Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02285327 1999-09-29
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Automated Surveving for Radiation
This invention relates to a method and an apparatus
for surveying an area for radiation from radioactive
materials, and for recording the locations at which
radiation is detected.
It iknown to survey an area for the presence of
nuclear radiation by scanning a geiger counter; an
operator can note down hip; Location (relati.ve to
convenient landmarks) i~ the count rate exceeds the
expected background level. Such an instrument is
portable, and might be used to search for radioactive
minerals, or for localised areas of contamination. A
more sophisticated survey instrument incorporates a gamma
spectrometer, combined with a locating instrument
utilising ground-based or satellite-based radio signals
(such as Decca, Loran, or GPS), but this is both
expensive and too large and heavy to be portable - it
might be moved by helicopter or in a vehicle.
According to the present invention there is provided
a portable apparatus for surveying an area for radiation,
the apparatus comprising a GPS locating instrument, to
enable the apparatus to be located to better than ~ 2.0
m, a gamma-sensitive scintillator to provide signals
corresponding to the energy of detected gamma rays, means
to classify the signals into a plurality of different
energy ranges, and to count the numbers of signals in
each energy range, and automatic memory means to record
the location and the said counts at intervals.
It is generally necessary to provide correction
signals to the GPS positional information, the correction
signals being derived from a fixed base station, to
enable the required positional accuracy to be achieved.
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It may be possible to achieve even better positional
accuracy, for example to better than ~ -0.5m.
In the preferred embodiment. there are three
different energy ranges. The thresholds between the
energy ranges may be set. in accordance with the
radionuclides which are to be detected, and may for
example be adjusted and calibrated in comparison to
signals from a gamma spectrometer prior to use of the
apparatus, using a standard gamma source.
The memory mean. is desirably arranged to recor:-d the
data at regular time inter-~.ral:~, for example once every
second. The apparatus can be carried by an operator
around an area to be surveyed, for example in a back
pack, and the stored data can subsequently be downloaded
from the memory means into a computer. The computer may
display a map of the area, indicating any areas where
radiation was detected, possibly displaying contours of
equal radiation; and also indicating any areas which have
not been surveyed.
Thus the invention also provides a method for
surveying an area for radiation from radioactive
materials, wherein an operator carries a portable
surveying apparatus around the area, the apparatus
comprising a GPS locating instrument to enable the
apparatus to be located to better than ~ 2.0 m, a gamma-
sensitive scintillator to provide signals corresponding
to the energy of detected gamma rays, means to classify
the signals into a plurality of different energy ranges,
and to count the numbers of signals in each energy range,
and automatic memory means to record the location and the
said counts, the location and the said counts being
recorded at sufficiently frequent intervals that the
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operator has moved no more than 2.0 m between successive
such records, and the method also comprising subsequently
downloading the recorded data, and thereby generating and
displaying a map of the area showing values of gamma rate
counts.
Tlle data concerning location and counts is
preferably recorded every second, which enables an
operator to walk over the area, at a normal walking pace,
to generate the data for a comprehensive survey. To
ensure that the entire area is surveyed, tlue operator
will usually walk to and fro from one side of the area to
the opposite side, successive paths being for example
parallel lines at a spacing of i.0 m. The count rate
data is preferably recorded even if no GPS signal is
obtained (for example because the operator passes under a
tree), and can also be subsequently downloaded; it may be
possible to calculate the corresponding locations by
interpolation or extrapolation from previous and/or
successive locations at which a GPS signal was obtained.
Thus all the count rate data is recorded, as unprocessed,
raw data, and this uncorrupted data can be retrieved
subsequently if desired.
The portable apparatus desirably also includes a
display to show the location as determined by GPS, so
that if the operator wishes to return subsequently to a
specific part of the survey area (for example because the
displayed map indicates that no measurements were taken
in that area) he can readily do so. It may also include
a display to show the count rate.
The invention will now be further and more
particularly described by way of example only and with
reference to the accompanying drawings in which:
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Figure 1 is a block diagram cf surveying apparatus;
and
Figure 2 is a circuit diagram of part. of the
surveying apparatu;> oB Fi gore 1 .
Referring to Figure 1, a surveying apparatus 10
includes a backpack 12 (indicated by a broken line) to be
carried by an operator. All the equipmentin the
backpack 12 is powered by batteuie~ (not shown). The
backpack 12, include: a date: logger: 14 wlncln ar one :second
interval~> stores digital data from a GPS receiver 16, and
a signal processor 18 (de.>cribed below in more detail
with reference to Figure 2).
The GPS receiver 16 is a 'I'rimble GPS Pathfinder
(trade mark) which has twelve channels to receive signals
via an aerial 20 from three or more GPS satellites at
once. The aerial 20 can also receive marine band (300
kHz) differential correction information from a base
station such as a lighthouse, if such information is
broadcast without encryption, so enabling the GPS
receiver 16 to determine its position to about ~ 1 metre.
In some parts of the world differential correction
information is broadcast in encrypted form (for example
by Trinity House/Scorpio Marine), or is broadcast as an
RDS signal on a non-marine band wavelength (for example
by Classic FM), the radio transmitter being indicated at
21, and the backpack 12 includes a radio receiver 22 for
receiving such signals and providing them to the GPS
receiver 16, so the receiver 16 can continuously and
accurately determine its position. The data logger-14
may incorporate a display to show the current position.-
If neither source of differential correction information
is available then the apparatus 10 would include a
second, identical GPS receiver 24 with an aerial 25 and a
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data logger 26, situated at an accurately known position,
referred r_o as a base station 27.
The data logger 14 in the backpack 12 not only
S stores signals from the GPS receiver indicating its
position, but also signals from the signal processor 18
which receives signals from a higlu efficiency 75 mm
diameter sodium iodide scintillator 30 with a
photomultiplier 32. The scintillator 30 detects ganuna
1(I rays, and the plnotomultiplier ~2 generates electrical
pulses whose magnitude is related to the energy of tlm
detected gamma ray;;. The signal processor 18 rec:eivea
this seiie;~ oL analogue pulses, county the numbers of
pulses in each of three different energy r-ange:~, and
15 provides these counts as digital signals to the data
logger 14 every second.
After an operator has walked to and fro with the
equipment in the backpack 12 over the area to be
20 surveyed, the data from the logger 14 is down-loaded into
a computer 34. The computer 34 analyses the data and can
for example provide a map output displaying the gamma ray
intensity at different parts of the area, highlighting
any parts of the area which have not been surveyed. The
25 gamma ray counts at different energy windows can allow
specific radio isotopes to be identified. Tf ir_ had been
necessary to use a base station 27, then the data from
the data logger 26 is also downloaded to the computer 34,
which corrects the positional information provided by the
30 GPS receiver 16. Thus the generation of a map displaying
the measured values of gamma counts can be performed
automatically by the computer 34. A contour map showing
variations in gamma intensity over the surveyed area may-
also be displayed.
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Referring now to Figure ~~, this show a circuit
diagram of part of the ~i.gn~~l processor 18. The analogue
pulses from the photomultipl.ier 32 are supplied via a
buffer (not shown) and input lead 40 to the positive
inputs of_ three differ==:tial amplifiers 41, 42 and 43.
The negative inputs cf one amplifiers 41, 42 and 43 are
connected to adju~;table resistor contact; 44, 45 and 46
respectively which form, part of a potential divider
network 47 between two =fixed voltage level ~ V, and V_,.
The contact 44 i.~ set .,~ the amplifies 41 give~~ an output
signal for any input pu'_~E-~ abovE_ the noi"e level . The
contacts 45 anc~ 46 ar.-a per ao tlec= amplify ei--:> 42 and 43
give output sigrmls fur- any inpmt pulse ~ large? than
respective thresholds, v.he threshold for the amplifier 43
being the higher of the two. The output ::signals from the
amplifier 41 are supplied vi.a a buffer 48 to
microcontroller 50, and the output signals from the
amplifiers 42 and 43 are supplied via respective buffers
48 to bistable latch units 52 which provide output
signals to the microcontroller 50.
When the microcontroller 50 detects the back edge of
a pulse from the amplifier 41 it reads the state of each
latch unit 52 to see if it has received a pulse, and then
transmits a reset. signGl via lead 59 to each latch unit
52. The microcontroller 50 counts the numbers of pulses
received via each amplifier 41, 42 end 43, and once a
second these three counts are transmitted (as digital
signals) to the data lcgger 14 via the output lead 55,
and the stored counts are reset to zero. The signal
processor 18 also includes a liquid crystal display 56
which may be switched to display the number pf counts per
second in both digital and pseudo-analogue form. It may
be arranged to display the total count rate (i.e. those
provided by amplifier 41), the count rate below the lower
T __ _... ___ __ T
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threshold (i.e. the count 'rom amplifier 41 minus the
count from amplifier 42), the count rate in the energy
windova between the two thresholds (i.e. the count from
amplifier 42 minus the count from amplifier 43), or the
count rate above the upper threshold (i.e. t1e count. from
amplifieu 4J). The microcontrolle~_- 50 also pr.-ovides an
output. digital signal for every pulse received from the
ampli.fiEr 41, which can be supplied to headphones 58 for
the operator.
It v,~ill be appreciatec: that a surveyinca apparatus
may differ from that descr~.bed above while remaining
within tire ;cope of the i r:-.-ention. For example the
scintillator might be of caesium iodide, and might be of
a different sire to tluat described. The location and the
count rates might; be recorced at different intervals,
such as every 10 seconds, or every 0.5 second. All the
components to be carried by the operator were described
as being in the backpack 12, but it will be appreciated
that an operator might instead carry some of the
components separately, connected electrically to the
components in the backpack 12. For example he might hold
the display 56 in one hand, to be able to see what count
rate is being recorded, and he might carry the
scintillator 30 and photomultiplier 32 on a shoulder
strap, adjusted so the scir:tillator 30 is about 0.5 m
above the ground.
A surveying apparatus might include other types of
detectors, for example a diode to detect beta radiation,
along with a signal processor similar to the processor 18
described above including a microcontroller to count the
detected beta rays. A surveying apparatus might thus
include a plurality of microcontrollers; and these may be
connected in cascade, so the stored counts from one are
transmitted to the next in the chain, to be transmitted
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on along the chain along with its own stored counts. The
last one in the chain would then transmit to the data
logger 14 the counts detected by each signal processor.
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