338
14.
ARTIFICIAL TRANSMUTATION
'
Immediately above the target, the tube F has been attened and
drilled with a hole of approximately one centimeter diameter.
’
Five wires of 0.024. centimeter diameter (the dots in gure 14—16)
are soldered across the hole to form a supporting grid for a very
thin mica window, W
,
made vacuum tight with a hard wax ring.
The mica should be as thin as possible and its stopping power, as
measured with the apparatus of gure 12—21, should not exceed
two centimeters, air equivalent. As close as possible above W
place a scintillation screen ]
,
with sensitive surface downward.
This screen must have very few spurious or background scintilla- .
tions.
These are to be counted (in a dark room) before the target
is bombarded with protons, but with the screen in place as shown,
and should not exceed one or two per minute.
Otherwise, the few
alpha particle scintillations produced by the protons accelerated at
the low voltages used will be masked. The counting of scintillations
is to be carried out over a period of ve or ten minutes and an average value used, to reduce statistical uctuations, both for the
background and when the target is under bombardment.
The voltage supply may consist of an X—ray power unit provided with rectication. Do not attempt the experiment with less
than 50 kV. Satisfactory results can be obtained in the range from50 to 70 kv.; the higher the voltage, the better.
The discharge tube is rst evacuated to as low a pressure as
possible. A Bunsen burner with a very low ame is clamped_near
_
the palladium tube and a small ow of hydrogen started into the
.
discharge tube, with the pumps in continuous operation. If the
,
rate of ow is too fast, the pressure in the tube rises, the Voltage
falls off and the tube and proton currents increase.
A series
_
'
kenotr0n, as used with the high—voltage supply of gure 3—6
, serves
to limit the current, protecting both the supply system and the dis_
-
charge tube.
_
The ow is decreased by moving the ame sightly
_
_
farther away
the palladium tube.
If the ow of hydrogen'îë
slow, the voltage drop across the discharge tube will rise, Which is
r
desirable, as may be seen from gure 14—11.
But, if the ow is ' _
too slow, the discharge current and the intensity of the
beam will decrease below the point where the number of alpha,
,
'
from the target is appreciably greater than the
'
background count. A pressure of appr0ximately 10“3 millimeter
'
is satisfactory, at which time the glass Walls show the CharaCter—z
14.
ARTIFICIAL TRANSMUTATION
'
Immediately above the target, the tube F has been attened and
drilled with a hole of approximately one centimeter diameter.
’
Five wires of 0.024. centimeter diameter (the dots in gure 14—16)
are soldered across the hole to form a supporting grid for a very
thin mica window, W
,
made vacuum tight with a hard wax ring.
The mica should be as thin as possible and its stopping power, as
measured with the apparatus of gure 12—21, should not exceed
two centimeters, air equivalent. As close as possible above W
place a scintillation screen ]
,
with sensitive surface downward.
This screen must have very few spurious or background scintilla- .
tions.
These are to be counted (in a dark room) before the target
is bombarded with protons, but with the screen in place as shown,
and should not exceed one or two per minute.
Otherwise, the few
alpha particle scintillations produced by the protons accelerated at
the low voltages used will be masked. The counting of scintillations
is to be carried out over a period of ve or ten minutes and an average value used, to reduce statistical uctuations, both for the
background and when the target is under bombardment.
The voltage supply may consist of an X—ray power unit provided with rectication. Do not attempt the experiment with less
than 50 kV. Satisfactory results can be obtained in the range from50 to 70 kv.; the higher the voltage, the better.
The discharge tube is rst evacuated to as low a pressure as
possible. A Bunsen burner with a very low ame is clamped_near
_
the palladium tube and a small ow of hydrogen started into the
.
discharge tube, with the pumps in continuous operation. If the
,
rate of ow is too fast, the pressure in the tube rises, the Voltage
falls off and the tube and proton currents increase.
A series
_
'
kenotr0n, as used with the high—voltage supply of gure 3—6
, serves
to limit the current, protecting both the supply system and the dis_
-
charge tube.
_
The ow is decreased by moving the ame sightly
_
_
farther away
the palladium tube.
If the ow of hydrogen'îë
slow, the voltage drop across the discharge tube will rise, Which is
r
desirable, as may be seen from gure 14—11.
But, if the ow is ' _
too slow, the discharge current and the intensity of the
beam will decrease below the point where the number of alpha,
,
'
from the target is appreciably greater than the
'
background count. A pressure of appr0ximately 10“3 millimeter
'
is satisfactory, at which time the glass Walls show the CharaCter—z