18504 — Halogen-Quenched Radiation Counter Tube
A self-quenching Geiger-Müller radiation counter tube featuring a thin mica window, designed for the detection and measurement of alpha, beta, gamma, and neutron radiation.
Overview
The 18504 is a halogen-quenched radiation counter tube designed for highly sensitive detection across multiple radiation types. It features a thin, delicate mica window at one end to permit the entry of low-energy alpha and beta particles, while its robust metal cathode and gas filling facilitate the detection of gamma rays and neutrons. The tube is filled with a mixture of neon, argon, and a halogen quenching agent, which ensures a self-quenching operation and an exceptionally long operating life.
With a compact physical profile, low operating voltage, and a wide operating temperature range, the 18504 is suitable for portable and laboratory radiation measurement equipment. Proper operation requires low-capacity mounting, clean and dry conditions to prevent leakage currents, and a high-value anode series resistor.
Construction and Physical Dimensions
The 18504 is built with a cylindrical metal cathode and a thin mica window sealed at one end.
- Cathode Material: 28% chromium and 72% iron.
- Cathode Dimensions: Inside diameter of 14.4 mm, effective length of 40 mm, and a wall thickness of 250 mg/cm².
- Mica Window: Features an effective diameter of 9 mm, an area of 63.5 mm², and a thickness of 2 to 3 mg/cm².
- Protective Cap: A removable aluminum cap (16.1 mm diameter, 4 mm length) is provided to protect the delicate mica window when the tube is not being used to measure low-energy radiation.
- Physical Dimensions: The body diameter ranges from 14.5 to 15.5 mm, with a maximum collar diameter of 17 mm. The overall length is 39 to 43 mm (excluding the anode pin, which has a maximum extension of 10 mm).
- Weight: 7 g.
Electrical and Operating Characteristics
The electrical characteristics of the 18504 are specified at an ambient temperature of 25°C. The tube operates on a relatively low voltage plateau, making it compatible with portable high-voltage power supplies.
Temperature and Life Ratings
- Temperature Coefficient of Starting Voltage: Approximately 0.5 V/°C.
- Operating Voltage (V_a or V_b): Arbitrary within the plateau region.
- Expected Life: Minimum of 5 x 10^10 counts.
- Ambient Temperature Range (t_amb): Minimum -55°C, maximum +75°C.
- Recommended Minimum Temperature: -40°C.
Circuit and Mounting Requirements
To maintain the high insulation resistance required for stable operation, specific circuit and physical mounting guidelines must be followed:
- Mounting: Low-capacity mounting with short connections is required to minimize stray capacitance, which can affect dead time and pulse characteristics.
- Maintenance: The tube must be kept dry and thoroughly cleaned. Accumulations of moisture or dirt can create leakage currents across the external envelope, leading to spurious counts or unstable operation.
- Measuring Circuit: The recommended anode resistor (R) is 10 MΩ. The output signal is typically taken via a coupling capacitor from a tap between the anode resistor R and a series resistor of 1/10 R (i.e., 1 MΩ).
Qualitative Curve Observations
- Count Rate vs. Anode Voltage: The characteristic curve of count rate versus anode voltage (measured with R_a = 10 MΩ) shows a flat, stable plateau region centered around 100 counts/sec. This plateau starts around 350 V to 400 V and extends past 650 V. Above 650 V, the count rate rises sharply, indicating the end of the plateau and the transition into continuous discharge.
- Dead Time vs. Anode Voltage: The dead time is voltage-dependent. At an anode voltage of 300 V, the dead time starts near 40 µs, rises to a peak of approximately 80 to 85 µs around 500 V to 550 V, and then gradually decreases to about 70 µs at 700 V.
Radiation Detection Applications
The 18504 is a versatile detector capable of measuring several types of radiation:
- Alpha and Beta Radiation: The thin mica window (2 to 3 mg/cm²) allows low-energy alpha and beta particles to penetrate into the active volume of the tube, where they initiate ionization.
- Gamma Radiation: Gamma rays are detected via interaction with the cathode wall and gas filling.
- Neutron Detection: For measuring thermal neutrons, the tube must be wrapped in a cadmium foil of approximately 0.5 mm thickness. The cadmium foil acts as a converter, capturing thermal neutrons and emitting secondary radiation that is subsequently detected by the tube.
Technical Summary
The 18504 is a compact, 7-gram, halogen-quenched Geiger-Müller radiation counter tube featuring a 9 mm effective diameter mica window and a chromium-iron cathode. Operating with a starting voltage between 275 V and 325 V, it provides a stable plateau of at least 225 V (typically 250 V) with a very low slope of 0.01%/V to 0.02%/V. It is highly resilient, offering an expected lifetime of at least 5 x 10^10 counts and operating reliably from -55°C to +75°C. Proper operation requires a 10 MΩ anode resistor, low-capacity mounting, and clean, dry conditions.
