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The Amptek PX4 is an interface between Amptek’s XR100 series of X-ray and gamma-ray detectors and a personal computer with data acquisition, control, and analysis software. The PX4 includes three major components: (1) a shaping amplifier, based on a state of the art, high performance, low power DP4 digital pulse processor, (2) a multichannel analyzer, and (3) power supplies. It replaces both the previous generation PX2 shaping amplifier and power supply and the separate MCA. The pulse processing and MCA function of the PX4 are based on Amptek’s DP4 digital pulse processor. The PX4 offers several performance advantages over traditional analog systems, including higher energy resolution, reduced ballistic deficit, higher throughput, better pile-up rejection, enhanced stability, and the ability to adjust shaping time parameters over a wide range to optimize performance. The PX4 includes a USB interface. The power supply portion of the PX4 provides all of the power necessary for the detector, preamplifier, and the PX4. The PX4 offers several advantages over the previous generation PX2: (1) a single unit interfaces with all XR100 variants; (2) many parameters may be adjusted to optimize performance, such as shaping time constant and HV bias; (3) the pulse processor offers enhanced baseline stability, throughput, pile-up rejection, and Rise Time Discrimination (RTD); and (4) the MCA is integrated with the complete system. The signal input to the PX4 is the preamplifier output. The PX4 digitizes the preamplifier output, applies real-time digital processing to the signal, detects the peak amplitude (digitally), and bins this value in its histogramming memory, generating an energy spectrum. The use of digital signal processing offers several important performance advantages compared to previous systems. The spectrum is then transmitted over the PX4’s USB interface to the user’s computer. The PX4 hardware is controlled over the USB interface, permitting the user not only to start and stop acquisition but to select shaping times, select the HV bias, etc. The PX4 complete pulse processing system and power supply
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Figure 2.
| Gain Settings | 28 user selectable coarse gain settings from x4 to x560. Fine gain is adjustable between 0.75 and 1.25. |
| Pulse Shape | Trapezoidal. A semi-gaussian amplifier with shaping time t has a peaking time of 2.2t and is comparable in performance with the trapezoidal shape of the same peaking time. |
| Peaking and Flat Top Times | Twenty-four programmable peaking times between 0.8 and 102 µsec. For each peak time, sixteen flat top durations are available, > 0.2 µsec |
| Rise Time Discriminator (RTD) | The digital pulse processor can be programmed to select input pulses based on their rise time properties. |
| Throughput | The pulse processing electronics have a cycle time of 1 µsec. With a peaking time of 0.8 µsec, a 1MHz periodic signal can be acquired. Dead time is 1.25 x peaking time. |
| Pile-Up Reject | Pulses separated by more than the fast channel resolving time, 600 nsec, and less than 1.25 x peaking time are rejected. |
| Number of channels | Commandable to 256, 512, 1k, 2k, 4k, or 8k channels. |
| Analog Input (BNC) | The analog input accepts pulses from the XR100 or any other detector with preamplifier reset or resistive feedback. |
| XR100 Power (6 pin LEMO) | Provides power to preamp and detector. Includes HV bias, thermoelectric cooler power, and preamp power. |
| USB and RS-232 Serial Interface | Standard USB 1.1 (USB 2.0 compatible) and RS-232 interface to personal computer. Used for data acquisition and hardware control. |
| DAC Output (BNC) | This output is used in oscilloscope mode, to view the shaped pulse and other diagnostic signals. Range: 0 to 1 V. |
| Input Power | 5 VDC (500 mA max) via power jack. It mates with a center positive 5.5 mm x 2.1 mm Power Plug. |
This is an analog output used in oscilloscope mode to view the shaped pulse and other diagnostic signals. Range: 0 to 1 V. NOTE: The DAC output will roll over if the voltage is greater or less than the permissible range. For this reason, an output offset of 100 mV or so is recommended (and may be set via software on the Misc. tab).
| Shaped | This is the most common setting and displays the shaped output, which should be trapezoidal. |
| Decimated | This displays the input to the ADC, which should be a simple exponential decay with a 3.2 µs time constant. The input to the ADC should not go below zero, and so a DC offset of 100 to 200 mV is recommended. This can be verified using the decimated output, but note the additional offset introduced by the DAC. |
| Test | Used for Amptek factory calibration and setup. |
| Fast | This shows the output of the fast channel shaper. |
This is a digital output which can be set (via software) to show various diagnostic signals. It is commonly used to gate an oscilloscope or is input to a counter. Options include:
| ICR (Incoming Count Rate) | This is the output of a discriminator at the fast threshold. It is HIGH whenever the fast channel signal exceeds its threshold. If set above the noise, it triggers on all incoming counts. |
| PILE-UP | This is the output of the pile-up indicator, i.e. a piled-up event was detected. |
| MCS Timebase | Toggles every time the MCS time base increments. |
| ONESHOT | Period during which a 2nd event would be considered piled-up. |
| DET_RES | Active-Low detector reset lockout period, i.e. indicates when a preamplifier reset is detected. |
| MCA_EN | High=MCA Enabled, Low=MCA Disabled. |
| TRIGGER | Peak detect fired. |
| SCA8 | Single Channel Analyzer output #8 |
| 1 | Temperature |
| 2 | Bias (up to 1500V) |
| 3 | -8.5 or -5 VDC |
| 4 | +8.5 or +5 VDC |
| 5 | Cooler - (grounded) |
| 6 | Cooler + |
| Ground on shield | |
| 1 | Gnd |
| 2 | RS232 - TX |
| 3 | RS232 -RX |
| 4 | SCA 6 Out |
| 5 | SCA 5 Out |
| 6 | Gnd |
| 7 | Gate (input) |
| 8 | Buffer Sel (input) |
| 9 | SCA 8 Out |
| 10 | External Power On |
| 11 | SCA 7 Out |
| 12 | SCA 1 Out |
| 13 | SCA 2 Out |
| 14 | SCA 3 Out |
| 15 | SCA 4 Out |
Digital I/O Notes:
Standard USB 1.1 type B connector.
The MCA portion of the PX4 supports 256, 512, 1024, 2048, 4096 or 8192 channels. The PX4 uses 3 bytes per channel, which allows up to 16.7M counts per channel. The MCA has two spectral buffers available, A and B, which can be selected either via software, or by a hardware signal on the Digital I/O connector (BUFFER_SEL).
The GATE input on the Digitial I/O connector can also control the MCA. It can be used with external logic to control when events are counted. It can be set active high, active low, or disabled via software. When counts are gated off, the clock is also gated off so that an accurate count rate can be determined. Amptek recommends that the gate input duration be equal to or greater than the sum of the peaking and flat top durations.
GATE and BUFFER_SEL inputs are: VIL (logic low): -0.5 to + 0.7 V, VIH (logic high): +2.4 to +5.5 V.
The PX4 contains eight (8) single channel analyzers (SCAs). Each SCA has an upper and a lower threshold. If an event occurs with a shaped output within the range defined by these thresholds, then a logic pulse is generated and is output to the I/O connector on the PX4, where it can be connected to external hardware. The upper and lower limits of the 8 SCAs can be set independently in the software. SCA8 serves a dual purpose – not only does it operate like the other SCAs, but it is also used to set the Region-of-Interest (ROI) for the "Preset Count" mode of MCA operation. That is, when a Preset Count is selected, the MCA will stop after the programmed number of counts occurs in the SCA8 ROI.
All SCA outputs on the Digital I/O connector are 50 ns wide, VOL (logic low): 0.1 V @ 100 µA, VOH (logic high): +3.1 V @ 100 µA
The PX4 is provided with the ADMCA display and acquisition software application. This software controls and sets all acquisition parameters and plots the data. It also sets all of the power supplies. This software has an integrated interface to the XRF-FP quantitative analysis software.
For user who wish to write custom applications, an Application Programming Interface (API) is provided.
Click here to download PX4 software
Figure 3. PX4 throughput for various peaking times. Taken with an Amptek XR100CR x-ray detector.
Figure 4. Complete XRF system.
PX4 Specifications in PDF
Digital Pulse Processor FAQ
PX4 FAQ
Glossary
Revised July 10, 2007
