ) in the energy domain, determining the count rate R(μ) of the first group of events, determining the count rate R(n,μ) of the second group of events, determining the background count rate R(back) from the expected count rate R(μ).sub.exp of interactions of the cosmic radiation with the crystal in the energy range of the window W(n,μ), said expected count rate R(μ).sub.exp being calculated from the count rate R(μ) and the known energy distribution of high energetic cosmic radiation, determining the count rate R(n) by subtracting the background count rate R(back) from the count rate R(n,μ), the count rate R(n) being a measure for the neutron flux.

2. The method of claim 1, wherein the crystal is selected from a group of sodium iodide and cesium iodide.

3. The method of claim 1, wherein the first threshold is 10 MeV.

4. The method of claim 1, wherein the second threshold is 3 MeV.

5. The method of claim 1, wherein the third threshold is 5 MeV.

6. The method of claim 1, wherein the count rate R(back) is further corrected by adding the count rate of pileup events R(n,μ).sub.pileup within the window W(n,μ).

7. The method of claim 6, wherein the count rate of pileup events R(n,μ).sub.pileup is calculated by multiplying the calculated pileup count rate R(n,μ).sub.sum in the Window W(n,μ) with the mean count rate r and the pulse pair resolving time t.sub.ppr.

8. The method of claim 7, wherein the expected count rate of pileup events R(n,μ).sub.sum is calculated from the count rate of events built by the sum of two consecutive random events in the window W(n,μ).

9. The method of claim 1, wherein an alarm is generated when the count rate R(n) is increasing above a threshold, said threshold being calculated from the actual background count rate R(back), within a predetermined time.

10. A portable radionuclide identification device (RID), comprising at least an inorganic crystal, said crystal comprising iodine, a light detector and electronics for the evaluation of the output signals of the light detector, characterized in that the electronics is set to evaluate signals being equivalent to energies deposited in the crystal of more than 5 MeV, preferably more than 10 MeV and that the RID is set to conduct a method for determining the neutron flux according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0018] An embodiment of the invention is described, referring to

[0019] FIG. 1 showing a measured background spectrum in the energy domain (‘energy spectrum’) with an extended energy range up to 1 GeV, and

[0020] FIG. 2 showing two measured spectra in the energy domain with an extended energy range of up to 100 MeV, namely a background spectrum with and without additional neutron source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The inventor found that the relevant events can be separated into three regions of interest (ROI), separated by the energy deposed in the crystal. The first ROI—ROI(γ)—covers energies up to 2.6 MeV. In this region, events originating from external gamma radiation (gamma events) dominate. It is the main purpose of an RID to evaluate those gamma events and to determine the origin of the gamma radiation source on the basis of the energy and the distribution of said gamma events. Other than a detector used for example in nuclear research, an RID only needs to detect gamma events originating from natural sources. Embodiments of the invention therefore makes use of the fact, that there is more or less no gamma radiation from natural sources, i.e. from decaying nuclei, with gamma energies of more than 2.6 MeV. Being more exact, the highest gamma energy expected from such sources is E(γ).sub.max=2,615 MeV (2.615 keV). The inventor also found that the sum energies, that is the sum of one or more gamma quanta being emitted from excited states coincidently, of gamma events from relevant coincident decays, i.e. from .sup.60Co or .sup.137Cs, is smaller than 2.6 MeV also when originating from natural sources.

[0022] As a consequence, an event with energies above 2.6 MeV, preferably above 3 MeV, measured by an RID, can be treated as not originating from external gamma radiation, but from heavy particles like neutrons, protons or muons, interacting with the crystal only. In order to be able to evaluate events with energies above 2.6 MeV, the electronics controlling the RID needs to be modified in a first step, enabling the RID to detect and to evaluate events with energies above 2.6 MeV. A standard RID is not set accordingly as the crystal is only used to detect gamma radiation events from natural sources, i.e. events with energies lower than 2.6 MeV.

[0023] Some neutrons will be captured by the iodine nuclei present in the crystal. This will lead to excited states in iodine which will decay to ground level by emitting gamma radiation, said gamma radiation having sum energies starting at around 2.6 MeV, mostly in the energy range between 3 and 8 MeV with a significant peak between 3 and 5 MeV.

[0024] The mean energies of muons at the earth's surface is around 4 GeV. Those muons will deposit an energy in the order of 4 MeV/cm in the crystal. For a diagonal trajectory in an 5×5 cm NaI crystal, the maximum energy deposed would be in the order of 28 MeV. However and due to other effects, muons will deposit up to 200 MeV of energy in the crystal (‘muon energy’ E(μ)).

[0025] As a consequence, events with measured energies of E.sub.T1=10 MeV and above are more or less solely events, originating from interactions between cosmic radiation—mainly muons (μ)—and the crystal, whereas events with energies above E(γ).sub.max=2,615 MeV and below E.sub.T1=10 MeV may originate from either neutron events or muon events. Those thresholds define the next two ROIs of relevance here: ROI(μ) is the region above event energies of 10 MeV, that region dominated by interactions of muons with the crystal. The third ROI, ROI(n,μ), ist the region between a second threshold E.sub.T2 and a third threshold E.sub.T3, E.sub.T2 being larger or equal E(γ).sub.max and a third threshold E.sub.T3, being lower or equal the first threshold E.sub.T1 but higher than E.sub.T2.

[0026] In the present embodiment, E.sub.T1 is set to 10 MeV, E.sub.T2 to 3 MeV and E.sub.T3 to 5 MeV. FIG. 1 shows the resulting energy spectrum measured overnight without a radiation source, using a RID with a NaI(Tl) crystal, the crystal having a standard size of 2×2 inch and set to detect and evaluate events with energies up to 1 GeV. The region with energies above E.sub.T1=10 MeV, ROI(μ), is marked with C. The ROI(n,μ) region between E.sub.T2 and E.sub.T3 is marked with B1 and covers the region between E.sub.T2=3 MeV and E.sub.T3=5 MeV, the gap between 5 and 10 MeV is marked with B2. Setting such a gap ensures a clear separation of the events.

[0027] The events below E.sub.T2=3 MeV are gamma events, resulting from background radiation.

[0028] FIG. 2 shows two measured spectra with an RID, comprising a 2 inch×2 inch NaI(Tl) crystal, set to detect and evaluate events with energies up to 100 MeV. Spectrum 100 is again a spectrum showing the background radiation, whereas spectrum 200 shows a background spectrum with a neutron source present in addition. Region A in FIG. 2 again covers energies up to 3 MeV, therefore showing the background gamma events. Region C, the ROI(μ), covers energies between about 7 MeV and 100 MeV, i.e. the region where cosmic (muon) radiation begins to dominate. Region B between 3 MeV and about 7 MeV is covering the neutron capture region, where cosmic radiation is present in addition, i.e. the region ROI(n,μ).

[0029] It can be seen from FIG. 2 that the count rates in regions A and C are more or less identical in the two spectra 100 and 200, whereas a significant difference can be seen in region B. The events measured in region B are, as far as the background of spectrum 100 is concerned, cosmic muons and, in spectrum 200, the sum of neutron- and muon-events.

[0030] It has to be mentioned further that the spectrum shown in FIG. 2 has been collected over several hours, therefore providing good statistics which allows to clearly distinguish between spectra 100 and 200 in region B—ROI(n,μ). During real measurements with an RID, the RID has to issue an alarm within seconds and therefore has to use spectra with significant fewer events. As a consequence, spectra 100 and 200 would hardly be distinguishable if compared like in FIG. 2.

[0031] In order to quickly determine the neutron count rate as required for an RID in a spectrum measured like spectrum 200, the measured count rate has to be corrected by the background count rate, i.e. the muon count rate.

[0032] As an RID is to be used in different surroundings with changing background count rate, it is not possible to determine the background radiation in advance in a separate measurement as one would do in a laboratory or when using a non-portable detector. Therefore, the total height of the muon induced background events in ROI(n,μ) is, like the corresponding background count rate R(n,μ), unknown, as they are dependent at least from the position of the RID and the time in the year, as the muon intensity is varying over the year.

[0033] Embodiments of the invention make use of the fact that the energy distribution of the muons interacting with the crystal is known and can be calculated for the region ROI(n,μ) for each crystal. The events within ROI(μ) are deemed to be muon events only. The same is true for the respective count rate R(μ). With the knowledge of the number of muon events in ROI(μ) over a predetermined time—the muon count rate R(μ) in ROI(μ)—and the muon energy distribution in the NaI(Tl) crystal used, the number of expected muon events in ROI(n,μ) over time—the expected muon count rate R(μ).sub.exp in ROI(n,μ)—can be calculated. Subtracting this number from the total count rate R(n,μ).sub.total of events in ROI(n,μ) provides the number of neutron events over time, i.e. the neutron count rate R(n) which is a measure for the actual neutron flux:

R(n)=R(n,μ).sub.total−R(μ).sub.exp  (1)

[0034] As the detection efficiency for neutrons can be easily determined for a specific RID in advance, the actual neutron flux can be determined from the measured neutron count rate R(n). The RID according to this embodiment will issue an alarm as soon as the neutron count rate R(n) increases above a predetermined value.

[0035] The measurement of the neutron count rate R(n) can be further improved when pileup information is taken into account also. Known means against pileup are count rate limitation and a tight pileup rejection with minimum pulse pair resolution time t.sub.ppr, but there will still be an inevitable statistical pileup which cannot be avoided completely.

[0036] The RID of the preferred embodiment is using state of the art pulse processing for the evaluation of the events measured, namely pulse height discrimination (PHD) for distinguishing gamma radiation from neutron capture events from other events. When the count rate of events is high, pulse pile up may occur so that pile up events originating from several different gamma events within a small time window, so called random coincidences, may look like events with higher energies. Some of those will increase the number of events in ROI(n,μ) without originating from interactions with neutrons and/or muons, thus increasing the measurement error.

[0037] The inventor used statistical considerations, making use of the fact that the emission of gamma quanta is Poisson distributed. A pileup occurs when the time between two pulses is shorter than the pulse pair resolving time t.sub.ppr of the detector, here the portable RID. In order to estimate the pileup rate in the neutron capture region, that is in ROI(n,μ), one has to know the total (artificial) coincidence probability and the sum spectrum energy distribution.

[0038] This pileup can be estimated by creating an artificial sum spectrum from the measured gamma events. This sum spectrum is set up by adding the energies of two consecutive events and creating a new spectrum from those summed up events only. The rate of sum events measured in ROI(n,μ), that is the rate of sum events with E.sub.T2<E.sub.γ(sum)<E.sub.T1, is R(n, μ).sub.sum. A good estimation of the rate of false pileup events caused by statistical pileups in the region ROI(n,μ)−R(n,μ).sub.pileup—can be calculated by scaling that rate of sum events by r t.sub.ppr, r being the mean count rate:

R(n,μ).sub.pileup−R(n,μ).sub.sum*rt.sub.ppr  (2)

[0039] In order to further correct the measured count rate of neutron events, it can be further corrected by taking the estimated pileup count rate within ROI(n,μ) into account also:

R(n)=R(n,μ).sub.total−(R(μ).sub.exp+R(n,μ).sub.pileup)  (3)

[0040] It has to be mentioned that both, R(n, μ).sub.sum and r, are obtained from actual measurement data during the measurement in real time so that this further correction is accurate and adapted to the actual measurement situation.

[0041] Whereas embodiments of the invention relate to the evaluation of events with energies above E(γ)max, it goes without saying that the portable RID described herein is at the same time capable of measuring gamma radiation with energies below E(γ).sub.max and to evaluate those in order to identify the radionuclides being the source of said gamma radiation. Those events lying in the ROI(γ) are evaluated in parallel according to standard evaluation methods for gamma radiation detected with RIDs so that the detection and identification of gamma radiation will occur in completely parallel to the measurement of the neutron flux.

[0042] One big advantage of embodiments of the inventive methods is that they work with a standard RID without a separate neutron detector. It is therefore possible to enable existing RIDs without neutron detection capability by adding such a neutron detection capability without hardware modification. A software update, implementing the disclosed method, may be sufficient.

[0043] At the same time, the inventive method is not limited to those standard RIDs. It may also be used to improve already available neutron detection by combining the measurement results. This is especially true when a scintillator, comprising iodine, is used in combination with a surrounding neutron absorbing material as known in the state of the art, that material capturing neutrons and emitting gamma radiation which is then used to detect the neutron radiation. Any such capture material with a (n,γ) reaction, for example .sup.6Li, may be used. Such a neutron absorbing material may also be placed within the crystal itself, which is sometimes the case with .sup.6Li.