Theory

The UltraLo-1800 employs the patented technique of electronic background suppression, which drives achievable background rates to 0.0001 alphas/cm2/hr and below. This is a factor of 50 or more better than can be achieved by the conventional proportional counter systems that are currently available.

Through the use of novel, dual channel pulse-shape-analysis techniques, the UltraLo- 1800 is able to distinguish between alpha particles emitted from the sample under test, and those emitted from other parts of the counter structure.

Keep reading to learn more about why backgrounds are important in alpha particle counting, and how the technique of electronic background suppression works in the UltraLo- 1800.

Alpha Counting Challenges

  • Materials used to construct alpha counters will emit alpha particles, contributing to the background signal of the instrument.
  • The general approach is to make two measurements, with and without the sample, and subtract them to remove the background.
  • But what happens when sample activity levels are much lower than background levels?
    • Published proportional counter backgrounds are 3-5 alphas per hour.
    • Yet wafer level materials are currently screened at 6 alphas per day.
  • Detecting such a small signal hidden in such a large background requires very long counting times and is prone to error from statistical fluctuations.
Imagine if you could eliminate the background by only counting alpha particles coming from the sample, and not those coming from the counter!

Current proportional counter technology

  • Sample sits under thin window.
  • Array of fine anode wires biased at HV creates a high field region and provides a gain mechanism (through electron multiplication) for better signal-to-noise.
  • But this means that all alpha emissions look the same, because the pulse shape is dominated by the gain mechanism!

UltraLo-1800 takes a novel approach

  • Fundamental difference: UltraLo-1800 operates in ionization mode, not proportional mode.
  • Sample sits within the active counting region – no entrance window.
  • Geometry of counter is designed to exaggerate differences in signals generated by alphas emitted from various parts of the counter.
  • Two signal channels to further distinguish between alpha emission locations.

UltraLo-1800 Technology

  • Emissions from different areas look different, because pulse shape isn’t dominated by gain, but rather by drift distance.
  • Dual-channel pulse shape analysis reveals origin of alpha particles.
  • UltraLo-1800 actively rejects events not coming from the sample.
  • Further pulse analysis gives energy of the alpha particle.