Detection and Measurement of Nuclear Radiation (1962) / Chapter Skim
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Ionization Chambers
Ionization Chambers
Pages 45-57
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From page 45... ...
Aluminizing the covering film inproves the light collection efficiency and may be used to protect the phototube from ambient light. The total thickness of material covering the scintillator proper should be < 1 mg/cm2.
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From page 46... ...
dE/dx (7) where w is the average energy to produce one ion pair ("total ionization")
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From page 47... ...
Further, ionic mobilities are relatively insensitive to changes in the applied electric field strength and the gas pressure. Therefore, in the interest of fast response, pulse ionization chambers are almost invariably arranged for electron collection.
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VGOS Electrode -S i/ -=VP -=r R To Electrometer 1 HIGH INTENSITY SOURCE >SATU RATION CURRENT LOW INTENSITY SOURCE APPLIED POTENTIAL VP Fig.
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From page 49... ...
are compared, the saturation current is proportional to the source strength N This is the basis of the many ionization chamber instruments used for monitoring and assay purposes.
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From page 50... ...
Here, the details of the collection process and the transient response of the ancillary equipment are both very important, since the complicated signal from the chamber is always observed distorted by the measuring system. Consider a parallel-plate ionization chamber in which a single ion pair has just been formed (cf., Fig.
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From page 51... ...
22. The pulse profile shown makes the simplifying assumption that n ion pairs were formed at a point x0 ; actually the ionization is produced along a track, and the qualitative pulse shape in the figure will be distorted by the spacial distribution of ion pairs.
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From page 52... ...
22. Idealized voltage pulse in a parallel-plate ionization chamber with plate spacing d, after production of n ion pairs a distance x0 from the collecting electrode.
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From page 53... ...
. To avoid some of the difficulties encountered when total ionization pulses are collected, only the portion of the pulse due to electron collection is employed.
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From page 54... ...
Even materials of high resistivity may develop serious leakage currents if the surfaces are permitted to acquire a charge from mechanical stresses; from rubbing one surface against the other; or from the electric field, which can induce an image charge or cause ions from the active volume to be collected on the insulating surfaces. On the other hand, ordinary surface leakage is not very important in fast pulse chambers at voltages of 1 or 2 kv.
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From page 55... ...
5. Counting and Assay Applications Ionization chambers are widely used for measuring the strengths of sources of heavy charged particles.
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From page 56... ...
To avoid random summing of low-amplitude pulses when a high beta- or gamma-ray activity is present in the alpha-particle source, a short clipping time is needed. A useful shortening of the detector rise time can be obtained by increasing electron drift velocity; in argon, a common filling gas, the rise time is improved by the addition of 5% C02 .
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From page 57... ...
STAINLESS STEEL GUARD RING BAKELITE TO VIBRATING REED ELECTROMETER COPPER WASHER FLUOROTHENE 0-RINGS -GUARD RING CONNECTOR (To Ground )
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