XIA Pixie-4 Express User manual

User's Manual
Digital Gamma Finder (DGF)
Pixie-4
Version 2.69, November 2015
XIA LLC
31057 Genstar Road
Hayward, CA 94544 USA
Phone: [phone]; Fax: [phone]
http://www.xia.com
Disclaimer
Information furnished by XIA is believed to be accurate and reliable. However, XIA assumes
no responsibility for its use, or for any infringement of patents, or other rights of third parties,
which may result from its use. No license is granted by implication or otherwise under the
patent rights of XIA. XIA reserves the right to change the DGF product, its documentation,
and the supporting software without prior notice.

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Table of Contents
1 Overview.............................................................................................................................................3
1.1 Features........................................................................................................................................3
1.2 Specifications...............................................................................................................................4
2 Setting Up............................................................................................................................................ 5
2.1 Installation....................................................................................................................................5
2.2 Getting Started.............................................................................................................................6
3 Navigating the Pixie Viewer..............................................................................................................10
3.1 Overview....................................................................................................................................10
3.2 Setup Group............................................................................................................................... 11
3.3 Run Control Group.....................................................................................................................16
3.4 Results Group.............................................................................................................................16
3.5 Optimizing Parameters...............................................................................................................18
3.6 File Series...................................................................................................................................20
4 Data Runs and Data Structures..........................................................................................................24
4.1 Run Types...................................................................................................................................24
4.2 Output Data Structures...............................................................................................................26
5 Hardware Description........................................................................................................................ 32
5.1 Analog Signal Conditioning.......................................................................................................32
5.2 Real-time Processing Units........................................................................................................33
5.3 Digital Signal Processor (DSP)..................................................................................................33
5.4 PCI Interface..............................................................................................................................34
6 Theory of Operation..........................................................................................................................35
6.1 Digital Filters for -ray Detectors ..............................................................................................35
6.2 Trapezoidal Filtering in the Pixie-4........................................................................................... 37
6.3 Baselines and Preamplifier Decay Times ..................................................................................38
6.4 Thresholds and Pile-up Inspection............................................................................................. 39
6.5 Filter Range................................................................................................................................42
6.6 Dead Time and Run Statistics....................................................................................................42
7 Operating Multiple Pixie-4 Modules Synchronously........................................................................ 55
7.1 Clock Distribution .....................................................................................................................55
7.2 Trigger Distribution....................................................................................................................57
7.3 Run Synchronization .................................................................................................................60
7.4 External Gate and Veto (GFLT) .................................................................................................61
7.5 External Status ...........................................................................................................................63
7.6 Coincident Events......................................................................................................................64
8 Using Pixie-4 Modules with Clover detectors...................................................................................68
9 Troubleshooting.................................................................................................................................69
9.1 Startup Problems........................................................................................................................69
9.2 Acquisition Problems................................................................................................................. 71
10 Appendix A...................................................................................................................................... 73
10.1 Front end jumpers for termination and attenuation.................................................................73
10.2 Clock Jumpers.........................................................................................................................74
10.3 PXI backplane pin functions...................................................................................................75
10.4 Control and Status Register Bits.............................................................................................76

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1 Overview
The Digital Gamma Finder (DGF) family of digital pulse processors features unique
capabilities for measuring both the amplitude and shape of pulses in nuclear spectroscopy
applications. The DGF architecture was originally developed for use with arrays of multi-
segmented HPGe gamma-ray detectors, but has since been applied to an ever broadening range
of applications.
The DGF Pixie-4 is a 4-channel all-digital waveform acquisition and spectrometer card based
on the CompactPCI/PXI standard for fast data readout to the host. It combines spectroscopy
with waveform capture and on-line pulse shape analysis. The Pixie-4 accepts signals from
virtually any radiation detector. Incoming signals are digitized by 14-bit 75 MSPS ADCs.
Waveforms of up to 13.6 μs in length for each event can be stored in a FIFO. The waveforms
are available for onboard pulse shape analysis, which can be customized by adding user
functions to the core processing software. Waveforms, timestamps, and the results of the pulse
shape analysis can be read out by the host system for further off-line processing. Pulse heights
are calculated to 16-bit precision and can be binned into spectra with up to 32K channels. The
Pixie-4 supports coincidence spectroscopy and can recognize complex hit patterns.
Data readout rates through the CompactPCI/PXI backplane to the host computer can be over
100Mbytes/s. The PXI backplane is also used to distribute clocks and trigger signals between
several Pixie-4 modules for group operation. With a large variety of CompactPCI/PXI
processor, controller or I/O modules being commercially available, complete data acquisition
and processing systems can be built in a small form factor.
1.1 Features
•Designed for high precision γ-ray spectroscopy with HPGe detectors.
•Directly compatible with scintillator/PMT combinations: NaI, CsI, BGO, and many
others.
•Simultaneous amplitude measurement and pulse shape analysis for each channel.
•Input signal decay time: as fast as 150ns and up to 10ms, exponentially decaying.
•Wide range of filter rise times: from 53ns to 109μs, equivalent to 27ns to 50μs
shaping times.
•Programmable gain and input offset.
•Excellent pileup inspection: double pulse resolution of 50 ns. Programmable pileup
inspection criteria include trigger filter parameters, threshold, and rejection criteria.
•Digital oscilloscope and FFT for health-of-system analysis.
•Triggered synchronous waveform acquisition across channels, modules and crates.
•Dead times as low as 1 s per event are achievable (limited by DSP algorithm
complexity). Events at even shorter time intervals can be extracted via off-line ADC
waveform analysis.
•Digital constant fraction algorithm measures event arrival times down to a few ns
accuracy.
•Supports 32-bit 33 MHz PCI data transfers (>100 Mbytes/second).

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1.2 Specifications
Front Panel I/O
Signal Input (4x)
4 analog inputs. Selectable input impedance: 50Ω and 5kΩ, ±5V pulsed,
±2V DC. Selectable input attenuation 1:7.5 and 1:1 for 50Ω setting.
Logic Input/Output
General Purpose I/O connected to programmable logic (Rev. C, D only).
Currently can be either used as input for global backplane signals Veto or
Status, as an input for module specific logic level reported in the data
stream, or as an external trigger.
Logic Output
General Purpose output from Digital Signal Processor. Function to be
determined.
Backplane I/O
Clock Input/Output
Distributed 37.5 MHz clock on PXI backplane.
Triggers
Two wired-or trigger buses on PXI backplane. One for synchronous
waveform acquisition, one for event triggers.
Status
Global logic level from backplane reported for each event
Token
Global logic level from backplane used for coincidence tests
Synch
Wired-or SYNC signal distributed through PXI backplane to synchronize
timers and run start/stop to 50ns.
Veto
Global logic level to suppress event triggering.
Channel Gate
Individual GATE to suppress event triggering for each channel with use of
PXI-PDM (Rev. D only)
Data Interface
PCI
32-bit, 33MHz Read/Write, memory readout rate to host
over 100 Mbytes/s.
Digital Controls
Gain
Analog switched gain from 0.97 to 11.25 in max. 10% steps.
Digital gain adjustment of up to ±10% in 15ppm steps.
Offset
DC offset adjustment from –2.5V to +2.5V, in 65535 steps.
Shaping
Digital trapezoidal filter. Rise time and flat top set independently: 53ns –
109μs in small steps.
Trigger
Digital trapezoidal trigger filter with adjustable threshold. Rise time and
flat top set independently from 26ns to 853ns.
Data Outputs
Spectrum
1024-32768 channels, 32 bit deep (4.2 billion counts/bin).
Additional memory for sum spectrum for clover detectors.
Statistics
Real time, live time, filter and gate dead time, input and throughput
counts.
Event data
Pulse height (energy), timestamps, pulse shape analysis results,
waveform data and ancillary data like hit patterns.

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2 Setting Up
2.1 Installation
2.1.1 Hardware Setup
The Pixie-4 modules can be operated in any standard 3U CompactPCI or PXI chassis. Chassis
following only the CompactPCI standard can be used to operate modules individually. To
operate several modules together, a backplane following the PXI standard must be present. Put
the host computer (or remote controller) in the system slot of your chassis. Place the Pixie-4
modules into any free slots with the chassis still powered down, then power up the chassis
(Pixie-4 modules are not hot swappable). If using a remote controller, be sure to boot the host
computer after powering up the chassis
1
.
2.1.2 Drivers and Software
System Requirements: The Pixie software is compatible with Windows XP, Vista, or
Windows 7. Restrictions apply to the 64 bit version of Windows 7
2
. Please contact XIA for
details on operating Pixie-4 modules with Linux.
When the host computer is powered up the first time after installing the controller and Pixie-4
modules in the chassis, it will detect new hardware and try to find drivers for it. (A Pixie-4
module will be detected as a new device every time it is installed in a new slot.) While there
is no required order of installation of the driver software, the following sequence is
recommended (users with embedded host computer skip to step 4):
1. If you have a remote controller, first install the driver software for the controller itself.
Otherwise, skip to step 4.
Unless directed otherwise by the manufacturer of the controller, this can be done with
or without the controller and Pixie-4 modules installed in the host computer and/or
chassis. If the modules are installed, ignore attempts by Windows to install drivers until
step 7.
NI controllers come with a multi-CD package called “Device Driver Reference CD”.
For simplicity it is recommended to install the software on these CDs in the default
configuration.
2. Unless already installed, power down the host computer, install the controller in both
the host computer and chassis, and power up the system again (chassis first).
3. Windows will detect new hardware (the controller) and should find the drivers
automatically. Verify in Window’s device manager that the controller is properly
installed and has no “resource conflicts”.
4. Install Igor Pro
1
In some systems, “scan for hardware changes” in the Windows device manager may detect and install a
remote chassis when the PC was booted first.
2
At the time of writing, these restrictions are: National Instrument's MXI-3 and MXI-4 controllers do not
support Windows 7 (64 bit) on systems with more than 4 GB RAM. Embedded PC controllers and the “MXI
Express” series of controller bridges appear to be fully compatible with Windows 7 (64 bit).

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5. Install the Pixie-4 software provided by XIA (see section 2.2.3)
6. Unless already installed, power down the host computer and install the Pixie-4 modules
in the chassis. Check the input jumper settings for the appropriate signal termination:
50 Ωor 5 kΩ(see section 10.1 for details). Then power up the system again (chassis
first).
7. Windows will detect new hardware (the Pixie-4 modules) and should find the drivers
automatically. If not, direct it to the “drivers” directory in the Pixie-4 software
distribution installed in step 5. Verify in Window’s device manager that the modules
are properly installed as “PLX Custom (OEM) PCI 9054 Boards (32)” or “... (64)” and
have no “resource conflicts”. Currently, the driver must be version 6.5.0.
3
The
previously used driver version 4.1 will identify the modules as “Custom (OEM) PCI
9054 Boards” without the “PLX”
2.1.3 Pixie User Interface
The Pixie Viewer, XIA’s graphical user interface to set up and run the Pixie-4 modules, is based
on WaveMetrics’ IGOR Pro. To run the Pixie Viewer, you have to have IGOR Version 5.0 or
higher installed on your computer. By default, IGOR Pro will be installed at C:\Program
Files\WaveMetrics\IGOR Pro Folder.
The CD-ROM with the Pixie-4 software distribution contains
1. an installation program Setup.exe,
2. the Pixie-4 software in the folder XIA\Pixie4 and its subfolders.
The Pixie-4 software can be installed by running its installation program. Follow the
instructions shown on the screen to install the software to the default folder selected by the
installation program, or to a custom folder. This folder will contain the IGOR control program
(Pixie4.pxp), online help files and 8 subfolders (Configuration, Doc, Drivers, DSP, Firmware,
MCA, PixieClib, and PulseShape). Make sure you keep this folder organization intact, as the
IGOR program and future updates rely on this. Feel free, however, to add folders and
subfolders at your convenience.
For the latest version of the Pixie Viewer software, go to support.xia.com and search for “Pixie
release”.
2.2 Getting Started
To start the Pixie Viewer, double-click on the file “Pixie4.pxp” in the installation folder. After
IGOR loaded the Pixie Viewer, the START UP
4
panel should be prominently displayed in the
middle of the desktop.
In the panel, first select the chassis type and number of Pixie-4 modules in the system. Then
specify the slot number in which each module resides.
3
For information on using the older PLX drivers (version 6.3.1) with Windows 2000, see the “readme” file in
the Drivers folder of the software distribution.
4
In the following, SMALL CAPS are used for panel names; italic font is used for buttons and controls.

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Click on the Start Up System button to initialize the modules. This will download DSP code
and FPGAconfiguration to the modules, as well as the module parameters. If you see messages
similar to “Module 0 in slot 5 started up successfully!” in the IGOR history window, the Pixie-
4 modules have been initialized successfully. Otherwise, refer to the troubleshooting section
for possible solutions. If you want to try the software without a chassis or modules attached,
click on Offline Analysis.
Figure 2.1: The Pixie-4 START UPpanel (above) and MAIN Panel (right)
After the system is initialized successfully, you will see the MAIN
control panel that serves as a shortcut to the most common actions and
from which all otherpanels are called. Its controls areorganized in three
groups: Setup, Run Control, and Results.
In the Setup group, the Start System button opens the START UPpanel in
case you need to reboot the modules. The Open Panels popup menu
leads to four panels where parameters and acquisition options are
entered. They are described in more detail in section 3 and in the online
help. To get started, select Parameter Setup, which will open (or bring
to front) the PARAMETER SETUP panel shown in Figure 2.2. For most of
the actions the Pixie Viewer interacts with one Pixie module at a time.
The number of that module is displayed at the top of the MAIN panel
and the top right of the PARAMETER SETUP panel. Proceed with the steps below to configure
your system.
Note: The More/Less button next to the Help button on the bottom of the PARAMETER SETUP
panel can be used to hide some controls. This may be helpful to first-time Pixie users who only
want to focus on the most essential settings.

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For an initial setup, go through the following steps:
Figure 2.2: The PARAMETER SETUP Panel, Energy tab shown
1. If not already visible, open the PARAMETER SETUP panel by selecting Parameter Setup from
the Open Panel popup menu in the MAIN panel.
2. At the bottom of the PARAMETER SETUP panel, click on the Oscilloscope button. This
opens a graph that shows the untriggered signal input.
In the OSCILLOSCOPE panel, click Refresh to update the display. The pulses should fall
in the display range (0-16K). If no pulses are visible or if they are cut off at the upper
or lower range of the display, click Adjust Offsets to automatically set the DC offset. If
the pulse amplitude is too large to fall in the display range, decrease the Gain. If the
pulses are negative, toggle the Invert checkbox.

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Figure 2.3: OSCILLOSCOPE panel with typical pulses from a pulser.
3. In the Energy tab of the PARAMETER SETUP panel, input an estimated preamplifier
exponential RC decay time for Tau, and then click on Auto Find Tau to determine the
actual Tau value for all channels of the current module. You can also enter a known
good Tau value directly in the Tau control field, or use the controls in the OSCILLOSCOPE
to manually fit Tau for a pulse.
4. Save the modified parameter settings to file. To do so, click on the Save button at the
bottom of the PARAMETER SETUP panel to open a save file dialog. Create a new file
name to avoid overwriting the default settings file.
5. Save the Igor experiment using File -> Save Experiment As from the top menu. This
saves the current state of the interface with all open panels and the settings for file paths
and slot numbers (the settings independent of module parameters).
6. Click on the Run Control tab, set Run Type to “0x301 MCA Mode”, Poll time to 1
second, and Run time to 30 seconds or so, then click on the Start Run button. Aspinning
wheel will appear occasionally in the lower left corner of the screen as long as the
system is waiting for the run to finish. If you click the Update button in the MAIN panel,
the count rates displayed in the Results group are updated.
7. After the run is complete, select MCA Spectrum from the Open Panels popup menu in
the Results group of the MAIN panel. The MCA SPECTRUM graph shows the MCA
histograms for all four channels. You can deselect other channels while working on
only one channel. After defining a range in the spectrum with the cursors and setting
the fit option to fit peaks between cursors, you can apply a Gauss fit to the spectrum by
selecting the channels to be fit in the Fit popup menu. You can alternatively enter the
fit limits using the Min and Max fields in the table or by specifying a Range around the
tallest peak or the peak with the highest energy. To scale the spectrum in keV, enter the
appropriate ratio in the field keV/bin.

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At this stage, you may not be able to get a spectrum with good energy resolutions. You may
need to adjust some settings such as energy filter rise time and flat top as described in section
3.5.
3 Navigating the Pixie Viewer
3.1 Overview
The Pixie Viewer consists of a number of graphs and control panels, linked together by the
MAIN control panel. The Viewer comes up in exactly the same state as it was when last saved
to file using File->Save Experiment. This preserves settings such as the file paths and the slot
numbers entered in the START UPpanel. However, the Pixie module itself loses all
programming when it is switched off. When the Pixie module is switched on again, all
programmable components need code and configuration files to be downloaded to the module.
Clicking on the Start Up System button in the START UPpanel performs this download. Below
we describe the concepts and principles of using the Pixie Viewer. Detailed information on the
individual controls can be found in the Online Help for each panel. The operating concepts are
described in sections 4-7 .
The controls in the MAIN control panel are organized in three groups: Setup, Run Control, and
Results. In the Setup and Results groups, popup menus lead to the panels and graphs indicated
in Figure 3.1.

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Figure 3.1: Block diagram of the major panels in the Pixie Viewer. Numbers in brackets point to
the corresponding section in the usermanual.All panels are described in detail in the online help.
3.2 Setup Group
In the setup group, there is a button to open the START UPpanel, which is used to boot the
modules. The Open Panels popup menu leads to one of the following panels: PARAMETER
SETUP, OSCILLOSCOPE, CHASSIS SETUP, FILES/PATHS
3.2.1 Parameter Setup Panel
The PARAMETER SETUP panel is divided into 7 tabs, summarized below. Settings for all four
channels of a module are shown in the same tab. At the upper right is a control to select the
module to address. At the bottom of the panel is a More button, which will make all advanced
panel controls visible as well.
The Pixie-4 being a digital system, all parameter settings are stored in a settings file. This file
is separate from the Igor experiment file, to allow saving and restoring different settings for
different detectors and applications. Parameter files are saved and loaded with the
corresponding buttons at the bottom of the PARAMETER SETUP panel. After loading a settings
file, the settings are automatically downloaded to the module. At module initialization, the
settings are automatically read and applied to the Pixie module from the last saved settings file.

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In addition there are buttons to copy settings between channels and modules, and to extract
settings from a settings file. Two large buttons at the lower left duplicate the buttons to call the
START UPpanel and the OSCILLOSCOPE.
3.2.1.1 Trigger Tab
The Trigger tab contains controls to set the trigger filter parameters and the trigger threshold,
together with checkboxes to enable or disable trigger, to control trigger distribution (see section
7.2.1), and to set time stamping options for each channel. Except for the threshold, the trigger
settings have rarely to be changed from their default values.
The threshold value corresponds to ¼ of the pulse height in ADC steps, e.g. with a threshold
of 20, triggers are issued for pulses above 80 ADC steps. This relation is true if the trigger filter
rise time is large compared to the pulse rise time and small compared to the pulse decay time.
Apulse shape not meeting these conditions has the effect of raising the effective threshold. For
a modeled behavior of the trigger, you can open displays from the OSCILLOSCOPE and the LIST
MODE TRACES panels that show trigger filter and threshold computed from acquired
waveforms using the current settings. The threshold value is scaled with the trigger filter rise
time, therefore it is not limited to integer numbers.
Figure 3.2: The Trigger tab of the PARAMETER SETUP panel.
3.2.1.2 Energy Tab
The Energy tab contains the settings for the energy filter and the subsequent computation.
These settings aremost important for obtaining the best possible energy resolution with a Pixie-
4 system. The energy filter rise time (or peaking time) essentially sets the tradeoff between
throughput and resolution: longer filter rise times generally improve the resolution (up to a
certain optimum) but reduce the throughput because more time is required to measure each
pulse. The pulse decay time Tau is used to compensate for the decay of a previous pulse in the
computation of the pulse height. You can enter a known good value, or click on Auto Find Tau
to let the Pixie-4 determine the best value.

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The advanced controls in this tab contain functions to modify the energy computation and to
acquire a series of measurements with varying filter settings and decay times to find the best
settings. For a detailed description of the filter operation, see section 6.
Figure 3.3: The Energy tab of the PARAMETER SETUP Panel.
3.2.1.3 Waveform Tab
The Waveform tab contains the controls to set the length and pre-trigger delay of the waveforms
to be acquired. Advanced options include parameters for online pulse shape analysis
3.2.1.4 Gate Tab
The Gate tab contains the controls to set the window for gating acquisition with external
signals. We distinguish
-GATE, a dedicated signal for each individual channel. It is active for the rising edge of
the pulse e.g. to suppress a detector pulse with a coincident pulse from a BGO shield.
-VETO, a signal distributed to all modules and channels, but each channel is
individually enabled to require or ignore this signal. VETO is active during the
validation of a pulse (after pileup inspection), an energy filter rise time plus flat top
after the rising edge. With suitable external logic, the decision to veto a pulse can be
made from information obtained at the rising edge of the pulse (e.g. multiplicity from
several channels) and therefore this function is also called Global First Level Trigger
(GFLT).
For a detailed description of the GATE and VETO operation, see section 7.4.
3.2.1.5 Coincidence Tab
The Coincidence tab contains the controls to set the acceptable hit pattern, and the coincidence
window after validation during which channels can contribute to the hit pattern. There is a
checkbox for each possible hit pattern. For example, if the checkbox with pattern 0100 is
checked, events with a hit in channel 2 and no others are accepted. Selecting multiple
checkboxes accepts combinations of hit patterns, e.g. any event with exactly one channel hit.

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For a detailed description of the coincidence operation, see section 7.2.1. Controls for
coincidences between modules are located in the CHASSIS SETUP Panel and described in section
7.2.2.
3.2.1.6 Advanced Tab
The Advanced tab contains the controls for modifying the pileup inspection, histogram
accumulation, and baseline measurements.
3.2.1.7 Run Control Tab
The Run Control tab defines the settings for data acquisition. The “Run Type” popup menu
selects MCA or list mode runs, see section 4 for a detailed description. In addition, there are
controls
-to set the run time (length of data acquisition as measured by Igor),
-to set the polling time (period for checking if list mode data is available for readout
and/or run time is reached),
-to specify the data file name (a base name plus 4-digit run number that can be made to
increment automatically), and
-to specify the number of spills in list mode runs. (In list mode runs, data is accumulated
in on-board memory until full, at which time it is read out by the host PC. We call each
such readout a spill. The number of spills thus sets the amount of data to collect.)
The Start Run and Stop Run buttons from the MAIN control panel are duplicated here as well.
Advanced options include settings for synchronizing acquisition between modules, controls to
set a timeout for each spill, the number of events per spill, and the spill readout mode, and a
button to open a panel with advance record options.
Figure 3.4: The Run Control tab of the PARAMETER SETUP Panel.

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3.2.2 Oscilloscope
As mentioned in section 2.3, the OSCILLOSCOPE (Figure 2.3) is used to view untriggered
traces as they appear at theADC input and to set all parameters relating to the analog gain
and offset. There are controls titled
-dT [us], which sets the time between samples in the oscilloscope (there are always
8192 samples in the oscilloscope window),
-Offset [%], which sets the target DC-offset level for automatic adjustment,
-Gain (V/V), which sets the analog gain before digitization, and
-Offset (V), which directly sets the offset voltage.
The traces from different channels are not acquired synchronously but one after the other.
Therefore even if coincident signals are connected to the Pixie-4 inputs, the OSCILLOSCOPE
will show unrelated pulses for each channel.
There are also buttons and controls to
-open a display of the FFT of the input signal, which is useful to detect noise sources
-open a display of the waveforms of the trigger filter and energy filter computed from
the traces in the oscilloscope
-repeat the action of the Refresh button until a pulse is captured. This is useful for low
count rates.
-Fit the pulses in the OSCILLOSCOPE with an exponential decay function to determine
the decay time Tau, and to accept the fit value for the module settings.
-View the current input count rate and the current fraction of time the signal is out of
range. These values are updated in the DSP every ~2-3ms if a run is in progress or
not. Their precision is in the order of 5-10%, or 50 cps.
3.2.3 Chassis Setup
The CHASSIS SETUP panel is used to set parameters that affect the system as a whole. Examples
are trigger distribution between modules, coincidence settings between modules, and the
operation of the Pixie-4’s front panel input. See sections 7.2.2 and 7.6.2 for details.
3.2.4 Files/Paths
The firmware files, DSP files and settings files are defined in the FILES/PATHS panel. Changes
will take effect at the next reboot, e.g. when clicking the Start Up System button in this panel
or in the START UPpanel. There is also a button to set the files and paths to the default, relative
to the “home path” of the file Pixie.pxp.

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Figure 3.5: The FILES/PATHS Panel.
3.3 Run Control Group
The Run Control group in the MAIN control panel has the most essential controls to start and
stop runs, and to define or monitor the run time and the number of spills. For more options,
use the Run Control tab of the PARAMETER SETUP panel.
3.4 Results Group
The Results group of the MAIN control panel displays the count rates of the current or most
recent run. Click Update to refresh these numbers.
The popup menu Open Panels leads to panels to view the output data from the data acquisition
in detail. These panels are the MCA SPECTRUM display, the LIST MODE TRACES display, the
LIST MODE SPECTRUM display, the RUN STATISTICS, and a panel to display results from a series
of files.

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3.4.1 MCA Spectrum
Figure 3.6: The MCASPECTRUM display.
The MCA SPECTRUM display shows the spectra accumulated in on-board memory or from a
.mca file saved at the end of a run. Spectrum analysis is limited to fitting peaks with a Gaussian
and computing the peak resolution. There are several options to define the fit range, as
described in the online help. Spectra can be saved as text files for import into other applications.
3.4.2 List Mode Traces and List Mode Spectrum
Figure 3.7: The LIST MODE TRACES display.

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The LIST MODE TRACES display shows the data from the binary list mode files (.bin). If
waveforms were collected, they are shown in the graph section of the panel. Event and channel
header information –energy, time stamps, and hit patterns as described in section 4.1.2 –are
shown in the fields above the graph section. After specifying a data file with the Find button,
you can select an event to view by entering its number in the Event Number field.
The button Digital Filters opens a new plot that shows the response of the trigger filter and
energy filter computed from the list mode waveforms. This plot is more precise than the related
graph opened from the OSCILLOSCOPE since it uses the same full rate (13.3ns) data as the filters
implemented in the module, not the reduced rate sampled at the OSCILLOSCOPE’SdT. However,
unless long list mode traces are acquired or energy filters are short, there may not be sufficient
data to compute the energy filter properly.
The LIST MODE SPECTRUM display is a plot similar to the MCASPECTRUM, but it is computed
from the energies saved in the list mode data file. Since energies are stored there in full 16 bit
precision, binning can be made finer than in the MCASPECTRUM, which is limited to 32K bins.
See the online help for a detailed description of the controls.
3.4.3 Run Statistics
Figure 3.8: The RUN STATISTICS panel.
The RUN STATISTICS panel shows the live times and count rates measured by the Pixie-4. The
numbers can be updated by clicking the Update button and read from or save to Files. For a
detailed description of the definition of these values, see section 6.6.
3.4.4 File Series
See section 3.6 for a more detailed description

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3.5 Optimizing Parameters
Optimization of the Pixie-4’s run parameters for best resolution depends on the individual
systems and usually requires some degree of experimentation. The Pixie Viewer includes
several diagnostic tools and settings options to assist the user, as described below.
3.5.1 Noise
For a quick analysis of the electronic noise in the system, you can view a Fourier transform of
the incoming signal by selecting OSCILLOSCOPE →FFT. The graph shows the FFT of the
untriggered input sigal of the OSCILLOSCOPE. By adjusting the dT control in the OSCILLOSCOPE
and clicking the Refresh button, you can investigate different frequency ranges. For best
results, remove any source from the detector and only regard traces without actual events. If
you find sharp lines in the 10 kHz to 1 MHz region you may need to find the cause for this and
remove it. If you click on the Apply Filter button, you can see the effect of the energy filter
simulated on the noise spectrum.
3.5.2 Energy Filter Parameters
The main parameter to optimize energy resolution is the energy filter rise time. Generally,
longer rise times result in better resolution, but reduce the throughput. Optimization should
begin with scanning the rise time through the available range. Try 2s, 4s, 8s, 11.2s, take
a run of 60s or so for each and note changes in energy resolution. Then fine tune the rise time.
The flat top usually needs only small adjustments.For a typical coaxial Ge-detector we suggest
to use a flat top of 1.2s. For a small detector (20% efficiency) a flat top of 0.8s is a good
choice. For larger detectors flat tops of 1.2s and 1.6s will be more appropriate. In general
the flat top needs to be wide enough to accommodate the longest typical signal rise time from
the detector. It then needs to be wider by one filter clock cycle than that minimum, but at least
3 filter clock cycles. Note that a filter clock cycle ranges from 0.026 to 0.853s, depending on
the filter range, so that it is not possible to have a very short flat top together with a very long
filter rise time.
The Pixie Viewer provides a tool which automatically scans all possible combinations of
energy filter rise time and flat top and finds the combination that gives the best energy
resolution. This tool can be accessed by clicking the Optimize button on the Settings tab. Please
refer to the Online Help documentation for more details. A second option is to create a file
series where the energy filter parameters are modified for each file in the series. See section
3.6 for more details.
3.5.3 Threshold and Trigger Filter Parameters
In general, the trigger threshold should be set as low as possible for best resolution. If too low,
the input count rate will go up dramatically and “noise peaks” will appear at the low energy
end of the spectrum. If the threshold is too high, especially at high count rates, low energy
events below the threshold can pass the pile-up inspector and pile up with larger events. This
increases the measured energy and thus leads to exponential tails on the (ideally Gaussian)
peaks in the spectrum. Ideally, the threshold should be set such that the noise peaks just
disappear.

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The settings of the trigger filter have only minor effect on the resolution. However, changing
the trigger conditions might have some effect on certain undesirable peak shapes. A longer
trigger rise time allows the threshold to be lowered more, since the noise is averaged over
longer periods. This can help to remove tails on the peaks. A long trigger flat top will help to
trigger better on slow rising pulses and thus result in a sharper cut off at the threshold in the
spectrum.
3.5.4 Decay Time
The preamplifier decay time  is used to correct the energy of a pulse sitting on the falling
slope of a previous pulse. The calculations assume a simple exponential decay with one decay
constant. A precise value of  is especially important at high count rates where pulses overlap
more frequently. If  is off the optimum, peaks in the spectrum will broaden, and if  is very
wrong, the spectrum will be significantly blurred.
The first and usually sufficiently precise estimate of  can be obtained from the Auto Find
routine in the Energy tab of the PARAMETER SETUP panel. Measure the decay time several times
and settle on the average value.
Fine tuning of  can be achieved by exploring small variations around the fit value (±2-3%).
This is best done at high count rates, as the effect on the resolution is more pronounced. The
value of  found through this way is also valid for low count rates. Manually enter  , take a
short run, and note the value of  that gives the best resolution.
Pixie users can also use the fit routines in the OSCILLOSCOPE to manually find the decay time
through exponentially fitting the untriggered input signals. Another tool is the Optimize routine
in the Energy tab of the PARAMETER SETUP panel. Similar to the routine for finding the optimal
energy filter times, this routine can be used to automatically scan a range of decay times and
find the optimal one. Please refer to the Online Help documentation for more details. Afurther
option is to create a file series where  is modified for each file in the series. See section 3.6
for more details.
3.6 File Series
3.6.1 File Series to break up long data acquisition runs
When taking long data acquisitions, it may be beneficial to break up the run into smaller sub
runs. This helps to save data in case of power failure or system crashes, since only the most
recent sub run is lost. Also list mode files tend to get large and unwieldy for analysis in longer
runs.
The Pixie Viewer thus has a method to create a series of files at specified intervals. In the DATA
RECORD OPTIONS panel, opened with the Record button in the Run Control tab of the
PARAMETER SETUP panel, there is a checkbox named New files every, followed by a control
field to enter a spill (or time) interval N. If checked and a run is started, every N spills (or, in
MCA runs, every N seconds) the data file is closed, spectra, settings and statistics are saved,
and then a new run is started. This is equivalent to manually clicking first the Stop Run button
and then the Start Run button. It is recommended to enable the automatic increment and auto-
store options as shown in Figure 3.9 as well.