National R&D Laboratory Application Case Study - Data Acquisition Particle Physics

National Research & Development Laboratory Applications

Alpha Particle Counting

Customer Case

A customer has to characterize a new design for a solid state alpha particle detector. The detector is essentially a silicon diode with a large area face. Because alpha particles, which are high-speed helium nuclei, are electrically charged, they interact strongly with matter and lose their energy quickly upon entering a solid. When an alpha particle decelerates within the depletion region of the diode, it creates electron-hole pairs. The carriers are collected by the diode's electrodes and create a measurable current pulse.

The customer's experimental solid state detector has two implanted electrodes whose signals will be used to diagnose and characterize the detector. Ultimately, understanding derived from the customer's experiments will be used to guide the design of commercial alpha particle detectors.

Often, in particle beam experiments, analog detector signals are analyzed by a Multi Channel Analyzer (MCA). The height of a single particle pulse is measured and the bin or "channel" count corresponding to its voltage is augmented - a process call pulse height discrimination. Multiple channels, typically thousands, are displayed in a histographic form on the screen of the MCA. If the pulse height is linearly correlated with the alpha particle energy, then calibration allows a direct read-out of the measured particle energy distribution.

The customer wants to capture all the particle pulse signals from both test electrodes. Without dual inputs and pulse storage capability, most MCAs are unable to meet the requirement. Furthermore, for the customer's uncharacterized detector, pulse height may not depend linearly on alpha energy.

In the customer's setup, alpha particles arrive randomly at an average rate of 100 per second. The particle pulses last 2 us and a minimum of 100 points per data record are required for detailed pulse analysis. At least 2000 particle pulse events must be captured in order to characterize the detector for a given set of test conditions. A high dynamic range is also required for capture of particle pulses with high and low amplitudes.

GaGe Case Solution

The GaGe solution is the CompuScope 8012A with 1 MegaSample of on-board memory working in Multiple Record mode. The customer's two electrode signals are connected to the two inputs of the CS8012A. At the required 50 MS/s dual channel sampling rate, this 12-bit A/D board provides the highest dynamic range available.

The CS8012A allows triggering that is highly appropriate for the customer's application. The board can be software adjusted to trigger in Boolean OR mode. In this mode, data acquisition is triggered when the signal on either input channels exceeds a separately adjustable threshold level. Acquisition occurs, therefore, if either one of the electrode signals causes a trigger event.

The CS8012A will acquire data in Multiple Record Mode. In this mode, the board is automatically hardware re-armed with no software interaction within 4 data points. At 50 MS/s, this incredibly fast 80 ns re-arm time ensures that virtually no alpha particle pulse events are missed. Successive records are stacked sequentially in the CS8012A's on-board memory until it is filled.

The CS8012A is set to capture 128 points per channel at 50 MS/s when either signal passes its threshold level. This meets the customer's requirement of 100 points per 2 us pulse. Since each event generates 256 Samples, the CS8012A's on-board memory can accommodate 4000 particle events, or twice the customer's requirement.

Since particle events occur at an average rate of 100 per second, the time required to fill on-board memory is 4000 events / (100 events/s) = 40 s. The CS8012A can download data at 1 MS/s through the ISA bus and so requires only one second in order to flush the entire on-board buffer. This 1 second ISA transfer time is only 2.5% of the 40 second counting time. If run repetitively in this experiment, therefore, the CS8012A alpha detector has a respectable 2.5% dead-time.

Storage of all particle pulses allows the customer to do detailed pulse measurements under different test conditions. In fact, the functionality of a conventional 2048 channel MCA can be recovered by analyzing the raw 12 bit pulse data, value[i], with the following lines of C source code:

MAX = -2027;
for (i=0; i < NSAMPLES; i++)
{
     if (value[i])>MAX) MAX=value(I);
}
if (MAX<0)MAX=0;
Channel (MAX) = Channel(MAX] + 1;

This piece of code calculates the pulse height, MAX, and then augments the appropriate bin or channel count, CHANNEL[MAX], by one.

The time required for thresholding calculation is 150 ns per point. This means that conventional pulse height discrimination can be done on the complete 1 MS on-board memory buffer in only 150 ms. This is small compared to the 1 second ISA bus download time. Therefore, the 2.5% dead time for the application can be maintained.

This application shows how the CS8012A, a general-purpose A/D card, can be used for sophisticated particle pulse detection. The CS8012A can be programmed to function as a conventional MCA or as a tool to do even more detailed pulse analysis.

GaGe Case Recommended Products

  • CompuScope 8012A - 12-bit, 100 MS/s A/D Card for the ISA Bus

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