Geiger–Müller Plateau Study

Project Overview

This experiment characterised the operating plateau of a Geiger–Müller tube by measuring counts‑per‑minute (CPM) as a function of bias voltage for a beta source. The goal was to determine a safe, stable operating voltage and to illustrate the physics of gas amplification and threshold behaviour in radiation detectors.

Apparatus & Method

End‑window Geiger–Müller tube on adjustable stand
Adjustable HV supply & scaler counter (0–1.5 kV, 0–9999 counts)
Encapsulated beta source (Sr‑90)
Stop‑watch & Lab notebook for manual logging
Voltage stepped in 25 V increments; 60 s integration per point

Key Findings

Start Voltage

No counts observed until 610 V → threshold for first Townsend avalanche.

Stable Plateau

CPM flat (±3 %) from 620 V – 1000 V, ideal for routine measurements.

Over‑voltage Risk

Above 1000 V counts rose sharply, indicating onset of continuous discharge.

Tools & Software

Analog scaler counter & HV supply
Python + Matplotlib for post‑lab plotting & linear fits
Excel for quick CPM calculations

Sample Data

Voltage (V)   CPM
580 – 600     0
610           19 ← start voltage
700           756
800           782
900           796
1000          805
1125          980 ← onset of discharge

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