GB2077423A

GB2077423A - Alpha-radiation monitor

Info

Publication number: GB2077423A
Application number: GB8110919A
Authority: GB
Original assignee: Kernforschungszentrum Karlsruhe GmbH
Current assignee: Forschungszentrum Karlsruhe GmbH
Priority date: 1980-05-07
Filing date: 1981-04-07
Publication date: 1981-12-16
Also published as: DE3017436A1; DE3017436C2; JPH0145595B2; GB2077423B; FR2482312B1; FR2482312A1; JPS574570A

Abstract

An alpha-monitor for determining the alpha -activity of a process solution, comprises a measuring cell (2) having a trough containing the solution (8) through which a roller (3) rotates and collects a thin film which it carries to a position in front of a radiation detector (5). The detector is protected by a PVC-sheet (14) and a scavenging gas (He) is injected close to the sheet to prevent condensation thereon. A lateral opening (6) is provided into and out of which a calibration bead (4) is displaceable. To determine the background radiation the cell (2) is flushed with the scavenging gas to remove residual test solution and the cell is then charged with a zero activity solution. <IMAGE>

Description

SPECIFICATION Alpha monitor The present invention relates to an alpha monitor which operates in accordance with the principle of a drum analyzer, whereby in a trough of measuring liquid a roller rotates below an opening in which a detector is positioned and brings a liquid film for measurement of a-radiation to a position in front of the detector, and in which the detector surface is protected from the condensate by means of a constant gas flow.

From the a-activity of a process solution in a reprocessing plant its plutonium concentration may be very accurately determined, since other aradiators such as the higher transuranes are separated during the first extraction step with a decontamination factor of 200-1000. Uranium also contained in the solution on the other hand has a specific a-activity which is less by the factor 1 -.

The a-radiation now, because of its extensive reaction with material in solution, only has a range of about 0.05 mm, and in air of about 3.5 cm. Therefore, when measuring the a-activity it is necessary to bring a thin liquid film very close to an a-detector.

The p and y activity also prevailing, which loads the detector, may also be reduced by this. An instrument which fulfils these requirements is the drum analyzer cell. Such a drum analyzer cell is described in PWA report 20/75 (1975), "Alpha-Monitor" of P. Groll et al.

It consists substantially of a small trough through which flows the process solution. By means of a uniformly rotating roller a process liquid is brought from a trough to below a detector, whereby the detector is protected by a foil against the medium, solvent splashes and vapours, so that direct contamination of the counting tube is impossible. A coating of the foil is prevented by gas flushing.

Besides the actual measurement, remote control allows optionally zero or calibrating measurements to be carried out. Closely above the roller a threeposition diaphragm disc is arranged, which is rotatably mounted in the centre and driven unilaterally at the edge.

Owing to the two-point mounting of the diaphragm disc the latter can sweep along the roller.

The contamination so occurring caused by the creep effect of the solvent, is not limited to the underside of the diaphragm disc. This results in an increased background radiation which causes measurements of lower concentration to become more inaccurate.

Particularly serious is the fact that neither a repro- ducible background radiation determination, nor a calibration can be carried out. The cause is to be found in that both measurements are performed by the diaphragm disc which rotates between the roller and the detector. Low contaminations already lead to wrong results.

The object of the invention now resides in providing an alpha-monitor which will allow correct background radiation determination and calibration measurements to be carried out.

According to the present invention there is provided an alpha-monitor, which operates in accordance with the principle of a drum analyzer, having a trough for containing a measuring solution below an opening to a detector, and through which a roller is rotated which brings a film of measuring liquid in front of the detector to enable measurement of the a-radiation, a foil covering the detector, which foil is protected from the condensate by means of a constant gas flow, and wherein an agree zero solution in the trough is used to determine the background radiation from the whole measuring cell and a calibrating preparation is provided which may be located between the roller and the detector.

The particular advantages of the invention are to be seen in that any condensation on the foil and on the calibration preparation is stopped by flushing with helium (He) gas. The He gas flushing is effected so that only about 41.h-1 are required. The development of the trough as container having a low-level outflow allows that after 3 flushing operations at 1 5ml each the zero rate is reproducible. The calibration preparation may comprise a glass bead into which minute quantities of plutonium have been fused. This provides an energy distribution similar to the energy distribution from the solution and the best chemical resistance of the preparation.The measurement rate of the calibration preparation lies within the measurement rate of the solution to be expected, so that after zero measurement by carrying in the calibration preparation at any time the accuracy of the measurements can be checked. The use of a suitable rate meter with background subtraction and recorder output permits the given measurement value to be recorded on a print-out, whereby the usability of the instrument for controlling a continuous process flow is obtained. The use of helium as the scavenging gas increases the sensitivity by 3.5 fold.

The invention will now be described further, by way of example, with reference to the accompanying drawing, showing an embodiment of an instrument according to the present invention.

The drawing shows a section through a measuring cell 2 which substantially comprises a trough 1, a cover 7, a quartz roller 3 and a semi-conductor detector 5 located above the roller 3. The trough 1 may be filled to a predetermined level with measuring fluid or null solution 8, via supply and discharge conduits 9 and 10. The quartz roller 3 rotates in the direction of the arrow and hence collects a liquid film on its surface 11 which is carried to a position below an opening 12 in front of the detectorS. The a-radiation, emanating from this film, is recorded by the detector 5, whilst a-radiation and ,3-radiation directly from the measuring liquid 8 is screened by the roller 3.The region in front of the detector 5 is provided with decontamination ring 13 and the detector 5 itself is protected from direct contamination by means of a thin PVC sheet 14. The detector 5 is secured by means of a holder 15 over the opening 12 in the cover 7 of the measuring cell 2.

During the measurements a scavenging gas, He, is injected close to the protective sheet 14 for the detector 5 in the region in front of the detector 5, which gas prevents condensation of liquid on the foil 14. The scavenging gas, He, is discharged again from the trough 1 through the discharge conduit 16.

For calibration a calibration preparation 4 is used, which is brought in front of the detector 5 through the opening 6. For this is used a rod 17, to the tip of which the calibration preparation 4 is secured in the shape of a glass bead with Pu distributed therein, and which, for exampie, is pneumatically axially (double arrow) displaceable. During measurement the preparation 4 is located within the opening 6 and is screened.

To determine the background raidation, the measuring solution 8 is removed from the trough 1 and roller 3 by means of a treble scavenging and is decontaminated with an organic, agree solution.

During the determination of the background radiation, the whole residual background radiation from the cell 2 is recorded.

Claims

1. An alpha-monitor, which operates in accordance with the principle of a drum analyzer, having a trough for containing a measuring solution below an opening to a detector, and through which a roller is rotated which brings a film of measuring liquid in front of the detector to enable measurement of the a-radiation, a foil covering the detector, which foil is protected from the condensate by means of a constant gas flow, and wherein an agree zero solution in the trough is used to determine the background radiation from the whole measuring cell and a calibrating preparation is provided which may be located between the roller and the detector.

2. An alpha-monitor according to claim 1, in which it operates in a He gas flow.

3. An alpha-monitor according to claim 1, in which the calibration preparation is a glass bead having plutonium homogenously distributed therein which is displaceable through a lateral opening to a position in front of the detector.

4. An alpha-monitor substantially as herein described with reference to and as illustrated in the accompanying drawings.