432
18.
THE DETECTION OF PART ICLES
stored fOr several days over P205 or CaCl2.
The tube is then
sealed off and is ready for use.
‘
'
If, on the other hand, it is desired to make counter tubes which
are consistem‘ly sensitive, stable and of low background count and
particularly, if they are of large size (a foot or more long and several
inches in diameter) for the detection of feeble radiations, then a
more elaborate procedure is necessary.
One such method * is as
follows.
A seamless copper tube, say
15 inches long, 1—9€ inch in
diameter, with walls 8 mils thick is ared at the ends, carefully
washed with grit—free soap and distilled water. The tube is
assembled in its pyrex container With a 3 mil tungsten anode.
It is washed with cleaning solution, water, ammonium hydroxide,
water, 5 per
cent nitric acid for 10—15 minutes and then water.
It is evacuated with a diffusion pûmping system and baked at
500° C. for 15 hours (over night). The tungsten wire is ashed
very brightly (at 2600—3000° C.) for 30 seconds. Dry air is
admitted to atmospheric pressure through a P205 tube and a C02
trap.
The tube is then heated in an oven at 500° C. for 15 to
20 minutes (until the copper is heavily oxidized). The tube is
-
removed from the pumping system, the oxide coat is washed off
with 5 per cent nitric acid (perhaps one-half hour) followed by a
'
thorough washing in distilled water. The tube is then re-evacu—
ated and baked as before for 15 hours.
When cool, dry air (or
hydrogen) is admitted to a pressure of approximately 7 (or 10)
centimeters of mercury (V,; à 1100 volts).
When operated
an overvoltage of 150, the residual count will be about 135 counts
per minute.
—
'
‘
? The threshold voltage is higher when the gas pressure is higher
Ï
and Wheh;the anode is of larger diameter. Typical curves
15
are -
shown in gure 18—12 for the case of dry air and argon in tubes of
—.
oxidized copper & centimeters long and one centimeter in
&
;
The’àn0des Were 'tungsten wires whose diameters were 3, Sand
…
‘.150mils,jas indicated in the gure. The tubes were
Wlth
'
gammarays
It is to be noted that lower voltageS may be used
=
"
with air*
.
.
=
it has been stated that
tubew1th
The :äuthor s indsbted ,to F R--'Sh0nka for this technique-… Thenecsay
'SœPsriis…l_inanWn at
su…enæa With further study,— «
…
v
18.
THE DETECTION OF PART ICLES
stored fOr several days over P205 or CaCl2.
The tube is then
sealed off and is ready for use.
‘
'
If, on the other hand, it is desired to make counter tubes which
are consistem‘ly sensitive, stable and of low background count and
particularly, if they are of large size (a foot or more long and several
inches in diameter) for the detection of feeble radiations, then a
more elaborate procedure is necessary.
One such method * is as
follows.
A seamless copper tube, say
15 inches long, 1—9€ inch in
diameter, with walls 8 mils thick is ared at the ends, carefully
washed with grit—free soap and distilled water. The tube is
assembled in its pyrex container With a 3 mil tungsten anode.
It is washed with cleaning solution, water, ammonium hydroxide,
water, 5 per
cent nitric acid for 10—15 minutes and then water.
It is evacuated with a diffusion pûmping system and baked at
500° C. for 15 hours (over night). The tungsten wire is ashed
very brightly (at 2600—3000° C.) for 30 seconds. Dry air is
admitted to atmospheric pressure through a P205 tube and a C02
trap.
The tube is then heated in an oven at 500° C. for 15 to
20 minutes (until the copper is heavily oxidized). The tube is
-
removed from the pumping system, the oxide coat is washed off
with 5 per cent nitric acid (perhaps one-half hour) followed by a
'
thorough washing in distilled water. The tube is then re-evacu—
ated and baked as before for 15 hours.
When cool, dry air (or
hydrogen) is admitted to a pressure of approximately 7 (or 10)
centimeters of mercury (V,; à 1100 volts).
When operated
an overvoltage of 150, the residual count will be about 135 counts
per minute.
—
'
‘
? The threshold voltage is higher when the gas pressure is higher
Ï
and Wheh;the anode is of larger diameter. Typical curves
15
are -
shown in gure 18—12 for the case of dry air and argon in tubes of
—.
oxidized copper & centimeters long and one centimeter in
&
;
The’àn0des Were 'tungsten wires whose diameters were 3, Sand
…
‘.150mils,jas indicated in the gure. The tubes were
Wlth
'
gammarays
It is to be noted that lower voltageS may be used
=
"
with air*
.
.
=
it has been stated that
tubew1th
The :äuthor s indsbted ,to F R--'Sh0nka for this technique-… Thenecsay
'SœPsriis…l_inanWn at
su…enæa With further study,— «
…
v