194
9.
X—RAYS
scarcely passing through the human_hand. They are then said
to be soft and are found to have a comparatively long wave-length,,
perhaps one or more angstroms. At a gas pressure of approxi—
mately0.0003 millimeter, exCeedingly high voltages are needed
to operate the discharge, the x—rays are exceedingly penetrating,
and are
able to pass through the bones of the hand With very little
absorption. These _rays are of comparatively short wave—length,
say
0.1 angstrom and are called hard x-rays.
Gas-lled tubes are
_
normally evacuated to approximately 00005 millimeter pressure
and are frequently provided With a device whereby the pressure
may be altered.
As a tube is operated, it tends to harden because
of the clean-up action of the electrode materials.
As we have seen in a previous chapter, the voltage across a
discharge tube depends to a large extent upon the pressure of the
gas in thetube;
An increase in the battery B, operating the induction coil ] of gure 9—2, Will cause an increase in the intensity of
the emitted x-rays but will not affect to any great extent the
Voltage drop across the tube. At the lower gas pressures,
how—
ever, the emitted x—rays are not only more penetrating but also
of greater intensity. The intensity rises very rapidly as the
voltage across
the tube increases; it is very nearly proportional
to the square of the voltage, if the current is constant.
Ï
9-‘—3.
X—ray_ Tubes.——In order to avoid the.
changes due to variations in gas pressure and the
“
wandering
”
of the focal spot in the earlier tubes, Coolidge,2 in 1913, replaced
'
the gaseous source of electrons by a hot lament. A later form
of such a hot—cathode high-vacuum tube is shown in gure 9—3.
C consists of a spiral of tungsten Wire Surrounded by“
a metal
Serves to focus the electrons upon a
_
area, the focal spot, of the anode Â. The anode is constructed of”
Which
tungsten plug has been imbedded.
18
'
Cddledrin a
ways; _ by means of the radiatin‘g
01”,
Ÿ
upongthe removal of the rod carrying the ns, by a stream ofwater
_jg
_circulàted inside
'
the anode by either convection or pressure
The
is evaÏCuated to 0.000,05 millimeter or less,18
made ofthick, lead impregnated glass except
..r€äg‘ionïrOf:t—=he:small,—' th1n,lime glass Window W where the raysmay'“
emerge1napprec1able inftenSity; .
_
tubes, conStructed
9.
X—RAYS
scarcely passing through the human_hand. They are then said
to be soft and are found to have a comparatively long wave-length,,
perhaps one or more angstroms. At a gas pressure of approxi—
mately0.0003 millimeter, exCeedingly high voltages are needed
to operate the discharge, the x—rays are exceedingly penetrating,
and are
able to pass through the bones of the hand With very little
absorption. These _rays are of comparatively short wave—length,
say
0.1 angstrom and are called hard x-rays.
Gas-lled tubes are
_
normally evacuated to approximately 00005 millimeter pressure
and are frequently provided With a device whereby the pressure
may be altered.
As a tube is operated, it tends to harden because
of the clean-up action of the electrode materials.
As we have seen in a previous chapter, the voltage across a
discharge tube depends to a large extent upon the pressure of the
gas in thetube;
An increase in the battery B, operating the induction coil ] of gure 9—2, Will cause an increase in the intensity of
the emitted x-rays but will not affect to any great extent the
Voltage drop across the tube. At the lower gas pressures,
how—
ever, the emitted x—rays are not only more penetrating but also
of greater intensity. The intensity rises very rapidly as the
voltage across
the tube increases; it is very nearly proportional
to the square of the voltage, if the current is constant.
Ï
9-‘—3.
X—ray_ Tubes.——In order to avoid the.
changes due to variations in gas pressure and the
“
wandering
”
of the focal spot in the earlier tubes, Coolidge,2 in 1913, replaced
'
the gaseous source of electrons by a hot lament. A later form
of such a hot—cathode high-vacuum tube is shown in gure 9—3.
C consists of a spiral of tungsten Wire Surrounded by“
a metal
Serves to focus the electrons upon a
_
area, the focal spot, of the anode Â. The anode is constructed of”
Which
tungsten plug has been imbedded.
18
'
Cddledrin a
ways; _ by means of the radiatin‘g
01”,
Ÿ
upongthe removal of the rod carrying the ns, by a stream ofwater
_jg
_circulàted inside
'
the anode by either convection or pressure
The
is evaÏCuated to 0.000,05 millimeter or less,18
made ofthick, lead impregnated glass except
..r€äg‘ionïrOf:t—=he:small,—' th1n,lime glass Window W where the raysmay'“
emerge1napprec1able inftenSity; .
_
tubes, conStructed