GB2239943A - Environmental radiological monitoring - Google Patents

Abstract

Apparatus for environmental radiological monitoring comprises an atmospheric air sampler 10 consisting of an air pump 11 and a dust collecting sampling head 12, the head 12 being connected to a housing 14 containing a radiation detector 13 calibrated by a calibration/timing device 15. The housing 14 has an opening permitting the insertion of radiation filters of various thicknesses between the sampling head 12 and the detector 13. The activity concentrations of alpha and beta emitters in air, and the absorbed gamma dose, may be monitored. <IMAGE>

Description

ENVIRONMENTAL RADIOLOGICAL MONITORING This invention concerns environmental radiological monitoring apparatus primarily for use by public health and emergency planning organisations charged with responsibility for the detection, minotring and management of environmental radiological emergencies.

An environmental radiological emergency (ERE) is taken to be any occurance involving the release and dispersal of radioactive material which so contaminates the environment as to result in an exposure of the population likely to exceed the dose equivalent limit.

Within this definition the inventors recognised that an environmental radiological emergency can be both acute, short duration exposure to high levels of hazard requiring urgent action, and chronic exposure to relatively low levels of hazard were there is a considerable period allowable for the more sensitive anal isis of the relevant parameters before dose equivalent limits are reached.

Current arrangements for the management of radiological emergencies are based on the simplistic scenario that the duration of the emergency will be short, and that corrective action at the source will staunch the release of radioactive material in a matter of hours. Action in the event of an acute emergency is directed by reference to Emergency Reference Levels (ERLs), and Derived Emergency Reference Levels (DERLs).

ERLs are levels of exposure, and therefore risk, above which planned remedial action, e.g. sheltering, taking iodine tablets, evacuation etc., could be implemented with the benifit. Correspondingly, DERLs are derived levels for the parameters, e.g. integrated air concentration (Bq.s.k.-3), ground deposition (Bq.m-2) which relate to the many exposure pathways e.g.

inhalation or radionuclides in the plume, inhalation of resuspended material respectively.

A logical extention of this generally accepted thinking is that an environmental monitoring system which is capable of measuring the parameters which relate to all the exposure pathways is essential for the best practicable management of an ERE. In particular the system must be sensitive enough to measure the most critical parameter at levels which allows time for consideration and implimentation of counter measure.

Furthermore, any system which perports to be an environmental monitoring system which does not measure all pathway parameters should be rejected as unsuitable.

Examination of the exposure pathways in the "early phase" of an acute ERE and the parameters which must be measured to determine exposure by that route are described in the following table: TABLE 1 Early Phase Exposure Pathways Exposure Pathway Parameter Dimension Direct gamma Absorbed Gy/h radiation from dose-rate plume.

Inhalation Activity Bq.s.m.-3 in plume. in air Radiation from Activity Bq.s.m-3 beta emitters in in air cloud Irradiation Deposits Bq.s.m-3 from skin on skin deposit Inhalation of Activity Bq.s.m-3 resuspended matter. in air Radiation from ground Deposited activty Bq.m-2 Since the relative importance of each route is indeterminate in advance of any accident, equal weight has been given to each.

An object of the present invention is to propose some means of measuring external absorbed gamma dose, and the activity concentration of alpha emitters, pure beta emitters and beta-gamma emitters in air, the sensitivity of each measurment being such as to optimise the time available for decision making, and the implementation of emergency plans or remedial action.

The chronic ERE is characterised by the availability of time to make the much more sensitive measurements required to detect and measure it. As far as the inventors can discover there is no direct measuring system capable of measuring the relevant parameters with the required sensitivity for direct monitoring to be possible. There is a need to sample and radiometrically analyse, where possible the primary parameter. This sample also serves as an historical record which can be used for verification purposes.

The Chernobyl accident exposed serious shortcommings in the ability of regulatory bodies and their executives to adequately monitor, in a radiological emergency, the parameters which relate to the multiple pathways of population exposure. Even now, some four years on, the British Government's monitoring system RIMNET, set up with the advice of NRPB, falls a long way short of achieving the state of preparedness which would be required to provide best practicable public protection should another accident occur. The RIMNET system fails to acknowledge more than one exposure pathway since it relies exclusively on a detection system which, though excellent for its intended function, measures only one of the many parameters required to determine population exposure.

RIMNET measures only external gamma absorbed dose. It makes no attempt to measure radioactivity in air, and is indeed incapable of detecting, let along measuring, those parameters which in some accident scenarios would be of major importance e.g. alpha particle emitters, like the plutonium isotopes, in air, and pure beta particle emitters, like strontium 90, in air.

Many local authorities and public health departments have followed similar advice to that given to Government and purchased gamma absorbed dose measuring devices with the same inherent inadequacies.

Compounding these shortcommings is the lack of any tangible historical record of conditions during an emergency which can confidently be used to confirm or refute spurious readings or reports by enthusiastic amatures of observed increases in radiation levels.

The "early phase" exposure pathways and the parameters which are measured for the estimation of exposure via each pathway are shown in table 1. The problem addressed by the present invtors was to devise a system which will measure all the required parameters, or at least those which can be used to derive those which cannot be measured direction. Cost and simplicity of operation are also given a high priority.

It was decided to examine, within the self imposed constraints of cost and simplicity, the possibility of measuring with one detector and a simple scaler/timer absorbed gamma dose, and activity concentration in air.

The measurement of absorbed gamma dose during an acute ERE was not seen as a major problem. Instruments currently used for this purpose are relatively simple.

In a chronic ERE the inhalation routes of exposure are likely to be more critical.

The measurement of activity concentration in air with an appropriate sensitivity requires more thought.

Examination of the values for DERLs for integrated air concentration for alpha emitters and beta emitters shows a three orders of magnetude difference for the most restrictive nuclides. It became clear then that the errors inherent in any system which did not differentiate between alpha and beta activity would be unacceptable. However, the "on the safe side" errors in assuming that the alpha emitters present were all the most restrictive Pu239(W) and that all the beta emitters were Sr90 might be an acceptable trade off for the benefits of an early result.

It is therefore the primary object of the present invention to proposed monitoring apparatus which enables operators to determine in good time and with a known confidence whether or not an acute radiological emergency exists, and collects a sample related to the most important exposure pathway.

With this object in view the present invention provides environmental radiological monitoring apparatus comprising a radiation detector, a scaler/timing device for the radiation detector, and an atmospheric air sampler having an air pump and a holder for a dust collecting filter, said apparatus also including a housing, for the radiation detector, to which the air sampler is connectable so that the dust collecting filter is in close proximity to the radiation detector, the housing being adapted to enable the insertion of radiation filters of differeing radiation absorption capacity between the detector and the filter.

In addition to measuring external gamma absorbed dose (like RIMNET), the apparatus of the present invention measures alpha emitters in air, and beta emitters in air, gamma emitters in air, and estimates skin and ground deposition using assumed deposition velocities. Its operation depends upon the collection of airborne particulates on the filter which is advantageously a glass fibre filter paper, and the measurement of the activity on the paper with a single detector which differentiates between alpha and beta emitters. The detector also responds to and measures external gamma absorbed dose. Rapid, first stage, measurements are made daily with a sensitivity and known confidence appropriate for acute emergency monitoring.

These are followed by the analysis of the filter papers collected over a period. Here measurements of greater sensitivity are used which allow the detection and determination of chronic emergencies which by their nature allow time for this to be done.

The invention will be described further, by way of example, with reference to the accompanying drawings in which: - Fig. 1 is a schematic block-diagram of the monitoring apparatus of the invention; Fig. 2 is a side elevation of a housing for the radiation detector which forms part of the apparatus of Fig. 1; and Fig. 3 is a cross-sectional elevation of the housing of Fig. 2.

Referring firstly to Fig. 1 it will be seen that the apparatus of the invention comprises an atmospheric air sampler 10 which consists of a high volume rotary vane pump 11 and a dust sampling head 12 which has a holder for a 6cm GF/A filter paper supported on a wire mesh. The air flow-rate is chosen by balancing the benefits a large sample and the disavantages of a "thick" source which might seriously affect the alpha counting efficiency.

It also comprises a radiation detector 13 which in this case is thin window 2" G.M. counter type manufactured by Distron, which is placed in a specially designed housing 14 to which the sampling head 12 is connected so that the detector 13 sits above and close to the filter paper. In this position the counting efficiency for alpha particles is about 12 to 15 percent, and about 20 percent for beta particles. The background of the detector 13 is due to an inherent background plus the count due to the external gamma dose. The design and performance characteristics of the sampler 10 and detector 13 optimise the sensitivity of measurement.

The crux of the measurement technique is the use of counting by difference to achieve both the differentiation between alpha and beta emitting nuclides, and sensitivity required for what in some scenarios would be the most critical exposure pathway i.e. alpha emitters in air.

The operation of the apparatus of the present invention hinges on the design and elegant use of the sampler 10 and detector 13.

With no activity on the filter paper i.e. before sampling begins, the system operates as an external absorbed gamma dose meter with a sensitivity and precision comparable with the commonly used Mini 6-80, but with a smaller systematic error. A calibration (using a calibration/timing device 15 e.g. a simple scaler/timer) of absorbed gamma dose-rate vs. count-rate is used to determine absorbed gamma dose-rate. With the sampler 10 run at a known rate over a 23 hour period, with the aid of the calibration/timing device 15, the activity on the paper is measured. The alpha emitters are measured by the difference in count rate without and with a radiation filter of thin aluminium foil interposed between paper and detector 12.The housing 14 is adapted to receive the filter by virtue of the fact that it has an opening 16 (Figs. 2 & 3) in the region where the sampling head 12 connects to the housing. A correction is made for a pre-determined radon daughters contribution, and the net count-rate converted to worst case alpha activity concentration using a Pu-239 counting efficiency and the volume of air sampled.

The filter paper count rate is determined with a radiation filter which is a thick aluminium sheet between paper and detector 13 the further reduction in countrate being due to the stoping of beta particles. A Sr-90 standard is used to determine the Sr-90 efficiency which is then used to determine the beta activity concentration using the worst case assumption that the collected nuclide is Sr-90.

The count-rate with the thick filter or absorber in position is used with a Cs-137 gamma efficiency to determine gamma emitters in air.

Results are then used to determine ground deposition by applying a suitable factor for deposition velocity. NRFB-DL10 suggest that 3.10-3m.s1 for particulate, and 1.10-2m.s-1 iodine are appropriate.

Thus, the operational steps of the apparatus of the invention can be summarised as follows:1. Place a new glass fibre filter paper on the air sampler. run the pump for 23 hours 2. Stop the pump and insert the thick filter, count for 1OOOsecs. Calculate rate A cps 3. Insert thin filter and count for 1000 secs.

Calculate rate B cps 4. Remove all filters and count for 1000 secs Calculate rate C cps.

5. Place a new filter in position and count for 1000s.

calculate the count-rate. Let this count rate be D cps.

6. Restart the pump.

Calculations 1) Activity Concentration of Alpha Emitters (as Pu-239) in Air (C - B) x 100 = Bq / litre El x t x R Where El = Pu counting Efficiency t = Sampling time (hrs) R = Pumping Rate (1/her) 2) Activity Concentration of Beta Emitters (as Sr90) in Air ( (C - B) - A ) x 100 = Bq / litre E2 x t x R Where E2 = Sr-90 Counting Efficiency t = as in 1 R = as in 1 3) Activity Concentration of Beta/Gamma emitters (as Cs-137) in Air (A - D) x 100 = Bq / litre E3 x t x R Where E3 = Counting Efficiency for Cs-137 (with thick absorber present) t = as in 1 R = as in 1 4) Absorbed Gamma Dose Calculated from a linear relationship between absorbed dose and count-rate with constants determined by direct calibration.

Claims (7)

1. Environmental radiological monitoring apparatus comprising a radiation detector, a scaler/timing device for the radiation detector, and an atmospheric air sampler having an air pump and a holder for a dust collecting filter, said apparatus also including an housing, for the radiation detector, to which the air sampler is connectable so that the dust collecting filter is in close proximity to the radiation detector, the housing being adapted to enable the insertion of radiation filters of differeing radiation absorption capacity between the detector and the filter.

2. Apparatus as claimed in claim 1 wherein the radiation detector is a thin window 2" G.M. counter type.

3. Apparatus as claimed in claim 1 or 2 wherein the atmospheric air sampler consists of a high volume rotary vane pump and a dust sampling head housing the holder for filter paper.

4. Apparatus as claimed in claim 3 wherein the filter paper is of glass fibre.

5. A method of measuring environmental radiation using the apparatus as claimed in any one of claims 1 to 4 comprising the steps of: (a) placing a filter paper in the air sampler and operating the air pump for 23 hours, (b) stopping the pump and inserting a thick filter, counting for 1000 seconds, and calculating counts per second (cps), (c) inserting a thin filter, counting for 1000 seconds and calculating cps, (d) removing all filters, counting for 1000 seconds and calculating cps, (e) placing a new filter in position, counting for 1000 seconds and calculating cps, and (d) re-starting the pump.

6. A method of measuring environmental radiation substantially as hereinbefore described with reference to the accompanying drawing.

7. Environmental radiological monitoring apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.