Airmec 255 User manual
RADIATION
MONITOR
TYPE
255
AIRMEC
LIMITED
-
HIGH
WYCOMBE
°*
BUCKS
Airmec
Limited
RADIATION
MONITOR
TYPE
255
AP
.2466
Airmec
Limited
High
Wycombe
Buckinghamshire
England
High
Wycombe
2f201
Telex
83243
RADIATION
MONITOR
TYPE
255
ike.
RADIATION
MONITOR
is
fully
transistorised,
battery
operated
and
portable.
It
com-
bines
all
the
advantageous
features
of
a
field
instrument
with
the
accuracy
of
a
laboratory
ratemeter.
There
are
four
probes,
a
sample
holder
and
lead
castle
available
for
use
with
the
instru-
ment
which
also
contains
a
built-in
geiger
tube
for
survey
work.
The
equipment
may
therefore
be
used
for
a
wide
variety
of
purposes
of
which
the
following
are
examples
:—
1.
Checking
the
presence
and
amount
of
radio-active
contamination
of
benches,
walls,
floors,
clothes,
equipment,
glasswork,
etc.
Being
completely
independent
of
mains
supplies
the
monitor
can
be
used
instantly
in
passages,
store
rooms
and
parts
of
buildings
remote
from
power
plugs.
2.
Checking
atmospheric
contamination
or
‘fall-out’,
including
contamination
of
food,
animals,
crops,
milk
and
water
supplies
as
well
as
structures,
buildings,
etc.
Not
only
will
the
equipment
indicate
contamination
of
areas
but
it
can
also
be
used
in
the
field
to
assess
accurately
the
degree
of
contamination
of
samples
of
grain,
food,
liquids,
etc,
3.
Following
the
progress
of
radio-active
tracer
elements
in
river
sediments,
oil
pipe
lines,
and
applications
encountered
in
food
crop
research,
medical
research,
and
various
in-
dustrial
processes.
4.
Tracing
the
whereabouts
of
radio-active
areas
and
making
accurate
assays
of
samples
in
geological
surveys.
5.
Tracing
lost
samples
of
radio-active
materials
both
in
buildings
and
out.
PRINTED
IN
ENGLAND
DESCRIPTIVE
LEAFLET
No.4
77B
GENERAL
DESCRIPTION
The
Radiation
Monitor
Type
255
consists
of
a
Rate
Meter
Unit,
four
types
of
probe,
and
a
Sample
Holder,
and
a
leather
Probe
carrying
case.
A
Lead
Castle
is
also
available
for
use
with
the
End
Window
and
Liquid
Sample
Probes.
The
Rate
Meter
Unit
is
fitted
in
a
robust
case
with
a
removable
cast
aluminium
lid
and
base.
The
lid
is
secured
in
position
by
two
non-losable
bolts
the
heads
of
which
have
coin
slots
for
quick
fixing
and
release.
When
removed
the
lid
may
be
screwed
onto
the
base
of
the
unit
to
prevent
loss
and
ensure
complete
portability.
The
base
consists
of
a
similar
casting
to
the
lid
and
contains
the
batteries
and
a
geiger
tube
for
survey
work.
Changing
batteries
or
the
geiger
tube
does
not
therefore
entail
breaking
the
main
seal
of
the
unit.
Rubber
gaskets
are
employed
between
the
body
of
the
rate
meter
and
the
lid
and
base
to
ensure
good
sealing
and
weather
proofing.
A
webbing
strap
is
attached
for
carrying
purposes,
and
small
straps
are
provided
on
one
side
of
the
case
for
securing
a
probe.
The
four
probes
available
are
an
Alpha
Probe,
a
Beta
Gamma
Probe,
an
end
Window
Probe,
and
a
Liquid
Sample
Probe.
They
are
connected
to
the
Rate
Meter
Unit
by
means
of
a
6
ft.
con-
nector
which
is
suitable
for
use
with
any
probe.
The
connector
terminations
are,
however,
arranged
so
that
none
of
the
low
voltage
probes
can
be
accidentally
connected
to
the
high
voltage
Alpha
Probe
Socket
on
the
front
panel.
The
Sample
Holder
enables
Alpha,
Beta
and
Gamma
measurements
to
be
made
on
samples
of
a
known
size
and
shape.
A
separate
carrying
case
is
available
for
transporting
the
probes,
sample
holder,
probe
lead
and
sample
trays
when
more
than
one
probe
needs
to
be
carried.
This
is
made
of
tropicalised
leather
and
measures
approximately
12”
x
11”
x
5”.
RATEMETER
UNIT
Transistors
are
used
throughout
this
unit
and
the
instrument
is
switched
off.
A
socket
is
pro-
it is
therefore
extremely
rugged
and
reliable.
It
vided
for
connection
to
a
12
volt
accumulator
if
includes
a
geiger
tube
in
the
base,
and
can
be
used
desired.
without
any
external
probes
for
detecting
and
monitoring
gamma
radiation.
The
meter
incorporated
gives
a
clear
and
accu-
rate
indication
of
radiation
intensity
in
four
ranges
0-5000,
0-500,
0-50,
and
0-5
counts
per
second.
In
addition
an
electromechanical
counter
on
the
front
panel
can
be
switched
in
for
monitoring
low
rates
of
count
with
high
accuracy.
Audible
indication
is
provided
by
the
built-in
loudspeaker
which
may
be
switched
off
when
not
required
—
and
a
socket
for
headphone
connection.
Although,
as
will
be
seen
from
the
specification
given
below,
the
stability
of
the
unit
is
excellent
with
large
changes
of
battery
voltage,
a
control
is
provided
on
the
front
panel
to
compensate
for
the
gradual
fall
in
battery
voltage
so
that
the
highest
possible
accuracy
may
always
be
obtained
when
required.
The
battery
voltage
is
set
up
and
can
be
monitored
by
the
meter
in
one
of
the
meter
switch
positions.
The
batteries
employed
give
a
very
long
life
with
normal
usage
and
are
of
a
type
available
all
over
the
world.
Warning
that
the
instrument
is
switched
on
is
given
not
only
by
the
audible
back-
ground
count
and
meter
reading
but
by
a
neon
indicator
on
the
front
panel.
A
warning
plate
on
Figure
1.
Radiation
Monitor
Type
255
base
view
the
top
of
the
lid
also
reminds
users
to
check
that
showing
batteries
and
G.M,
tube,
177B
ALPHA
PROBE
A
scintillation
probe
is
used
for
the
detection
of
Alpha
particles.
It
employs
a
zinc
sulphide
scintillation
screen
the
light
from
which
is
con-
centrated
by
a
prismoid
light
guide
on
to
a
photo-
multiplier
tube.
The
light
excluding
screen
is
an
aluminised
terylene
film
having
a
density
of
|
mg/cm’.
The
effective
screen
area
of
the
probe
is
5
sq.
ins.
and
the
sensitivity
is
such
that
it
will
detect
Alpha
particles
having
an
energy
of
4
meV
or
greater.
Although
the
terylene
film
covering
the
probe
window
is
extremely
tough
and
will
stand
up
to
all
normal
usage
a
sliding
metal
cover
is
provided
in
addition
for
protection
purposes
when
the
probe
is
not
in
use.
The
removable
handle
can
be
fitted
on
the
end
of
the
probe
in
either
of
the
two
ways
illustrated.
For
manual
use
either
position
may
be
found
more
convenient
according
to
circumstances,
but
the
position
in
Figure
3
is
particularly
useful
for
attaching
the
probe
to
a
pole
or
other
extension
device
when
the
user
wishes
to
remain
at
a
dis-
tance
from
the
source
of
radio-activity,
BETA-GAMMA
PROBE
A
low
voltage
thin
walled
r
tube
Type
BI2H
is
mounted
in
a
cylindrical
container
for
beta
and
gamma
measurements.
The
wall
density
of
the
tube
is
30-35
mg/cm’,
the
sensitivity
is
0.5
MeV
beta
and
the
dead
time
is
of
the
order
of
300
microseconds.
A
shutter
on
the
side
of
the
probe
enables
beta
radiations
to
be
excluded
and
thus
allows
the
probe
to
be
used
for
monitoring
gamma
radiation
only.
The
Geiger
tube
is
coated
with
colloidal
graphite
to
exclude
light
when
the
probe
is
operated
with
the
shutter
open.
The
outer
casing
of
the
probe
may
be
com-
pletely
removed
to
allow
the
Geiger
tube
to
be
immersed
in
liquids
when
it
is
required
to
monitor
them
in
bulk.
For
accurate
measurements
on
small
liquid
samples
a
special
liquid
Sample
Probe
is
available
and
this
is
described
on
the
opposite
page.
The
removable
handle
which
is
identical
to
that
employed
on
the
Alpha
Probe
may
also
be
mounted
on
this
probe
in
either
of
the
ways
des-
cribed
in
the
previous
section.
Figure
2.
Alpha
Probe
Type
255
Figure
3,
Alpha
Probe
Type
255
showing
altern-
ative
method
of
fixing
handle.
Figure
4,
Beta
Gamma
Probe
Type
255.
177B
END
WINDOW
PROBE
This
probe
is
suitable
for
the
detection
of
high
energy
alpha
and
low
energy
beta
radiation.
It
employs
an
end
window
geiger
tube
Type
EW3H
having
a
window
density
of
1.5
to
2.5
mg/cm’,
and
a
sensitivity
of
0.1
MeV
beta.
The
dead time
of
the
tube
is
of
the
order
of
700
microseconds.
A
window
of
aluminised
terylene
film
with
a
density
of
1
mg/cm?
is
fitted
for
light
exclusion
purposes.
This
may
be
removed
to
obtain
the
full
tube
sensitivity
when
the
probe
is
used
with
the
Sample
Holder
or
if
light
can
be
otherwise
excluded.
The
handle,
which
is
identical
to
that
employed
on
the
Alpha
and
Beta
Gamma
Probes,
can
also
be
mounted
in
either
of
the
positions
described
above,
LIQUID
SAMPLE
PROBE
The
Liquid
Sample
Probe
consists
of
a
special
geiger
tube
Type
M2H
designed
to
hold
5
milli-
Figure
5,
End
litres
of
liquid
with
a
fixed
source
geometry
and
Window
Probe
mounted
on
a
rigid
base
casting.
The
tube
itself
is
=
7ype
255.
sheathed
in
rubber
and
has
a
rubber
top
cap
attached
by
a
light
chain.
‘
The
wall
density
of
the
tube
is
15
mg/cm?
the
sensitivity
is
0.5
MeV
beta,
and
the
dead
time
is
of
the
order
of
150
us.
SAMPLE
HOLDER
eS
A
sample
holder
has
been
specially
designed
for
,
i
O55,
use
with
either
the
End
Window
Probe
or
the
Alpha
Probe
to
give
a
fixed
source
geometry.
The
drawer
will
take
a
dish
of
13”
diameter
and
}”
deep
(available
from
Airmec)
or
a
standard
A.E.R.E.
2”
sq.
sample
tray.
Fig.
7.
Sample
Holder
Type
255
with
End
Window
Probe
Figure
8.
Sample
Holder
Type
255
with
Alpha
Probe.
177B
SPECIFICATION
INDICATION
:
COUNTING
RANGES
:
BACKGROUND
RANGE
:
ACCURACY
:
DEAD
TIME
:
STASILITY
:
TEMPERATURE
RANGE
:
OUTPUT
:
INTERNAL
GEIGER
TUBE
:
PROBE
VOLTAGES
:
Radiation
Monitor
Type
255
1.
A
25”
meter
on
the
front
panel
gives
an
indication
of
radiation
intensity
in
counts
per
second.
2.
A
four
figure
electromagnetic
counter
on
the
front
panel
is
used
for
background
counting.
3.
Audible
indication
is
provided
by
a
24”
loudspeaker
mounted
at
the
end
of
the
unit.
This
facility
is
available
on
all
ranges
except
the
background
range
and
may
be
switched
off
when
not
required.
4.
Headphones
or
a
hearing-aid
earphone
may
be
employed
by
connecting
to
a
socket
at
the
end
of
the
unit.
The
main
control
selects
one
of
the
following
meter
ranges
:—
0
-
5000
counts
per
second
(X100
range)
0
-
500
”
”
”
(X10
”
)
0
-
50
o
m
«#
Ul
»
)
0-5
«
~~
s
(RI
«
3D
A
fifth
position
on
the
main
control
selects
the
electromagnetic
counter
for
background
counting.
The
maximum
counting
range
for
regular
pulses
is
25
per
second,
and
for
random
inputs
the
resolution
time
is
20
milliseconds.
When
not
in
use
the
counter
is
completely
out
of
circuit
to
avoid
a
steady
drain
on
the
batteries.
The
accuracy
of
measurement
for
a
regular
pulse
input
is
better
than
+
2.5%
of
F.S.D.
on
the
X100,
X10,
and
XI
ranges,
and
better
than
+
3%
F.S.D.
on
the
X0.1
range.
The
circuit
dead
times
on
the
counting
ranges
are
:—
X100
range
20
microseconds
X10,
200
”
XI
s
2
milliseconds
KO
is
20
‘5
A
preset
control
on
the
front
panel
marked
‘SET
6V’
enables
com-
pensation
to
be
obtained
for
a
fall
in
battery
voltage.
The
electromechanical
counter
is
unaffected
by
a
fali
in
battery
voltage
of
25%
(12-9
volts).
The
equipment
operates
satisfactorily
over
the
temperature
range
0-
40°C,
the
accuracies
quoted
being
those
at
20°C
and
below.
At
the
limit
of
40°C
no
error
is
increased
by
more
than
a
factor
of
2.
A
negative
pulse
of
not
less
than
1
volt
amplitude
and
of
constant
level
is
available
at
a
single
way
plug
on
the
front
panel
for
operat-
ing
an
external
scaling
unit.
A
robust
low
voltage
geiger
tube
for
gamma
radiation
is
mounted
inside
the
Ratemeter
Unit
and
can
be
selected
by
means
of
a
panel
switch.
It
will
detect
gamma
radiation
down
to
0.1
MeV.
The
voltage
ranges
available
for
the
probes
are
300-500
volts,
500-
700
volts,
and
900-1100
volts.
Each
range
is
variable
in
steps
of
about
30
volts.
The
voltages
are
stabilised
and
do
not
fall
by
more
than
1.5%
for
a
fall
in
battery
voltage
of
25%
(12-9
volts).
177B
SPECIFICATION
(continued
)
PROBES
:
CONNECTOR
:
SAMPLE
HOLDER
:
BATTERIES
:
WARNING
:
DIMENSIONS
:
FINISH
:
WEIGHT
:
177B
The
following
probes
are
available
for
use
with
the
Monitor
:—
1.
Beta/Gamma
Probe
—
for
beta
and
gamma
measurements.
2.
End
Window
Probe
—
for
detection
of
high
energy
alpha
and
low
energy
beta
radiation.
3.
Alpha
Probe
—
for
detection
of
alpha
particles.
4.
Liquid
Sample
Probe
—
for
beta
and
gamma
measurements
on
liquids,
A
6
ft.
connector
is
provided
and
is
suitable
for
use
with
any
of
the
probes.
The
terminations
are
arranged
so
that
none
of
the
low
voltage
probes
can
accidentally
be
plugged
into
the
high
voltage
Alpha
Probe
socket
on
the
front
panel.
This
is
designed
for
use
with
either
the
End
Window
Probe
or
the
Alpha
Scintillation
Probe
to
give
a
fixed
source
geometry.
The
1”
drawer
will
take
x
13”
diameter
x
}”
deep
dish
or
a
standard
A.E.R.E,
2”
square
sample
tray.
The
instrument
is
powered
by
eight
1.5
volt
leak-proof
dry
cells
type
U2
arranged
to
form
a
12
volt
battery.
When
used
for
4
hours
per
day,
the
battery
life,
without
the
loud-
speaker
in
use,
is
at
least
120
hours.
The
life
with
the
electromagnetic
counter
or
loudspeaker
in
use
at
5
counts
per
second
is
more
than
60
hours.
Alternatively
the
instrument
may
be
operated
from
a
12
volt
accumulator
or
a
12
volt
power
pack.
A
neon
indicator
on
the
front
panel
provides
a
warning
that
the
instrument
is
switched
on,
and
a
warning
plate
on
the
top
of
the
lid
reminds
users
to
check
that
the
instrument
is
switched
off.
Ratemeter
Unit:
12”
long
x
54”
wide
x
8}”
deep
(30.5
x
14
x
21.6
cms.).
Beta/Gamma
Probe:
9”
long
x
2”
in
diameter
(22.9
x
5
cms.).
End
Window
Probe:
54”
long
x
2”
in
diameter
(14
x
5
cms.).
Alpha
Probe:
61”
long
x
31”
wide
x
3”
deep
(15.9
x
8.2
x
7.6
cms.)
Liquid
Sample
Probe:
33”
long
x
33”
wide
x
2}”
deep
(8.6
x
8.6
x
5.4
cms.).
(Geiger
height
4”
—
10.2
cms.).
Sample
Holder:
93”
long
x
33”
wide
x
14”
dee
(24.8
x
9.5
x
2.8
cms.).
All
units
with
the
exception
of
the
Sample
Holder
are
finished
in
stove
enamel
medical
cream.
The
Sample
Holder
is
anodised
matt
black.
Ratemeter
Unit:
16
lbs.
(7.25
kgs.)
(complete
with
batteries).
Beta/Gamma
Probe:
14
ozs.
(398
gms.)
less
handle.
End
Window
Probe:
7
ozs.
(199
gms.)
less
handle.
Alpha
Probe:
|
Ib.
1
oz.
(7.53
gms.)
less
handle.
Liquid
Sample
Probe:
12
ozs.
(341
gms.).
Sampt!e
Holder:
1
Ib.
7
ozs.
(7.45
kgs.).
Handle:
3
ozs.
(85
gms.).
Connector:
4
ozs.
(114
gms.).
LEAD
CASTLE
TYPE
295
T
HE
Lead
Castle
Type
295
is
designed
primarily
for
use
with
the
Airmec
Radiation
Monitor
Type
255
to
minimise
the
effects
of
all
other
sources
of
radiation
when
testing
radio-
active
materials.
Either
solid
or
liquid
samples
may
be
tested
by
the
use
of
the
appropriate
Geiger
tube
type
EW3H
or
M2H
respectively.
The
tube
type
M2H
holds
3.5
millilitres
of
liquid,
and
the
drawer
provided
for
checking
solid
samp!es
accommodates
the
standard
A.E.R.E.
2”
square
tray,
2”
or
13”
planchets,
or
13”
diameter
by
}”
deep
dishes.
Despite
the
light
weight
cf
45
Ibs.
which
enables
the
assembly
to
be
carried
by
one
man,
the
wall
thickness
shielding
the
sensitive
volume
is
14”
of
lead.
A
special
feature
is
the
ease
with
which
a
conversion
may
be
made
from
testing
solid
samples
to
liquid
samples
and
vice
versa,
merely
by
inverting
the
method
of
assembly
and
fitting
the
appropriate
type
of
Geiger
tube.
Stowage
for
the
unused
tube
is
provided
on
the
carrying
strap.
177B
Lead
Castle
Type
295
SPECIFICATION
Reduction
in
Radiation
:
The
castle
provides
a
reduction
of
X10
in
gamma
radiation
from
mixed
fusion
products.
Shielding
:
11”
thickness
of
lead
(4°/,
Antimonial
to
A.E.R.E.
Specification
420).
Solid
Samples
:
A
shielded
drawer
assembly
takes
the
standard
A.E.R.E.
2”
square
tray,
2”
or
13”
planchets,
or
a
13”
diameter
by
}”
deep
dish.
Liquid
Samples
:
The
liquid
jacket
of
the
Geiger
tube
type
M2H
has
a
capacity
of
\
7
approximately
3.5
millilitres.
ae
Geiger
tubes
:
20th
Century
EW3H
for
end
window
tests
on
solid
samples.
20th
Century
M2H
for
liquid
sample
tests.
Construction
:
Three
interlocking
lead
pieces
of
easily
manageable
weight.
Conversion
from
liquid
to
solid
samples
and
vice
versa
is
by
inversion
of
the
method
of
assembly
and
fitting
of
the
appropriate
tube
into
the
composite
holder.
Dimensions
:
The
outside
dimensions
are
4.4%”
diameter
x
93”
high
(122
x
245
mm.).
The
internal
cavity
is
13”
diameter
x
64”
high.
Weight
:
Approximately
45
Ibs.
(21
kg.).
Finish
:
Silicone
wax
direct
on
the
lead.
Carrying
Strap
:
A
webbing
strap
is
provided
which
enables
the
castle
to
be
carried
by
one
man.
Tube
Stowage
:
A
tubular
container
is
attached
to
the
carrying
strap
for
stowage
of
the
Geiger
tube
not
in
use.
These
specifications
are
representative
of
average
instruments
and
we
reserve
the
right
to
effect
modifications.
AIRMEC
LIMITED
HIGH
WYCOMBE,
BUCKINGHAMSHIRE,
ENGLAND.
Telephone:
High
Wycombe
2501
(7
lines)
Cables;
Airmec,
High
Wycombe
177B
AP
2466
Alpha
Probe
Connector
Socket
Beta-Gamma
and
a
End
Window
Probe
Connector
Socket
&
PROBE
Electromagnetic
Register
Probe
Selector
Switch
Probe
Voltage
Control
Warning
Neon
Meter
Range
Selector
Switch
Internal
Check
Selector
Switch
Set
6V
Control
RADIATION
MONITOR
TYPE
255
Location
of
Front
Panel
Controls
AP
.2466
CONTENTS
INTRODUCTION
INITIAL
ADJUSTMENTS
OPERATION
»
General
-
»
Ratemeter
with
built-in
geiger
tube
-
Beta-Gamma
probe
»
Alpha
probe
;
»
End
Window
probe
»
Liquid
Sample
probe
.
Corrections
.
Conversion
to
dosage
»
Calibration
service
-
10.
External
scaler
-
1].
Headphones
-
12.
External
power
supply
13.
Probe
storage
OANA
OA
WD
—
WWWWW
WW WW
WW
W
WH
CIRCUIT
DESCRIPTION
4.1.
General
4.2.
Ratemeter
unit
4.3.
Probe
VICING
.
1.
General
»
6V
line
»
Probe
voltage
ranges
.
Dead
time
.
Accuracy
.
Beta-Gamma
probe
»
End
Window
probe
.
Alpha
probe
ONAaARWNH
—
w
PERFORMANCE
SPECIFICATION
Page
ONO
UDA
ON
A
AAA
KR
AND
ND
AP
.2466
t.
INTRODUCTION
The
Radiation
Monitor
Type
255
is
a
portable
equipment
for
detecting
alpha,
beta,
and
gamma
radiation.
It
indicates
radiation
intensity
in
terms
of
"counts
per
second"
on
a
meter
with
four
switched
ranges,
and
for
aural
monitoring
a
built-
in
loudspeaker
and
a
headphone
jack
are
provided;
in
addition,
there
is
a
four-
digit
electromagnetic
register
which
can
be
switched
into
circuit
for
integration
or
for
background
counting.
The
instrument
is
fully
transistorised
and
is
powered
by
eight
U2-type
dry
batteries
with
a
life
in
use
of
at
least
120
hours
at
4
hours
per
day.
The
complete
equipment,
illustrated
opposite,
consists
of
a
Ratemeter
and
four
types
of
probe.
The
Ratemeter
is
contained
in
a
strong
sealed
case
fitted
with
a
carrying
strap
and
has
a
built-in
Geiger-Muller
tube
for
detecting
gamma
radiation.
A
fitted
leather
case
is
available
for
carrying
the
probes
and
a
lead
castle
can
be
supplied
for
reducing
the
effect
of
background
radiation
when
making
tests
on
small
samples.
_
rE
INITIAL
ADJUSTMENTS
The
Ratemeter
as
supplied
is
fitted
with
fresh
batteries,
and
both
top
and
bottom
lids
are
in
position.
IF
IT
1S
TO
BE
STORED
FOR
ANY
LENGTH
OF
TIME
BEFORE
USE,
TAKE
THE
BATTERIES
OUT.
To
use
the
Ratemeter,
take
the
top
lid
off
by
undoing
the
captive
hold-
down
screws
and
stow
it
by
clamping
it
over
the
lower
lid
with
the
same
screws.
Check
battery
voltage
by
setting
the
Internal
Check
switch
to
BATTERY
and
the
Range
switch
to
x
10
or
x
100.
The
meter
needle
should
read
above
the
red
9V
mark
on
the
scale.
AP
.2466
Check
the
internal
6V
line
by
setting
the
Intemal
Check
switch
to
6V
LINE.
The
meter
should
read
exactly
6V.
If
it
does
not,
take
the
black
cover
off
the
Set
6V
control
and
adjust
it
with
a
screwdriver
to
bring
the
needle
exactly
over
the
red
6V
mark
on
the
scale.
Turn
the
Internal
Check
switch
to
NORMAL
and
the
Monitor
to
ready
for
use.
No
warm-up
time
is
necessary
since
stable
operating
conditions
are
reached
within
10
seconds
of
switching
on.
3.
OPERATION
3.1.
General
The
instrument
may
be
used
either
by
itself,
using
the
built-in
Geiger
tube,
or
with
one
of
the
probes
connected.
The
probe
cable
is
fitted
with
different
sizes
of
plug
at
opposite
ends,
as
a
precaution
to
ensure
that
the
correct
supply
socket
is
used
for
the
different
types
of
probe.
The
beta-~gamma
probe
has
a
rotatable
shutter
for
stopping
beta
particles
and
enabling
gamma
radiation
only
to
be
measured.
Control
settings
for
using
each
of
the
probes
are
summarised
in
Table
1.
The
electromagnetic
register
on
the
Ratemeter
Unit
is
intended
for
use
at
low
counting
rates,
such
as
measuring
the
radioactive
background
or
taking
measurements
on
low~activity
sources.
It
is
brought
into
use
by
setting
the
Range
Selector
to
BG
and
the
Internal
Check
switch
to
NORMAL;
both
the
meter
and
the
loudspeaker
are
then
cut
out
of
circuit.
AP
.2466
TABLE
1.
MEASUREMENT
PROCEDURES
Application
|
Control
Settings
|
intemal
|
Pies
|
Range
|
Ege
[Selector
|
|
Selector
|
0,
then
advance
until
background
is
just
Ratemeter
being
counted
as
shown
by
intermittent
Only,
with
|
Normal
|
;
|
clicks
in
loudspeaker.
|
built-in
|
|
Then
advance
one
more
notch
(about
30V)
Geiger
tube
!
|
to
operate
Geiger
tube
on
its
plateau.
|
0,
then
advance
until
background
is
just
|
being
counted
as
shown
by
intermittent
|
Beta-Gamma
|}
Normal
|
|
clicks
in
loudspeaker.
Probe
|
Then
advance
one
more
notch
(about
30V)
|
to
operate
probe
on
its
plateau.
0,
then
advance
until
background
is
being
counted
at
a
rate
of
about
one
Alpha
Probe
|
Normal
|
|
click
per
second
in
the
loudspeaker.
|
Then
reduce
by
one
notch
(about
30V):
|
to
bring
operating
point
away
from
noise
region.
End
End
Window
kisi
Window
As
for
Beta-Gamma
Probe
Probe
As
for
Beta-Gamma
Probe
AP
..2466
3.2.
Ratemeter
with
built-in
Geiger
tube
Set
the
controls
as
given
in
Table
1.
For
measuring
gamma
radiation
it
is
not
necessary
to
remove
the
lower
lid
since
it
is
of
aluminium
alloy
and
is
virtually
transparent
to
this
type
of
radiation.
The
instrument
may
be
carried
by
its
shoulder
strap
or
placed
on
a
bench
for
the
general
monitoring
of
the
radioactive
background.
When
used
with
a
radioactive
source,
if
the
distance
between
the
geiger
tube
and
the
source
is
required
to
be
known,
the
longitudinal
centre
line
of
the
instrument
may
be
taken
as
the
centre
line
of
the
geiger
tube.
The
height
of
the
geiger
tube
above
the
bench
is
approximately
2.1/4"
or
1.1/4"
depending
on
whether
both
or
only
one
of
the
lids
is
fitted
on
the
underside
of
the
instrument.
The
reading
obtained
on
the
meter
will
depend
on
the
strength
of
the
source
and
its
distance
from
the
geiger
tube
and
the
Range
switch
should
be
set
accordingly.
Any
further
check
of
the
6V
line
should
be
made
with
the
signal
temporarily
removed
in
order
to
obtain
an
accurate
reading.
This
can
be
done
either
by
setting
the
probe
selector
switch
to
some
other
position
or
reducing
the
probe
voltage.
3.3.
Beta—Gamma
Probe
Connect
the
probe
to
the
socket
on
the
Ratemeter
Unit
with
the
special
cable
supplied
and
set
the
controls
as
given
in
Table
1.
For
measuring
beta
radiation,
open
the
probe
shutter
by
turning
the
end
cap,
and
aim
the
aperture
at
the
source
of
radiation.
For
measuring
gamma
radiation
the
shutter
may
be
in
either
position
unless
it
is
desired
to
exclude
beta
particles,
when
it
should
be
closed.
To
use
the
probe
for
dipping,
remove
the
shutter
assembly
by
unscrewing
the
three
retaining
screws
farthest
from
the
connector.
The
probe
can
then
be
immersed
up
to
the
base
of
the
geiger
tube,
3.4.
Alpha
Probe
Connect
the
probe
to
the
socket
on
the
Ratemeter
Unit
with
the
special
cable
supplied
and
set
the
controls
as
given
in
Table
1.
AP.
2466
The
probe
may
be
either
hand-held
or
used
on
the
sample
holder.
The
handle
may
be
fitted
in
one
of
two
positions
(over
the
top
of
the
probe,
or
projecting
rear-
wards
from
it)
by
undoing
the
knurled
retaining
nut.
The
second
position
enables
the
probe
to
be
easily
fastened
to
a
pole,
so
that
the
operator
can
keep
at
a
safe
distance
while
monitoring
a
highly
active
source.
To
use
the
probe
with
the
sample
holder,
remove
the
End
Window
adaptor
from
the
holder before
sliding
the
probe
onto
it.
3.5.
End
Window
Probe
Connect
the
probe
to
the
Rate
meter
Unit
with
the
special
cable
supplied
and
set
the
controls
as
given
in
Table].
The
probe
may
be
either
hand-held
or
used
on
the
sample
holder.
For
hand-
held
use,
aim
the
wire-mesh
end
at
the
source
of
radiation;
the
handle
can
be
fitted
in
one
of
two
positions,
as
for
the
beta-gamma
probe,
by
undoing
the
knurled
retaining
nut.
To
use
the
probe
with
the
sample
holder,
screw
it
into
the
End
Window
adaptor
and
then
slide
it
onto
the
holder.
The
thin
end
window
of
the
tube
is
protected
by
a
sheet
of
metallised
terylene
film
and
a
wire
mesh.
For
measurements
requiring
maximum
sensitivity,
the
terylene
film
and
wire
mesh
can
be
removed
to
expose
the
thin
mica
window
of
the
geiger
tube.
To
remove
these,
take
off
the
neoprene
sleeve
and
the
three
4BA
retaining
screws,
then
slide
the
base
assembly
and
geiger
tube
out
and
remove
the
terylene
film
and
wire
mesh.
Replace
the
base
assembly
and
geiger
tube
and
re-assemble
the
probe.
When
this
is
done,
take
care
to
avoid
damaging
the
mica
window
and
also
to
exclude
light,
for
this
will
falsify
the
measurement
and
is
liable
to
damage
the
tube.
3.6.
Liguid
Sample
Probe
Connect
the
probe
to
the
Ratemeter
with
the
special
cable
supplied
and
set
the
controls
as
given
in
Table
1.
Place
the
probe
on
a
flat
surface,
take
off
the
neoprene
cap
at
the
top,
and
pour
in
the
sample
of
liquid.
After
use,
pour
out
the
sample
and
then
wash
out
thoroughly.
The
geiger
tube
can
be
taken
out
and
washed
separately.
AP.2466
3.7.
Corrections
In
order
to
obtain
the
greatest
accuracy
of
measurement,
it
is
necessary
to
apply
corrections
to
the
observed
reading
to
allow
for
dead
time,
background
radiation,
and
statistical
factors.
3.7.1.
Dead
time
Dead
times
of
the
probes
are
approximately:-
Built-in
Geiger
in
Ratemeter
300uS
Beta-Gamma
probe
300uS
Alpha
probe
10yS
End
Window
probe
700uS
Liquid
Sample
probe
150uS
ia
These
dead
times
will
vary
with
temperature
and
counting
rate;
to
compensate
for
this
variation,
fixed
and
accurately
calibrated
dead
times
are
introduced
into
the
Ratemeter
circuit
to
enable
the
true
count
to
be
determined.
These
circuit
dead
times
are:-
20uS
on
the
x100
range
200uS
on
the
x10
range
2mS
on
the
x
1
range
20mS
on
the
—
10
range
By
selecting
the
range
to
give
a
longer
circuit
dead
time
than
the
probe
in
use,
any
variation
in
probe
dead
time
is
allowed
for
and
the
true
count
per
second
can
be
determined
from
the
expression:-
Where
n_
is
the
true
count
per
second
No
is
the
observed
count
per
second
t
is
the
circuit
dead
time
in
seconds
3.7.2.
Background
The
count
indicated
by
the
monitor,
with
a
radioactive
source
present,
is
due
to
the
source
plus
the
natural
radioactive
background.
To
correct
for
back-
AP.2466
ground,
first
measure
the
background
level
with
no
radioactive
source
present,
and
apply
dead
time
correction
to
the
background
count.
Then
take
a
measurement
with
the
radioactive
source
present,
and
apply
dead
time
correction
to
this
reading
also.
Finally
subtract
the
corrected
background
reading
from
the
corrected
total
reading,
to
obtain
a
true
reading
of
source
radiation
only.
3.7.3.
Statistical
Radioactivity
is
random
in
nature,
and
therefore
the
intensity
of
radiation
from
a
source
varies
from
instant
to
instant
in
a
random
manner.
As
a
result
of
this,
it
is
necessary
to
average
out
the
instantaneous
counting
rates
over
a
comparatively
long
time
in
order
to
get
a
satisfactorily
accurate
measurement.
To
arrive
at
a
practical
method
of
averaging,
it
is
necessary
first
to
consider
how
the
Type
255
Monitor
works.
Basically,
an
electronic
circuit
produces
one
electrical
pulse
for
each
atomic
disintegration
detected.
When
the
controls
are
set
so
that
the
meter
is
in
use,
additional
circuits
are
brought
into
operation
which
automatically
and
continuously
average
out
the
instantaneous
pulse
rates
from
this
circuit.
If
the
meter
range
switch
is
set
to
xl
or
—
10,
averaging
occurs
over
a
period
of
approximately
6
seconds;
if
the
switch
is
set
to
x10
or
x100,
the
period
is
approximately
0.6
seconds.
When
the
controls
are
set
to
drive
the
electromagnetic
register
or
an
external
scaler,
the
integrating
circuits
are
disconnected
and
no
averaging
occurs.
The
following
methods
of
averaging
have
been
found
to
give
satisfactorily
accurate
results
in
practice.
(a)
Using
Meter.
Take
a
measurement
over
a
period
of
approximatel
y
one
minute.
Estimate
by
eye
the
mid-point
of
any
fluctuations
of
the
meter
needle
and
take
this
as
the
reading.
(b)
Using
Register
or
External
Scaler.
Take
a
count
over
at
least
two
minutes,
or
longer
if
the
source
is
only
feebly
radioactive,
before
dividing
by
total
elapsed
time
to
get
the
average
count
per
second.
3.8.
Conversion
to
dosage
For
some
purposes
it
is
necessary
to
convert
the
counting
rate
into
terms
of
dosage,
measured
in
millirads/hour.
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