18517 — Halogen-Quenched Cosmic Ray Guard Counter Tube

A specialized gas-filled radiation detector designed for low-background measurements, operating as a guard tube in anticoincidence circuits.

Overview

The 18517 is a halogen-quenched cosmic ray guard counter tube. It is specifically engineered for low-background radiation measurement applications, where it typically functions as a guard tube in anticoincidence configurations. In these setups, it is commonly paired with a beta counter, such as the type 18515, to detect and reject cosmic ray background events.

The tube utilizes a gas mixture of neon, argon, and a halogen quenching agent to ensure stable operation and self-quenching capability. It features a robust metal-and-glass construction with a dome base flange and flexible wire leads for electrical connection.

Mechanical Construction and Mounting

The 18517 is constructed with a heavy-duty envelope featuring a wall thickness of 1 mm. The cathode and anode structures are fabricated from an alloy consisting of 28% chromium and 72% iron.

Physical Dimensions and Weight

The physical envelope is characterized by a dome base flange and a top exhaust nipple. The key mechanical specifications are:

Mounting and Maintenance Requirements

Due to the high operating voltages and sensitive charge-collection characteristics of the tube, low-capacitance mounting is required. To prevent electrical leakage across the terminals, the tube must be kept dry and thoroughly cleaned.

Electrical and Operating Characteristics

At an ambient temperature of 25 °C, the 18517 exhibits highly stable plateau characteristics and low background counts when properly shielded.

Typical Operating Parameters

The standard operating point is defined with a recommended anode resistor ($R$) of 10 MΩ.

Recommended Circuit Configurations

The 18517 can be integrated into radiation detection systems using two primary circuit topologies, depending on whether it is used as a standalone detector or as an anticoincidence guard.

Standard Detector Circuit

In the standard configuration, the tube is biased through a series anode resistor, and the output signal is derived from the cathode:

• Anode Connection: Connected to the positive high-voltage supply ($+V_b$) via a 10 MΩ resistor ($R$).
• Cathode Connection: Connected to ground through a cathode resistor equal to one-tenth of the anode resistance ($1/10\ R$).
• Signal Output: Taken from the junction between the cathode and the cathode resistor, decoupled through a suitable coupling capacitor.

The value of the cathode resistor is determined by calculation from the recommended anode resistor ($R = 10\text{ M}\Omega$): $\text{Cathode Resistor} = \frac{1}{10} \times R = \frac{1}{10} \times 10\text{ M}\Omega = 1\text{ M}\Omega$

Anticoincidence Guard Circuit (with Type 18515)

When operating as a cosmic ray guard in conjunction with a type 18515 beta counter, the biasing arrangement is modified to optimize shielding performance:

• Anode Connection: Connected directly to the positive high-voltage supply ($+V_b$).
• Cathode Connection: Connected to ground through a series network consisting of a 10 MΩ resistor ($R$) and a 1 MΩ resistor ($1/10\ R$).
• Signal Output: Taken from the junction between the 10 MΩ and 1 MΩ cathode resistors, decoupled through a coupling capacitor.

Environmental and Voltage Limits

To prevent damage and ensure reliable operation, the 18517 must be operated strictly within its absolute maximum ratings.

• Maximum Anode Supply Voltage ($V_b$): 1200 V (with a series resistance $R = 10\text{ M}\Omega$).
• Ambient Temperature Range ($t_{\text{amb}}$): −50 °C to +75 °C.

Performance Curves and Temperature Behavior

Starting Voltage vs. Temperature

The starting voltage ($V_{\text{ign}}$) of the 18517 is temperature-dependent and increases monotonically across its operating range. At the lower limit of −50 °C, the starting voltage is approximately 480 V. As the temperature rises to 0 °C, $V_{\text{ign}}$ increases to about 590 V, reaching approximately 610 V at the nominal room temperature of 25 °C. If the temperature is increased further, the starting voltage continues its upward trend, reaching approximately 680 V at +150 °C.

Plateau Characteristics

When plotting the count rate against the applied anode voltage ($V_a$) with a 10 MΩ anode resistor, the tube exhibits a very flat plateau. This plateau begins near 800 V and extends up to 1200 V, maintaining a stable count rate of approximately 50 counts per second. When the anode voltage exceeds the absolute limit of 1200 V, the curve begins to bend upward, indicating the onset of continuous discharge or spurious pulsing.

Technical Summary

The 18517 is a specialized, halogen-quenched cosmic ray guard counter tube designed for low-background anticoincidence systems. Constructed with a 1 mm thick chromium-iron envelope and containing a neon-argon-halogen gas mixture, it provides a stable plateau from 800 V to 1200 V with a maximum slope of 0.03 %/V. Operating at a nominal voltage of 1000 V with a 10 MΩ anode resistor, the tube features a dead time of under 1 ms and a shielded background count rate of no more than 75 counts per minute. Its physical design, low interelectrode capacitance of 5.5 pF, and specific cathode-output circuit configurations make it highly optimized for integration with companion detectors like the type 18515.