Changing coaxial cable lengths between the detector and counting instrument can change the overall response of the detector. It is important to realize what the effects of increasing or decreasing detector cable lengths are and how to compensate for these changes to ensure the counting system reliability.
Coaxial cables have capacitance through the dielectric material between the current or signal carrying conductors. This capacitance is usually rated in pF/ft. or pF/m. A typical Belden RG-58U Low Noise coaxial cable has a nominal capacitance of 38 pF/ft. or 124.7 pF/m.
The cable connecting the detector to the counting instrument is typically configured to use the center conductor of the coax to carry the detector operating voltage (HV) and signal components of the detector. The coax shield or braided conductor is used as the detector ground which is referenced to the counting instrument chassis ground. Therefore, the signal which is a series of AC pulses corresponding to each detected radiation event, is attenuated between the center conductor through the dielectric to ground. The counting instruments have a threshold in which the detector pulse must cross to be counted. A typical threshold level, commonly referred to as input sensitivity, is 30 mV for a portable survey meter such as a Model 3; therefore, at least a 30 mV pulse amplitude is required from the detector before it will be recognized by the counting circuit.
Scintillation and Proportional Detectors
Scintillator or proportional detectors are the most affected by cable length changes. Considering that the detector pulse output varies in amplitude with radiation energy levels, any changes in pulse amplitude would be extremely critical. Using a Model 3 and Model 44-2 (2.5 x 2.5 cm [1 x 1 in.] NaI) scintillation detector for an example:
The Model 3 with the Model 44-2 is plateaued with a 241Am source using a 1 m (39 in.) cable. The operating point is set approximately 50 V above the knee of the plateau at 614 V and input sensitivity is 30 mV. If the connecting cable is changed from 1 m (39 in.) to 1.8 m (6 ft.), very little difference would be noticed in the meter reading, but if the operating voltage is decreased by 20 V, the count rate will start to fall off. If the 214Am plateau is re-plotted, the operating voltage would need to be increased to 650 V (34 V increase) to be at the same operating point as the 1 m (39 in.) cable connection. If a 6 m (20 ft.) cable was substituted, the operating voltage would have to be increased to 752 V. A 15 m (50 ft.) cable would require an 823 V operating voltage.
The above scenario illustrates that the detector operating voltage (HV) can be used to compensate for gain losses or increases due to the change in cable length. Due to the wider operating voltage range of most scintillation detectors, adjusting the HV is the most efficient way to compensate for cable length changes. But in the situation of the proportional detector, the detector breakdown or saturation point remains constant which limits the operating voltage range of the detector.
In addition to increasing the operating voltage for an increase in cable length, the threshold (input sensitivity) can be lowered to achieve the cable length compensation. Since the proportional detector saturation point does not change with cable length variations, the cable lengths are usually limited to less than 25 ft.
The parameters affecting detector operating voltage and sensitivity changes due to cable length variations are: cable capacitance, input impedance of the counting instrument, detector impedance, and detector gain. With this many variables, there cannot be a direct ratio between cable length and instrument voltage/threshold compensation. Count rate versus detector operating voltage plateaus is the tool for changing the cable lengths on scintillation and proportional detectors.
GM Detectors
Increasing or decreasing the coaxial cable lengths from a Geiger-Müller (GM) detector to a counting instrument is more forgiving when compared to proportional and scintillation type detectors. Unlike the scintillation or proportional detector, increasing the coaxial cable length from 1 m (39 in.) to 2 m (6.6 ft.) would not require re-calibration or detector operating voltage adjustment to compensate for the increase in cable capacitance. The large pulse amplitudes, typically 1 to 6 V, produced by GM type detectors allow longer cable lengths to be substituted without reducing the signal below the counting instrument's threshold level.
A general "rule of thumb" for the maximum cable length between most GM detectors and the counting instrument is 46 m (150 ft.). Variations in the instrument input sensitivity and input impedance may increase or decrease the maximum cable length. Problem symptoms and precautionary measures to watch for when increasing GM detector connecting cable lengths are:
Problem Symptoms: Non-linearity problems when the detector is exposed to radiation fields near the upper operating range of the detector. Detector pulse output may decrease or even appear "dead" (overload paralysis) when exposed to radiation fields above the upper operating range of the detector.
Precautionary Measures: When increasing cable lengths, check the detector/instrument calibration for non-linearity at the detector's upper operating range. If the detector appears to be non-linear, perform an operating voltage / count rate plateau in 25 V increments up to the maximum detector operating voltage limit. Determine if the operating voltage can be increased to improve counting linearity but still remain below maximum voltage limit. For dose equivalent readouts up to and including 10 R/h (0.1 Gy/h), expose the detector to 100 times the full scale, maximum decade reading to confirm that overload paralysis does not occur (e.g. full scale reading = 1000 mR/h, expose the detector to 100 R/h and ensure readout remains above full scale reading). For maximum readouts and detector limits greater than 10 R/h, expose the detector to 10 times the upper detection range to confirm that overload paralysis does not occur.
Detector operating voltage may be increased to compensate for pulse amplitude loss the same as with the scintillation and proportional detectors, but the overall improvement is much less and the maximum detector operating voltage usually restricts the voltage range. Input sensitivity can also be decreased to compensate for amplitude losses; however, caution must be observed to ensure that the counter threshold is set above the multiple pulsing region.
Special Applications
There are methods other than parameter adjustments for cable length compensation. Increasing detector operating voltage or decreasing the input sensitivity for some applications is often an impossible solution to cable length variations. Instruments, such as the Ludlum Model 3500 gate monitor series, have extreme cable length variations (typically from 30.5 to 305 m [100 to 1000 ft.]) from the scintillation detector(s) to the control console which makes parameter adjustment impractical.
The input capacitor (decoupling the HV from the signal) to the input amplifier is typically 100 pF for most Ludlum instruments. If the cable capacitance is 100 pF, then 50% of the signal will be attenuated by cable capacitance and 50% will be transferred to the input amplifier through the input capacitor (100 pF). If the cable length is increased and cable capacitance increases to 1000 pF (0.001 μF), 90% of the signal would be dropped across the cable capacitance and 10% would be transferred to the input amplifier. Therefore, the solution to the cable length problems is to select an input capacitor to match the cable length capacitance.
As with most problem solutions, there are restrictions to increasing the input capacitance — the recovery time of the counting circuitry is increased as the input capacitor is increased. Scintillation and proportional non-linearity problems may begin to appear between 50,000 to 100,000 cpm. Multiple pulsing (pulse over-shoot in amplifier circuitry) problems may occur if capacitance is increased too high for GM detector applications. Precautionary measures could be taken in ensuring that the detector operating voltage (H) is completely discharged when connecting/disconnecting coaxial cable(s) from the instrument to prevent a large transient from destroying the input amplifier. Consult the Ludlum engineering department when the counting instrument H and threshold parameters can no longer be adjusted to compensate cable length before modifying the instrument(s).
Coaxial Cable Length Limitations for Radiation Detectors
The detector cable lengths specified below are used with standard Ludlum instruments (survey meters and scaler/ratemeters) with input sensitivities around 10 to 35 mV. Model 3500 series gate monitors specially modified instruments incorporate special circuitry which compensates for extended cable lengths. Lower input sensitivities, photomultiplier tube gain (for scintillation detectors), instrument input impedance, and detector impedance may increase or decrease the maximum detector operating cable length.
| Detector | Maximum* Cable Length | Problem Symptoms and Precautionary Measures |
|---|---|---|
| Geiger-Müller (GM) | 46 m (150 ft.) | Non-linearity problems when the detector is exposed to radiation fields near the upper operating range of the detector. For survey meters with dose equivalent readouts, place the GM detector in a radiation field 100 times greater than the full scale/decade instrument operating point (for full scale/decade readings ≤ 10 R/h; use 10 times the upper limit for fields greater than 10 R/h) to ensure detector count does not decrease as radiation field intensity is increased. |
| Scintillators | 15 m (50 ft.) | Lower than normal counting efficiencies for low energy isotopes when increasing detector cable length. Confirm that the detector is operated in the plateau region when cable length is changed by plotting detector operating voltage/count rate operating curve. |
| Proportional (Including Neutron) | 8 m (25 ft.) | Same problem symptoms and precautionary measures as the scintillation detectors. The detector breakdown or saturation region remains the same for proportional detectors regardless of cable length; therefore, ensure that the detector operating point is below the breakdown region (50 to 100 V below). Gamma rejection point should be checked when shifting the operating voltage on neutron detectors. |
| Air Proportional | 2 m (6.5 ft.) | Same problem symptoms and precautionary measurements as the proportional detectors above. |
* The cable lengths above are suggested maximum cable lengths for Ludlum instruments in their standard design configuration; the cable length may be extended by modification to some of the instruments. Consult the Ludlum engineering department for more information for a specific application.