18514 — Self-Quenching Radiation Counter Tube
The 18514 is a self-quenching radiation counter tube designed for the detection of alpha particles, beta particles, and photons down to very low energy levels. Filled with a rare gas and featuring a thin mica window, the tube is utilized in laboratory, medical, and industrial tracer techniques, thickness measuring instruments, and general survey or laboratory monitoring equipment.
Mechanical Construction and Protection
The 18514 is constructed with a thin mica window at one end to permit the entry of low-energy ionizing radiation. The physical and mechanical specifications of the tube and its protective housing include:
- Mica Window Diameter: 27.8 mm
- Mica Window Area: 605 mm²
- Mica Window Density: 3.5 to 4 mg/cm² (uniform for each individual counter)
- Cathode Wall Thickness: 1.2 mm
- Tube Weight: 56 g (alone), 156 g (with container)
- Tube Dimensions: Maximum diameter of 30 mm, maximum length of 98 mm
- Container Dimensions: Maximum diameter of 54 mm, maximum length of 121 mm
The tube is delivered inside a protective aluminium container that is electrically connected to the cathode. For testing or for measuring gamma radiation and high-energy particles, the tube can remain inside this container. The container features a V-groove that can be cut through with a penknife to remove the counter when necessary. Additionally, a detachable rubber cap is supplied to protect the delicate mica window when the tube is not being used to measure low-energy radiation.
Electrical and Operating Characteristics
The electrical behavior of the 18514 is defined by its rare gas filling and physical geometry. The operating parameters under typical conditions are:
The dead time of the tube varies from 250 µs down to 100 µs depending on the applied working voltage. The background count is kept to a maximum of 40 counts per minute when the tube is shielded by a combination of 1/4-inch aluminium and 2-inch lead shielding.
Environmental and Life Ratings
The 18514 is rated for operation across a wide thermal range and exhibits a long operating life:
- Ambient Temperature Range: -55 °C to +75 °C
- Count Life Expectancy: Minimum 10¹⁰ counts
Detection Efficiency
The detection capabilities of the 18514 vary depending on the type of radiation and its incident energy:
- Gamma and Radium Sensitivity: When exposed to a radium source of 2.39 ± 1% millicurie, the tube produces a counting rate of 7,210 counts per minute for a radiation intensity of 2.5 m röntgen/hour.
- Beta Particles: The tube detects and counts all beta particles arriving at the mica window with an energy exceeding 50 keV. The actual counting rate is subject to the source type, geometry, and self-absorption factors.
- Alpha Particles: The stopping power of the mica window is equivalent to 3 cm of air. Alpha particles arriving at the window with an energy of approximately 5 MeV are successfully counted.
Circuit-Specific Operating Example
In a typical application circuit, the 18514 is integrated with an external amplifier stage. The cathode is grounded and connected to the protective aluminium container. The recommended circuit values are as follows:
- Anode Series Resistor: 2.7 MΩ (connected to the high-voltage HT+ supply)
- Coupling Capacitor: 300 pF
- Grid Series Resistor: 330 Ω (connected directly to the grid of the amplifier tube)
- Grid Leak Resistor: 1 MΩ
Technical Summary
The 18514 is a robust, rare-gas-filled, self-quenching radiation counter tube featuring a 27.8 mm diameter mica window. It is designed to detect alpha, beta, and gamma radiation across a temperature range of -55 °C to +75 °C with a minimum life expectancy of 10¹⁰ counts. Operating with a threshold voltage of 600 V and a plateau length of 300 V, the tube provides reliable radiation measurement capabilities in laboratory, medical, and industrial environments.
