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

National Research & Development Laboratory Applications

Nuclear Decay Experiment

Customer Case

A customer wants to measure nuclear decay rates of various radioactive isotopes. With each nuclear decay, the isotope under study simultaneously emits a beta particle, which is a high-speed electron, and a gamma particle, which is a high-frequency photon of light. Beta particles are detected with an efficiency of over 90% by a proportional gas counter that surrounds the radioactive isotope. Surrounding only one quarter of the isotope, a sodium iodide scintillator crystal detects gamma particles with about 25% efficiency. When it impinges on the scintillator crystal, a gamma creates a flash of visible light that is detected by a photo-multiplier tube. Upon detection of a particle, both detector systems produce a unipolar analog pulse of ~10 us duration. The customer wants to digitize these pulses at 10 MS/s with 12-bit resolution.

The customer must capture both detector signals when either detector detects a particle. The customer must also know the time of occurrence of each of these events within 100 ns. By determining the number of events in which either a beta, a gamma or both are detected, the customer can calculate the efficiencies of each detector. Using the occurrence time data, the customer will then be able to determine the true nuclear decay rate of the isotope.

Decays occur in a random fashion at an average rate of 1000 decays/second. The customer wants to capture detector signals for all decays occurring during a counting time period of at least 10 seconds for good counting statistics.

GaGe Case Solution

The GaGe data acquisition solution is the CompuScope 1012/PCI and the Trigger Marker Board (TMB). A wiring diagram is shown in the illustration below. The CS1012/PCI is connected with the beta signal on CHANNEL_A and the gamma signal on CHANNEL_B. Data capture at 10 MS/s is triggered by the Boolean condition: CHANNEL_A OR CHANNEL_B. With this setup, data capture will begin whenever the signal on either channel exceeds the software-adjustable trigger level.

Nuclear Decay System Diagram

The CS1012/PCI is to be operated in PCI Real Time / Multiple Record mode. In this mode, captured data is continuously transferred to PC RAM through the PCI bus. Also, after data capture, the digitizer is hardware-rearmed to await the next trigger within only four data sampling intervals or 0.4 us.

The CS1012/PCI will capture two channels of 12-bit (2 byte) data. At a 10 MS/s sampling rate, therefore, the board will capture:

(2) * (2 Bytes/Sample) * 10 MSample/s = 40 MBytes/s

This is well below the sustained PCI transfer rate of 100 MB/s that the CS1012/PCI can attain. Therefore, even if all the data (and not just the particle detection events) were captured continuously, the CS1012/PCI would be able to sustain PCI Real Time Transfer throughout the counting period.

The customer believes that records 50 us in length will ensure the capture of all likely detector pulses. The individual record length, therefore, will be:

10 MS/s * 50 us = 500 Samples

Since events occur at an average rate of 1000 events/s, the average rate of data generation is:

2 * (2 Bytes/Sample) * (500 Samples/event) * (1000 events/s) = 2 MB/s

In order to capture all events that occur during a 10 second counting period, therefore, the customer will only need 20 MB of free PC RAM. After the counting period has finished the data can be saved to hard disk.

The Trigger Marker Board (TMB) consists of a 40-bit counter clocked by a 50 MHz crystal. On the rising each of the TTL signals on its INDEX input, the TMB resets the counter value to zero. On the rising edge of the TTL signal on its EVENT input, the TMB stores the counter value in its onboard memory. In this way, the timing of all EVENTs can be referenced to the time at which the INDEX pulse was sent.

Since absolute time is irrelevant to the customer's application, the counter can be reset through software at the beginning of the counting period. In experiments that must be synchronized with other experiments, however, the INDEX pulse can be provided by some synchronizing pulse. The customer's CS1012/PCI will be modified to provide a buffered output of the TTL trigger signal that triggers its ADC chip. This TRIGGER OUTPUT on the CS1012/PCI is connected to the EVENT input of the TMB.

In this way, every event that triggers data capture, in this case the triggering condition CHANNEL_A OR CHANNEL_B, is time-stamped by the TMB. The 20 MHz TMB clock provides time stamping with an accuracy of 50 ns-twice the customer's requirement. After the counting period, the event time data is downloaded from onboard memory to PC RAM through the ISA bus.

Since the TMB is only accessed by the controlling software before and after the counting period, its operation does not inhibit PCI data transfer. This is a good example, therefore, of how two different GaGe boards can be externally wired to meet the requirements of the customer's application.

GaGe Case Recommended Products

  • CompuScope 1012/PCI - 12-bit, 10 MS/s A/D and Scope Card for PCI Bus
  • Trigger Marker Board (TMB) - ISA Bus Card to Measure Time Interval Between Two Input Pulses

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