ECONOMIC SOLID STATE ALPHA DETECTOR FOR MONITORING RADON GAS AND ITS DAUGHTER PRODUCTS

DETECTEUR ALPHA PEU COUTEUX A SEMICONDUCTEUR POUR DETECTER LE RADON ET SES DESCENDANTS RADIOACTIFS

English Abstract

Abstract
In the detection of Radon gas it is common for the radon sample
to be admitted to a dark chamber the sides of which are coated
with a 'phosphor' (silver activated zinc sulphide or equivalent).
the resultant light flashes(caused by the alpha emanation striking
the 'phosphor') are then counted by a highly sensitive light
detector such as a photo multiplier tube. The auxiliary circuitry
for the photo multiplier tube is expensive, there is a problem of
the phosphor being contaminated by build up of Radon daughters
and a problem of light contamination in changing chambers in the
field .
In this invention, a less sensitive but economical solid state
optical sensor is used and an increase in sensitivity obtained by
depositing the 'phosphor' directly upon the optical detector. This
may then be sealed with a thin metallic deposit that is opaque to
light but semi transparent to alpha emanation. The reduced sensit-
ivity due to reduced surface area may be compensated for by longer
periods of observation.

Claims

Reference Application #292, 152
Claim 1, the only claim on file is hereby amended to read
as follows:-
1. A photosensitive device such as a photodiode upon the
surface of which a thin layer of alpha sensitive phosphor
such as silver activated zinc sulphide is deposited, and
that a thin metallic deposit is in turn provided upon this
thin layer of phosphor, with the result that the phosphor
is sealed from light.

Description

107897~ Richard Brancker Research Ltd
ECONOMIC SOLID STATE ALP~A DETECTOR FOR MONITORING
RADO~ GAS AND I~S_DAUGH~ER PRODUC~S _
Specification
~his invention relates to an econimical device for detecting
alpha emanation from Radon ga~ and its daughter products.
It is common in the detection of Radon gas (Rn 222 or Rn 220)
for the sample to be admitted to a dark chamber, the sides of
which are coated with Silver activated Zinc Sulphide (or an
equivalent 'phosphor') with the resultant 'light flashes'
(produced as the alpha emanations from the radon gas strike
the phosphor) being counted by a sensitive optical detector
such as a photo multiplier tube.
In the above it is advantageous to use as lar~e a chamber as
possible with respect to the mean free path of alpha in air
so as to contain as much radon as possible (usually 4 to 5 cm
in diameter). In order to detect weak light flashes at the
extremities the optical detector has to be very sensitive and
is usually of necessity a photo multiplier tube. The photo
multiplier tube, though considerably more sensative than a
solid state optical de~ector, requires expensive auxilliary
circuitry in the form of a stable high voltage supply, overload
protection circuitry, etc.
Specification page 1 of 3
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i~ ~897'~ Richard Brancker Research Ltd.
The second disadvantage of the current method is that the
chamber must at all times be kept 'dark' since light also
activates the 'phosphori and the activity due to exposure
to light may take some 5 to 20 minutes to decay to a useable
background. In certain field instruments where the chamber
has to be periodically changed due to radon daughter contam-
ination, this can prove a maJor problem.
I believe that it is possible to achieve reasonable sensitivity
with a simple solid state optical detector such as a photo diode
providing the 'phosphori (Silver activated Zinc Sulphide) is
deposited as a thin layer directly upon the optical detector.
~he effects of light may now also be overcome by adding a thin
metalic deposit over this layer of 'phosphor'. ~his metallic
layer will be transparent to alpha emanations and yet opaque
to light.
In general therefore the invention consists of taking a standard
photo-diode (or other optical sensor) and depositing upon its
surface a thin layer of silver activated zinc sulphide and in
turn depositing upon this a thin metalic coating of a thickness
aust sufficient to satisfactorily inhibit the entry of light.
Specification page 2 of 3
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