Model SF-TF-6


  • Ideal for wipe testing counting low energy beta emitters (H-3, C-14, S-35, etc.)
  • Highly sensitive: 0.5 nCi (18.5 Bq) / sample
  • Metal drawer with two 1.25” (31mm) diameter sample cavities surrounded by 1” lead shielding
  • Low background (typically <10 CPM) windowless gas proportional detector
  • Includes six decades of measurement (1-100,000 cps or other unit of choice)
  • Audible and visible alarms, RS-232 data output


The Model SF-TF-6 is a small portable gas proportional counter with a shielded drawer system for measuring surface contamination collected on wipes, filter papers, or other thin samples.  It is suitable for measuring any alpha or beta emitter, but is designed for measuring low energy beta emitters such as tritium and C-14.  Contaminated wipes are inserted into a 2” wide metal drawer system with two sample cavities of 1.25” (31mm) diameter and 0.18” (4.5mm) depth and surrounded by 1” lead shielding.  Two cavities allow the user to save time unloading and loading samples.  Usable sample diameter is limited by the 1” (25mm) diameter of the detector face.

The detector is a low background windowless gas flow proportional detector with a nominal outside diameter of 1.38” (35mm) and an inside diameter of 1” (25mm).  P-10 or similar counting gas is required for use of the instrument.  Steady-state gas usage is about 250cc/min, and some users turn the flow rate up to 600cc/min between samples as a purge.  Six feet of ¼” tubing is supplied with the instrument for connecting to a P-10 gas supply.  The instrument body is a lead shield approximately 3.87” x 3.5” x 3” (98x87x76mm) to reduce background radiation in the detector cavity, and a carrying handle attaches to the shield block for easy carriage.

Includes PRS-7 scaler/counter/analyzer, which is a digital display pulse counter that covers a 6-decade range (1-100,000 CPS or other unit of choice).  The PRS-7 is battery-powered, with an internal lithium cell and charger, and an external 12V AC power input.  The PRS-7 has a preamplifier with a built-in single-channel analyzer so that pulses may be selected by their pulse height and cancel out other noise.  It can also scale the counter display by a multiplier over a wide range, and the display can show both pulse rate and total pulse count, with a timer that will stop the counter after a preset interval.  Includes an adjustable high voltage power supply with a range of 200-2500 Volts, audible/visible alarms, and RS-232 data output.

Gas proportional probes generate signal pulses of heights proportional to the energy of the detected radiation events.  The single-channel analyzer (SCA) in the PRS-7 consists of two comparators with adjustable thresholds.  The thresholds are called “discriminator” and “window” and are controlled by 10-turn potentiometers on the front panel.  The SCA selects which pulses to pass along to the counter based on the two thresholds and the mode setting.

Discriminator Mode: Only the discriminator threshold is used and all pulses with heights exceeding the threshold are passed then counted.  A dial setting of 100 is typically used to block low-level pulses which are predominately noise.  Note: A 3-way toggle switch allows you to select from 3 different adjustable preset discriminator levels for convenience when measuring different isotopes.

Window Mode: Both the discriminator threshold and window threshold are used, and only pulses that are above the discriminator and below the window will be passed then counted.  When performing measurements where there is significant background or the possibility of more than one radioactive emitter, use window mode to eliminate higher energy from the counted pulses.

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