Fluke Biomedical TNT 12000 - Manual (Page 118)

TNT 12000
Users Manual
D-2
Applications
It has been shown 1 that in a phantom, integration of the radiation exposure profile
produced by a single scan from a CT scanner along a line normal to the slice, divided by
the table increment, is equal to the exposure to a central slice at that point produced by a
series of scans. The line of integration must be of sufficient length to intercept not only
the primary beam, but also the Compton scatter produced in the phantom. This integral is
then expressed as R •cm.
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 model 500-200 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. Thus, the correction factor provided is
stated in terms of R •cm/Coulomb.
Specifications
Detector Type ........................................................ Vented air ion chamber
Volume ................................................................... 10.1 cc
Sensitive Length ................................................... 10.0 cm
Rated Length ......................................................... 8.2 cm (IEC 61674)
Effective Length .................................................... 9.6 cm (IEC 61674)
Chamber Material.................................................. Acrylic (PMMA)
Chamber Outside Diameter.................................. 12.7 mm
± 0.4 mm (0.5 in ± 0.015 in)
Chamber Inside Diameter..................................... 11.44 mm (0.45 in)
Chamber Wall Thickness ..................................... 77 mg/cm
2
Electrode Material ................................................. Aluminum, 1100
Nominal Sensitivity............................................... 2.8 mGy/nC
(0.32 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 active length
Beam Orientation .................................................. Normal to chamber axis
Angular Response ................................................ <3 % for
± 180 ° in the plane perpendicular to the ion chamber axis
Leakage Current (300 V collection potential)..... Less than 10-14 A at 10 min polarization time
Intensity Limits, Continuous beam ..................... 270 mGy/sek
(31.6 R/sec) (1 % recombination loss)
Pulsed Beam ......................................................... 138 μ Gy/Impuls (15.8 mR/pulse) (1 % recombination loss)
Collection Time ..................................................... 0.478 mSec
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
Typical Energy Dependence ................................ See Figure D-2
1
R.A. Jucius, G.X. Kambic, “Radiation Dosimetry in Computed Tomography (CT)”