Fluke Biomedical TNT 12000 - Manual (Page 114)

TNT 12000
Users Manual
C-2
A long, thin radiation probe can be used to make this measurement. The probe may be
calibrated in a uniform field covering its entire length, generating a correction factor in
the conventional manner. Subsequent probe readings, when multiplied by the
conventional correction factor, and then by the probe’s sensitive length, will be in units of
R •cm.
Since the 3.2 cc probes are intended mainly to be used to integrate radiation exposure
profiles produced by the CT scanners, this length (10 cm) factor has, for the user’s
convenience, been built into the calibration.
The correction factor is stated in terms of
R •cm/Coulomb. In the now special case of
using this probe to measure uniform field exposures in terms of R , merely divide the 50-
100 correction factor by 10.
Specifications
Detector Type ........................................................ Vented air ion chamber
Volume ................................................................... 3.2 cc
Sensitive Length ................................................... 10.0 cm
Rated Length ......................................................... 8.9 cm (IEC 61674)
Effective Length .................................................... 10.1 cm (IEC 61674)
Chamber Material.................................................. Polystyrene
Chamber Inside Diameter..................................... 6.4 mm
Chamber Wall Thickness ..................................... 54 mg/cm
2
Electrode Material ................................................. Aluminum
Nominal Sensitivity............................................... 8.76 mGy/nC
(1 R/nC)
Standard Calibration............................................. 100 kVCP, 5.5 mm Al HVL (NIST Tech. M100), 100 KVCP, 3.5 mm Al
HVL (DV Beam Quality)
Response Uniformity Along Axis........................ ± 3 % over central 90 % of rated length
Angular Response ................................................ <3 % for ± 180 ° in the plane perpendicular to the ion chamber axis
Beam Orientation .................................................. Normal to chamber axis
Phantom Adaptor OD ........................................... 1.27
± 04 cm (.50 ± 0.015 in)
Leakage Current (300 V collection potential)..... Less than 10-13 A at 10 min. polarization time, less than 10-14 A at
2 hr polarization time
Intensity Limits, Continuous beam ..................... 42.6 Gy/min (4.86 kR/min) (1 % recombination loss)
Pulsed Beam ......................................................... 0.45 mGy/pulse (51.5 mR/pulse) (1 % recombination loss)
Maximum Pulse Repetition Rate ......................... 3.3 kHz
Cable Length ......................................................... 0.9 m
(3 ft)
Operating Voltage ................................................. - 300 V
Termination ........................................................... Triax BNC
Air Density Correction.......................................... To perform air density corrections when using a calibration factor with
a reference temperature of 22 ° C, multiply the ion current by the
following correction factor, F: F = (273.15 + T) / (295.15 x P).
Where T is the temperature in
° C and P is the pressure expressed as a
fraction of a standard atmosphere (1013 hPa). For chambers with a
calibration factor normalized to 20
° C, the denominator is 293.15 x P.