Note: Descriptions are shown in the official language in which they were submitted.
~2~363S
This invention relates to an instrument for measuring levels of
concentration of tritium and tritium oxides in environmental air.
The invention is applicable, for example, to the monitoring of
tritium levels in the vicinity of nuclear power stations or other
establishments where tritium may present a potential hazard.
Tritium may be present in air in an elemental form or in a combined
form as tritiated water vapour, and since it presents a much
greater health hazard in the latter form it is important that a
tritium monitoring system should be capable of distinguishing
between the two forms. A measuring instrument in accordance with
the present invention is particularly adapted to accomplish this.
Moreover, in cases where the air has to be monitcred in the
presence of background radiation such as gamma radiation and beta-
radiation emitting radioactive gases, it is important that the
system should be capable of distinguishing signals attributable to
the tritium from signals attributable to the background radiation.
A measuring instrument in accordance with the present invention is
particularly adapted to accomplish this.
~ '~
Basically, a measuring instrument in accordance with one aspect of
the invention for measuring levels of concentration of tritium
oxides and/or elemental tritium in air comprises: an instrument
housing providing an inlet and an outlet for air to be monitored, a
light sensing device mounted therein, circuit means connected to
the light sensing device for deriving output signals proportional
to the amount of light sensed therefrom, a thermally conductive
mounting plate providing a mounting surface spaced from the light
sensing device for mounting a scintillator in viewing relationship
thereto, means for flowing air to be monitored through the space
between the mounting plate and the light sensing device, means for
selectively cooling and warming the mounting plate Eor condensing
air moisture onto the scintillator and evaporating condensed mois-
ture therefrom and means for selectively exposing the scintillator
to the light sensing device at timed intervals and for timed
durations.
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The light sensing device may be a photomultiplier having a window.
The means for selectively exposing the scintillator to the light
sensing device may comprise a shutter which is selectively movable
across the field of view of said device, and means for selectively
operating the shutter at timed intervals.
Preferablyl means for selectively exposing the scintillator to the
light sensing device at timed intervals includes a shutter in the
form of an opaque disc having an aperture which in one position of
the shutter is aligned with the window of the photomultiplier for
exposing the scintillator thereto, and further providing means for
locating a reference scintillator at a position such that in a
second position of the shutter the reference scintillator is
exposed to the window, the shutter being operated so as to expose
the scintillators to the window sequentially at timed intervals and
for timed durations.
The output signals from the light sensing device must be processed
to derive a signal representative of tritium oxide concentration,
which signal can be applied to a suitable display device. The
signal processor and the display device may be separate components
or they can be incorporated into the instrument.
,~
According to another aspect of the present invention there is
provided an instrument for measuring levels of concentration of
tritium oxides in air, comprising an instrument housing providing
an internal air flow path for air to be monitored, said air flow
path having an inlet and an outlet, a light sensing device mounted
in the housing, the light sensing device having a window, circuit
means connected to the light sensing device for deriving output
signals therefrom proportional to the amount of light sensed, a
temperature sensor located in the air flow path for sensing the
temperature of air being monitored, a humidity sensor located in
the air flow path for sensing the humidity of air being monitored,
a thermally conductive mounting plate providing a mounting surface
spaced from the light sensing device window for mounting a first
scintillator in viewing relationship thereto, means for selectively
cooling and warming the mounting plate for condensing air moisture
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3Ei3~i
onto the scintillator and evaporating condensed moisture therefrom,
a shutter which is movable across the field of view of said window,
the shutter having an aperture which in one position of the shutter
is aligned with the window Eor exposing said first scintillator
thereto, and further providing means for locating a reference
scintillator at a position such that in the second position of the
shutter the reference scintillator is exposed to the window, means
for selectively operating the shutter at time intervals for
sequentially exposing the scintillators to the window for timed
durations, and signal processing means responsive to the outputs of
the temperat~re sensor and humidity sensor and sequential output
signals from the light sensing device for deriving therefrom a
signal representative of tritium oxide concentration.
In order that the invention may be more readily understood, one
such measuring instrument will now be described, by way of example,
with reference to the accompanying drawings, in which:
Figure 1 is a schematic block diagram of a monitoring system for
; measuring levels as of concentration of tritium and tritium oxides
in environmental air;
Figure 2 is a sectional view of an instrument used in the
monitoring system of Figure 1;
Figure 3 is an underneath sectional plan view taken on line 3-3 in
Figure 2;
Figure 4 is a perspective view of the instrument with the housing
removed to reveal internal parts; and
Figures 5a, 5b, 5c and 5d are fragmentary sectional views of the
shutter showing a sequence of steps in a measuring operation.
Before considering the details of the instrument itself it will be
appropriate to consider the measurement procedure for which it is
particularly designed and also the calibration procedure suitable
for this instrument.
~ 3 ~
5~6;~S
The Measurement Procedure
The procedure involves bringing the scintillators sequentially to a
scintillation detector, such as a photomultiplier tube, in order to
detect and count the scintillations produced in each of them during
predetermined times. The first scintillator comprises a thin film
of scintillator material, more than 0.7 mg/cm2 thick but not more
than 5 mg/cm2 thick, mounted on a light reflecting substrate whose
temperature can be controlled for the purpose of condensing air
moisture onto the scintillator and evaporating condensed moisture
from it. The second scintillator (referred to herein as the
"reference scintillator") also comprises a thin film of scintil-
lator material of the same thickness as the first, and completely
covered by a transparent layer of non-scintillating material more
than 0.7 mg/cm2 thick but not more than 5 mg/cm2 thick, for the
purpose of preventing the sampled air from reaching the surface of
the scintillator.
The measurement procedure for measuring the concentration of
tritium oxides in air and the concentration of elemental tritium in
air consists of the following steps, wherein the scintillators are
exposed to the detector sequentially in the presence of the sampled
air:
(a) Measure the signal from the light sensing device resulting
from scintillations in the first scintillator while it is covered
with moisture condensed from the sampled air. Let the measured
value be K1-
,~
(b) Measure the signal from the light sensing device resultingfrom scintillations in the first scintillator after the moisture
has been evaporated from it. Let this value be K2.
(c) Measure the signal from the light sensing device when
neither the first nor second scintillator is exposed to it. Let
this value be K3.
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~.~53635
Then the concentration (Hw) of tritium oxides in the water present
in the sampled air is given by
Hw = mlK1 + m2K2 + m3K3
where ml, m2 and m3 are calibration factorsO
The concentration (Ha) of tritium oxides in the air is therefore
given by
Ha = Hw x G(RH,T)
where the function G(RH,T) is a known function of relative humidity
and temperatureO The relative humidity and temperature may already
be known or they may be separately measured during the procedure.
The concentration (Ht) oE elemental tritium in the air is given by
: Ht = n1Kl + n2K2 + n3K3
:;
where n1, n2 and n3 are calibration factors.
;~
Where the measuring procedure is to be carried out in the presence
of beta-emitting radioactive gases, the measurement procedure for
measuring the concentration of tritium oxides in air and the
concentration of elemental tritium in air consists of the following
steps, wherein the scintillators are exposed to the detector
sequentially in the presence of the sampled air:
~` :
(a) Measure the signal from the light sensing device resulting
~-~ from scintillations in the first scintillator while it is covered
~ with moisture condensed from the sampled air. Let the measured
; value be K1-
(b) Measure the signal from the light sensing device resulting
from scintillations in the first scintillator after the moisture
has been evaporated from it. Let thls value be K2.
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. ~
(c) Measure the signal from the light sensing device resulting
from scintillations in the second scintillator (i.e. reference
scintillator). Let this value be K3.
(d) Measure the signal from the light sensing device when
neither the first nor the second scintillator is exposed to it.
Let this value be K4.
Then the concentration (Hw) of tritium oxides in the water present
in the sampled air is given by
Hw = m1K1 + m2K2 + m3K3 + m4K4
where m1, m2, m3 and m4 are calibration factors.
The concentration (Ha) of tritium oxides in the air is therefore
given by
H~ = Hw x G(~H,T)
where the function G(RH,T) is a known function of relative humidity
and temperature. The relative humidity and temperature may already
be known or they may be separately measured during the procedure.
The concentration (Ht) of elemental tritium in the air is given by
Ht = n1K1 ~ n2K2 + n3K3
where n1, n2, and n3 are calibration factors.
In order to determine the calibration factors one must follow the
calibration procedure outlined below, which may be performed
manually or by a programmed controller.
The Calibration Procedure
(a) Provide to the instrument a sample of clean air, not
containing any radioactive substances, under constant and normal
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3635
conditions of temperature and relative humidity. Perform Step (b)
of the measurement procedure and let the result be B.
(b) Provide to the instrument a sample of a mixture of clean
air and a radioactive beta-emitting gas emitting beta particles of
maximum energy not less than lOOKeV. Perform Steps (a), (b), (c)
and (d) of the measurement procedure and let the results be ~1~ A
A3 and A4 respectively.
:
(c) Provide to the instrument a sample of a mixture of clean
air and elemental tritium such that the concentration of elemental
tritium is one unit. Perform Steps (a), (b) and (d) of the
measurement procedure and let the results be Bl, B2 and B4
respectively.
(d) Provide to the instrument a sample of air containing
tritiated water vapour such that the concentration of tritium
oxides in the water is one unit. Perform Step (a) of the
measurement procedure and let the result be Cl.
:~
The calibration factors are calculated as follows:
ml = _ (a4 - a ~b2 ~ (a2 - a3)b4
clb4(a2 - a3) - clb2(a4 - a3)
. '
m2 = (a~L -~ a3) bl - (al - a3)b4
clb4 (a2 - a3) - (a~ - a3) Clb2
m3 = (al - a4) (b2 - b~) - (a2 - a4) (bl - b4)
C1b4 (a2 - a3) - (a4 - a3) clb2
m4 = (al - a~)b2 ~ (a2 - a~)bl
~ clb4(a2 - a3) - C1b2(a4 - a3)
: -~ n1 = O
n2 = - (a~ - a3)
b4 ( a 2 ~ a3 ) - b2 ( a 4 - a 3 )
635
n3 = (a2 - a4) ,__
b4(a2 ~ a3) - b2(a4 - a3)
n~ = ~a~ - a3)
b4(a2 a3) - b2(a4 - a3)
where
ai = ~i - B
bi = Bi - B
cl = C1 - B
The Monitoring System
Referring to Figure 1, at the heart of the monitoring system is a
sampling assembly 10. The sampling assembly will be described in
detail with references to Figures 2, 3 and 4, but for the present
it is sufficient to mention that the sampling assembly comprises a
photomultiplier having an output circuitt a thermally conductive
mounting plate providing a light-reflecting mounting surface for
mounting a scintillator in viewing relationship to the window of
; the photomultiplier, and means for selectively cooling and warming
the mounting plate for condensing air moisture onto the scintilla-
tor and for evaporating condensed moisture therefrom. A shutter is
movable across the field of the view of the photomultiplier, and in
the present example provides means for mounting a reference scin-
tillator at a position such that movement of the shutter through a
complete cycle across the field of view will expose the scintil-
lators to the window in sequence. The shutter and the above-
mentioned cooling and warming means are, of course, controlled so
as to effect the various measurements described above under "The
Measurement Procedure".
~:
~; Air to be monitored is supplied to the sampling assembly via an air
~- flow path shown as a duct 11, so as to flow across each of the
` 30 scintillators when in the viewing position. The incoming air is
filtered by a filter 12, and sensors 13, 14 are positioned in the
air flow path for measuring the temperature and relative humidity
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~Z~i~6;35
of the incoming air. The temperat~re sensor 13 in the present
example is a solid state device, AD590 supplied by Analog Device
Incorporated; this is an integrated circuit device which furnishes
an analogue output proportional to temperature, and is described in
U.S. Patent No. 4,123,698. The relative humidity sensor 14 is also
a solid state device, component PC-2101 supplied by Thunder
Scientific Corporation, which is an integrated circuit device
furnishing an analogue output signal proportional to relative
humidity. The signals from these devices and the output signals
from the photomultiplier are processed by a microprocessor 15,
which is suitably programmed so as to furnish the numerical values
of Ha and Ht in accordance with the formulae discussed previously.
The numerical outputs of the microprocessor are also applied to a
conventional display device 1~. A control panel 17 provides the
usual controls ON/OFF, CLEAR, and RESET.
A power supply for the photomultiplier is shown at 18.
Besides processing the data which is applied to it, the
microprocessor is programmed also to furnish control signals for
controlling the shutter of the sampling assembly and controlling
the cooling and warming of the scintillator mounting plate so that
the various steps of the measurement procedure will be carried out
in the required sequence and for the required durations.
Microprocessors suitable for the purpose are available commercially
from numerous suppliers and these can be programmed routinely
according to the functions required of them.
The Measuring Instrument
Referring now to Figures 2-4, the measuring instrument of the
present example comprises essentially an instrument housing 20 of
aluminium providing an internal flow path 21 for air to be
monitored, the housing providing an air inlet 22 and an air outlet
23. An air filter 24 is connected to the inlet 22 for filtering
incoming air which is pumped along the flow path by a pump 25. A
photomultiplier tube 26 (Model D90~D supplied by EMI~ is mounted in
the housing 20, and a power supply 27 for operating the
g _
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~2~36~5
photomultiplier forms part of the electrical systern of the
instrument. The electrical system, denoted generally by numeral
23, further comprises an output circuit for the photomultiplier
including a preamplifier.
llhe temperature sensor 13 and relative humidity sensor 14 of Figure
1 may be located anywhere along the air flow path 21.
Mounted on a thermally conductive plate 45 on the floor of the
housing 20 is a thermally conductive mounting block 29, preferably
of aluminium, which incorporates a thermoelectric device 39 con-
trolled by the signal processor 15 (Figure 1) so as to cool or warmthe mounting block as required. The mounting block 29 provides a
light reflecting upper mounting surface spaced from the window 30
of the photomultiplier for mounting a scintillator 31 in viewing
relationship to the window. A shutter 32 in the form of an opaque
disc rotatable about its axis is positioned so as to interrupt the
field of view between the photomultiplier window 30 and the scin-
tillator 31. The shutter has a circular aperture 33 which, in one
position of the shutter, is aligned with the photomultiplier window
so as to expose the scintillator 31 to it. The shutter 32 also
provides on its upper side a well 34 which is spaced 120~ from the
aperture 33. This well, the base of which is light reflecting,
serves to locate and mount a reference scintillator 42, and is po-
sitioned so that in a second position of the shutter the re~erence
scintillator 42 is exposed to the photomultiplier 30, the scintil-
lator 31 being shielded by the shutter. ~n a third position 35 of
the shutter 32 the scintillator 31 is shielded from, and the re~
ference scintillator is remote from, the photomultiplier window 30.
The shutter 32 thus has three operative positions, a first and a
second of which a respective one of the scintillators is exposed to
the photomultiplier window, and a third of which neither
scintillator is so exposed. For the purpose of moving the shutter
to each of its three operative positions in the required sequence
an electric motor 36 is mounted in the instrument housing and is
drivingly coupled to the shutter by reduction gearing. The
reduction gearing takes the form of a pinion 37 on the output shaft
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~53635
.
of the motor 36~ the pinion meshing with the toothed periphery 38
of the shutter disc. The electrical system 28 includes an
energizing circuit for the motor 36, and this in turn is controlled
by the microprocessor 15 (Figure 1) so as to energize the motor at
timed intervals and so align each of the three operative positions
in sequence with the photomultiplier window for timed durations.
As previously stated, the operation of the thermoelectric device 39
for cooling and warming the mounting plate 29, so as to condense
air moisture onto the mounting plate or evaporate condensed
moisture from it, is controlled by signals from the microprocessor
15. All electrical connections to the instrument for supplying
power, providing control signals and deriving output signals are
led through suitable couplings 40.
A heat sink ~1, consisting of a finned block of aluminium and the
thermally conductive plate 45, is bolted to the base of the
instrument housing 20, the mounting block 29 being in good heat
conductive relation to it.
.
Figures 5a, 5b, 5c and 5d illustrate the four steps of the
measurement procedure in the presence of beta-emitting radioactive
gases. This is easily adapted to conditions where no beta-emitting
radioactive gases are present, as described above. The first
- shutter position, in which scintillator 31 is exposed to the
photomultiplier window is illustrated in Figures 5a and 5b, while
the second shutter position, in which the reference scintillator 42
is exposed to the photomultiplier window, is illustrated in Figure
5c. The third shutter position in which neither scintillator is
exposed to the photomultiplier window is illustrated in Figure 5d.
The scintillator 31 consists of a thin film of plastic scintillator
material (Type NE102 manufactured by Nuclear Enterprises) which is
deposited on an aluminized Mylar ("Mylari' is a trade mark)
substrate, the substrate being 1 to 8 mg/cm2 thick and having good
light reflecting properties as well as good mechanical properties
for supporting the film. The reference scintillator 42 is similar
to the scintillator 31 in all respects except that it is covered by
a thin, transparent foil of clear Mylar 5 mg/cm2 thick such that no
air can come into contact with the film of scintillator material.
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~ ;3635
In Figure 5a the mounting plate 29 has been cooled by the
thermoelectric device 39 so as to condense a film of air moisture
44 onto the upper surface of the scintillator 31. The shutter 32
is shown in the first of its three operative positions with the
aperture 33 aligned with the photomultiplier window and the
scintillator 31. In these conditions the first measurement of the
measurement procedure is taken.
Figure 5b also shows the shutter 32 in the first operative
position, with the aperture 33 aligned with the scintillator 31 and
the window of the photomultiplier 26. However, the thermoelectric
device 39 has now been operated so as to warm the mounting plate
and evaporate the film of moisture 44. In these conditions the
second measurement of the measurement procedure is taken.
In Figure 5c the shutter 32 has been rotated to the second
operative position so as to mask the scintillator 31 (not shown in
Figure 5c) while exposing the reference scintillator 42 to the
window of the photomultiplier. The third measurement of the
measurement procedure is now taken.
- In Figure 5d the shutter 32 has been rotated to the third of its
operative positions with neither of the scintillators 31, 42
~; exposed to the photomultiplier window. In this condition the
~` fourth measurement of the measurement procedure is taken.
The output signals representative of four measurements are
processed, along with the temperature and relative humidity signals
from the sensors 13, 14 (Figure 1) by the microprocessor 15 to
provide a numerical display indicating the concentration levels in
air of elemental tritium and tritium oxides, respectively.
It will be appreciated that the four steps of the measurement
procedure can be carried out repeatedly in cyclic succession,
thereby providing a display which is continually updated as
frequently as is required.
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