18526 — Halogen-Quenched Radiation Counter Tube

The 18526 is a halogen-quenched radiation counter tube designed with an end window for the detection of alpha, beta, and gamma radiation. It features a gas filling of neon and argon combined with a halogen quenching agent. The tube is constructed with a thin mica end window and a robust chromium-iron cathode, making it suitable for sensitive radiation measurement applications.

Construction and Physical Dimensions

The 18526 is built with specific materials and dimensions optimized for radiation detection:

• Mica Window: The tube features an end window made of mica with an effective diameter of 27.8 mm and an effective area of 6.1 cm². The window thickness is extremely thin, ranging from 1.5 to 2 mg/cm², allowing alpha and low-energy beta particles to penetrate into the active volume.
• Cathode: The cathode is constructed from an alloy of 28% chromium and 72% iron. It has a wall thickness of 1.3 mm, an inside diameter of 27.8 mm, and an effective length of 37 mm.
• Gas Filling: The active volume is filled with a mixture of Neon (Ne) and Argon (Ar), along with a halogen quenching agent to terminate the ionization discharges.
• Physical Dimensions:
  • Body diameter: 29.9 to 30.9 mm
  • Window flange diameter: 33 to 34 mm
  • Body length: 38 mm
  • Total length (excluding pins): 41 to 45 mm

Electrical Characteristics and Operating Parameters

At an ambient temperature of 25 °C, the typical operating parameters of the 18526 are characterized by a wide plateau and low background count rate:

• Interelectrode Capacitance: The tube has a capacitance of 3.5 pF.
• Starting Voltage ($V_{\text{ign}}$): The maximum starting voltage is 375 V.
• Operating Voltage ($V_{\text{b}}$): The operating voltage can be set arbitrarily within the plateau range.
• Plateau Range ($V_{\text{pl}}$): The plateau extends from 450 V to 750 V.
• Plateau Slope ($S_{\text{pl}}$): The maximum slope of the plateau is 0.02 %/V, ensuring stable counting performance despite minor fluctuations in the high-voltage supply.
• Background ($N_0$): When shielded with 5 cm of lead (Pb) and 3 mm of aluminum (Al), the maximum background count rate is 20 counts per minute.
• Dead Time ($\tau$): The maximum dead time is 200 µs at an operating voltage ($V_{\text{b}}$) of 600 V.

Absolute Maximum Ratings

To prevent damage and ensure reliable operation, the 18526 must be operated within the following absolute limits:

• Maximum Anode Voltage ($V_{\text{a}}$): 750 V
• Minimum Anode Resistor ($R$): 2 MΩ
• Ambient Temperature Range ($t_{\text{amb}}$): -50 °C to +75 °C

Circuit Design and Mounting Recommendations

For optimal performance, the physical installation and electrical circuit of the 18526 must minimize stray capacitance and leakage currents:

• Mounting: The connection between the anode and the anode series resistor must be kept as short as possible. Additionally, the capacitance between the anode and earth must be minimized to prevent pulse distortion and prolonged dead times.
• Recommended Circuit: The anode should be connected to the high-voltage supply ($+V_{\text{b}}$) through a series anode resistor ($R$) of 10 MΩ. The signal output is ideally taken from a 1/10 R tap (a voltage divider tap at one-tenth of the series resistance).
• Maintenance: The tube must be kept dry and clean during storage and operation to prevent external leakage currents, which could degrade the signal-to-noise ratio or cause spurious counts.

Environmental and Operating Curves

The performance of the 18526 varies systematically with temperature and operating voltage:

• Starting Voltage vs. Temperature: The starting voltage ($V_{\text{ign}}$) is highly stable, remaining around 340 V to 350 V across a wide temperature range from -40 °C to +80 °C. However, at temperatures below -40 °C, the starting voltage drops sharply, reaching approximately 290 V at -60 °C.
• Plateau Curve: The characteristic curve of counts per second versus operating voltage ($V_{\text{b}}$) exhibits a flat, stable plateau region extending from approximately 450 V to 750 V.
• Dead Time vs. Voltage: The dead time ($\tau$) is voltage-dependent. It peaks at approximately 200 µs near the beginning of the plateau at 450 V, and then decreases steadily as the voltage increases, falling to approximately 130 µs at 800 V.

Technical Summary

The 18526 is a halogen-quenched Geiger-Müller radiation counter tube featuring a 27.8 mm mica end window (thickness 1.5 to 2 mg/cm²) and a chromium-iron cathode. It is designed for detecting alpha, beta, and gamma radiation. Operating with a 10 MΩ anode resistor, it features a wide plateau from 450 V to 750 V with a maximum slope of 0.02 %/V and a maximum starting voltage of 375 V. The tube exhibits a maximum dead time of 200 µs at 600 V and a low background count rate of 20 counts/min under shielding. It is rated for operation between -50 °C and +75 °C, requiring clean, dry conditions and minimal anode-to-earth capacitance for optimal pulse resolution.