Introduction

The SiPM-Counter is ideal for applications where the developer would like to migrate from a simple counter to an IoT-ready device with many built-in capabilities – while retaining the simplicity of just having a plain counter.

At the minimum, this smart counter powers an SiPM array and provides programmable, but fully autonomous, gain stabilization.

There are built-in functions to provide sample vs background counting, alarm computations and even to act as a Portal Monitor Appliance where alarm computations update 10 times per second.

Brief Specifications

• Ideal for NaI and slower scintillators
• Accurate count rate measurements; dead time <0.9µs
• Sample vs background measurements with alarming
• Portal Monitor mode with background tracking and dynamic alarming
• Two-channel logger to record transient phenomena in 50ms time steps
• Time histogram for data quality monitoring and scientifically accurate count rate measurement
• Built-in gain stabilization
• Power: 4.3V to 5.5V, 15mA
• Open source application programmer's interface; Python API
• wxMCA graphical user interface for Windows and Linux
• SiPM supply: 37V/5mA DC/10mA surge
• Uses 2N3904 as external temperature sensor.
• USB 1.2 interface compatible with USB 2.0
• Serial UART interface up to 3MBd (300kB/s)
• On-board software is secure against reverse engineering.

Capabilities

SiPM-Counter Capabilities
Capability	Description
Analog	Factory-configurable options include charge-integrating preamplifier with 50µs tail pulse, or I-2-V converter.
Gain stabilization	The SiPM-Counter uses either built-in or user-programmable look up tables to adjust the SiPM operating voltage as a function of temperature. Non-standard embedded software may also adjust the trigger threshold as required by some scintillators.
Trigger	Instead of the analog output, users can activate a line driver for digital pulse outputs. Both, the trigger threshold and the output pulse width are programmable.
Counter	The SiPM-Counter can count pulses in either of two active banks, one for samples to be measured and one for storing a background measurement. The device reports count rates and statistical 2-σ errors.
Net Counter	The SiPM-Counter reports the difference between sample and background count rate together with the combined statistical 2-σ errors.
Analysis	The SiPM-Counter reports the probability that the measured sample count rate is compatible with the background count rate.
Dynamic alarming	The SiPM-Counter can analyze and report count rates in time slices of 100ms, ie at a rate of 10/s. The device automatically tracks slowly changing backgrounds and will alarm on a passing source. Its digital output can be used to drive an audio or visual alarm.
Loss-less counting	The SiPM-Counter implements a read-and-clear command, in which the microcontroller clears the counters right after copying data to the output buffer – for near loss-less reading of count rates.
Time histogram	The SiPM-Counter can histogram the time difference between successive events. This is a great analystic tool to check if counts are truly due to randomly arriving radiation pulses – and are not caused by electromegnetic interference. Further, it can be used to accurately measure a count rate independent of assumptions about the dead time.
Two-channel logger	The SiPM-Counter can can log two channels of status parameters in programmable time steps of 50ms to 12.75s with a history length of 1023 samples.
Communication	The SiPM-Counter implements a USB-2.0 compatible USB 1.2 interface and a serial interface

Gain stabilization

The SiPM-Counter can use a 20-point lookup table that describes the desired operating voltage vs temperature behavior. The embedded processor applies this to counteract the SiPM vs temperature gain drift. Typically, the lookup table starts at lut_tmin=-30°C and increments in lut_dt=5°C steps up to 65°C. However, the developer can configure that to meet their requirements. And the developer can program a lookup table of their own choice into the non-volatile memory of the SiPM-Counter.

Count rate measurements

For negative-going output pulses, the SiPM-Counter can use a built-in discriminator to accomplish two tasks. For one, it can for each scintillator pulse create a digital output pulse and send that to a remote receiver. Given its strong line driver, and the fact that the output pulse width can be adjusted between 50ns and 1.37ms, this feature allows to create a system that is very robust against electromagnetic interference from the outside.

Secondly, the discriminator trigger threshold can be software adjusted and the embedded ARM processor can count trigger pulses in real time. The feature is used to establish two alternative counters for count rate measurement. Typically, one may be used to establish a background count rate, while the other is used to measure a current sample. The SiPM-Counter not only reports the count rates, but also the statistical uncertainty of the measurement.

In addition, there is built-in software to compare the sample count rate against the background count rate and compute the statistical probability that the sample counts are caused by the same activity as the established background rate.

The developer can set an alarm threshold and let the trigger output of the SiPM-Counter indicate an alarm when the sample is decidedly more radioactive than the background.

Time-slice operation

There are dynamic situations, where a radioactive source can be measured only for a brief moment. Examples are a vehicle passing through a radiation portal monitor, or a person with a backpack detector walking past a stationary source.

The time-slice operation supports these cases. The built-in software tracks slow changes in the environmental background. An alarm is created when during a summation time (L) of typically 4 seconds, the accumulated counts are significantly more than what is expected from the background. The alarm threshold is defined as the probability that the measured counts (N) during a period L, could have been caused by the established background rate over the same period (B). A threshold of 1.0e-4 means that we alarm when P(Counts ≥ N|BCK) < 1.0e-4.

For example, assume a summation time of 4 seconds and a background rate of 500cps for BCK=2000. Now assume that we count 2500cps in a particular 4s-period. The probability of the established background to cause 2224 counts or more in 4s is P(Counts ≥ 2224|BCK=2000) = 2.86e-7. This smaller than the alarm threshold of 1.0e-4, and the embedded program will generate an alarm.

If the alarm condition is permanent, the software resets all the logic after a period of H time slices and starts counting again. It now will accept the suddenly higher level of radioactivity as the new normal background.

Finally, a 'wait' parameter tells the system to wait a number of time slices after turn-on or reset before being ready to alarm. This is necessary so that the background will be known with sufficient accuracy.

All told, the time-slice firmware provides an unprecedented, and highly configurable, but fully autonomous alarming system for portal monitors. This is ideal for very low-cost mass-produced pedestrian monitors, hand-held sweepers and similar applications.

Performance

The SiPM-Counter includes a two-channel logger which allows to log any two parameters from the arm_status data array.

Use it to observe rapidly changing situations without having to read data from the device at high speed.

We show recorded examples of operating voltage response to required changes and alarming in response to count rate changes.

SiPM: 3.24cm²

Response to a 0.5V required change in operating voltage

Measured net count rate above background over time and alarm response as recored by the two-channel logger.

Time histogram

A time histogram is a great tool to ascertain data quality.

Easily recognize contamination from repetitive pulse sources caused by outside interference.

Compute count rates accurately from a time histogram witht assumptions about dead time.

Here we show a histogram of the time difference between successive events.

For truly random radiation pulses the probability density function of the time differences t is pdf=exp(-t/cr) where cr is the count rate.

For such radiation pulses the semi-logarithmic plot of the time histogram is a straight line.

A linear fit reveals the true count rate, independent of any instrument dead times.

SiPM: 3.24cm²; NaI(Tl): 50×50mm

A histogram of arrival time differences at 76kcps.

Portal Monitor Feature

The SiPM-Counter, just like the SiPM-Counter (PMT-1000 and SiPM-1000) has a built-in portal monitor capability.

It can evaluate count rates 10 times per second and issue an alarm when the count rate exceeds the background rate.

The unit continuously monitors the count rate and updates the measured background count rate to keep up with slow changes in the environment due to rain, wind and dust. For a portable system the background may slowly change with the terrain without causing an alarm.

The SiPM-Counter performs a statistical analysis every 100ms and alarms if the alarm probability exceeds a programmed threshold of typically 1:10k to 1:1000k (1k=1000)

It is possible to set a lower trigger threshold to focus the attention to a certain kind of radiation.

The panels on the right show the result of a walk by of 3.8µCi (140kBq) near a 50mm NaI(Tl) detector.

In one measurement we used a high trigger threshold near the Cs-137 full-energy peak, in the other we used a low threshold to allow all pulses above 30keV

In both cases the SiPM-Counter responds immediately with an alarm.

In blue we show he number of counts per 100ms time slice. In purple we show the alarm pulse, which was programmed to last at least 10 seconds.

A more detailed description can be found in the user's manual .

Alarm computed by the SiPM-Counter

Plot of Number of events per 100ms time slice and the resulting alarm over time.

Plot of Number of events per 100ms time slice and the resulting alarm over time. This is for the high trigger threshold, and the count rate is much lower.

Downloads and Pricing

Prices may change without prior notice.

For best results, we encourage customers to use our assemblies of Counter and SiPM carrier board.

For first time customers, we reserve the right to only sell the combination of Counter, SiPM carrier board and mounted in the housing.

First time customers are encouraged to buy the first system with one of our crystals so that we can present a known good unit and guarantee its performance prior to sale.

• Flyer: English,
• User's manual
• Connector pin outs
• MCA Selection Guide
• Self-extracting Windows driver installer: Version 3.0
• Software and documentation for all MCA wxMCA.zip
• On-line software documentation