Start: September 1997 (month 19)
End: April 1998
Responsible partner: UICE.DG
This task is at a final stage. The beginning of the project was delayed, first because expected funds showed not to be available for the completion of the project (Task 2), and then it was delayed due to some bureaucratic difficulties in transferring funds and resulting problems in scheduling the work.
The design and construction of a new system for automatic monitoring of radon in ground water for earthquake prediction is now in its final phase. The work was delayed seriously because of the late transfer of the grant, more than half a year later than expected. The main designer and constructor of the system left for graduate study in Copenhagen in the middle of the work, but he could continue part time on the project in Copenhagen and work on it during two short periods when he returned to Iceland. Despite this, the progress of the work is satisfactory and the system will hopefully be finished in the middle of April 1998. In order to optimize the radon monitoring work we have emphasized:
The key to the present solution is a novel way of suppressing interfering background pulses, a method which seems not to have been put into routine use before. This is based on delayed counting, i.e. to count only pulses if a pulse has arrived in the preceding millisecond period. Only pulses from Po-214, a radioisotope in the radon decay chain, fulfil this requirement. This secures both low background and strict control over the proper functioning of the system. 200 ml water samples will be collected at the sampling stations and sent to Selfoss, where the radon system will be operated. The radon will be transferred to a toluene scintillation cocktail by bubbling 0.3 liter of air through the water sample and then through the scintillator in a conventional 20 ml liquid scintillation vial. The automatic sample changer is of the carousel type: 8 vials sit on the carousel, which is driven by a step motor. The scintillations are short light bursts that occur each time (1) a radioactive isotope emits an alpha- or a beta particle, (2) when a cosmic ray particle traverses the scintillator or (3) when an external gamma ray is absorbed. The scintillations are detected by a photomultiplier tube, which nearly touches the side of the sample vial. The tube gives an electric pulse each time a light burst arrives. The electronic system consists of a PC-computer and a dedicated interface unit, which is inside the sample changer chassis. This unit contains a power supply, the stepping motor driving circuit, regulated high voltage supply, pulse amplifier and a pulse size threshold detector. The PC-computer reads continuously the threshold detector and records delayed pulses. A major part of the work, the production of the carousel sample changer, is now finished. The electronic system has been designed, printed circuit boards have been made and electronic components are now being soldered to the boards. According to our plan the system will be fully assembled in the middle of April and then tested in our laboratory.
The new LSC apparatus will be set up at Selfoss, in the South Iceland seismic zone, in summer of 1998, where the routine radon measurements will be made. Sampling will be organized at some of the tested sampling sites of our 1977-1993 program, Flúdir, Hlemmiskeid, Kaldárholt, Laugaland, Thorleifskot and Bakki, possibly also at Öxnalækur and Ormsstadir. Tests for consistency and diurnal variations will be run in the beginning, and then regular sampling will begin, at least twice a week. See next section.