Digitizers

Gammasphere's digitizers, as the name suggests, digitize signals from each detector. They take signals that have been conditioned in the pickoff card and routed from the collector box, and communicate with the triggers to determine which signals are worthy enough to digitize.

Digitizers Before Upgrades

The experiments at Argonne National Laboratory (Gammasphere, FMA, DSSD, HELIOS) have instrumented their detectors with digital data acquisition systems using the digitizer module developed for the GRETINA experiment. Although the hardware is the same, the differing architectures of the experiments and different detector structures necessitated completely new and unique firmware in both the digitizer and trigger modules. A generic firmware design from which specific variants for each of the Argonne Tandem Linear Accelerator System (ATLAS) experiments can be derived is described herein. It is our hope that this firmware development, where all channels are independent and equal, may be utilized in other experiments as well. As of 2018, Digital Gammasphere (DGS), Digital FMA (DFMA), HELIOS (Helical Orbit Spectrometer) plus the X-Array all use this firmware.

The digitizer module design and the original GRETINA firmware developed for it are products of Lawrence Berkeley National Laboratory (LBNL). The trigger module and its firmware are products of Argonne National Laboratory (ANL). We at Argonne thank our LBNL collaborators for sharing the source code and schematics of the digitizer module, both of which were invaluable references in this totally new firmware development.

Data from the ADCs of the digitizer is signed 14-bit values that arrive at 100MHz (10ns period). Each digitizer module implements ten channels that implement independent input and energy integration blocks. The purpose of the input and energy integration firmware block, implemented as a pipeline, is to receive the data, internally trigger on edges of signals and calculate the energy of the input signal. Following each energy integrator is a pending event queue and event acceptance logic block that places accepted events into channel-specific FIFO buffers. At the back end, a channel readout machine collects all data from the channels and stuffs accepted events into a large external readout FIFO, accessible over the VME backplane.

Two versions of firmware have been developed, a Master build and a Slave build. These operate identically in virtually all ways save that the Master build directly interfaces to the trigger system and the Slave build receives its clock and control signals from the trigger over a front panel cable driven by the Master. The bulk of this "Beginner Guide" describes the Master design.

Hardware Interface Notes

While this is a firmware manual, some general reference information regarding how signals are connected to the digitizer is in order. The detector system this firmware was designed for is named Gammasphere. The Gammasphere consists of many (up to 110) individual detector modules. Each of the individual modules contains a central piece of high-purity Germanium (HPGe) surrounded on the sides by six Bismuth Germanate (BGO) shield scintillators and at the back by a 7th BGO ‘backplug’. Each detector module contains its own preamplifier electronics for the HPGe and BGO sections that create electrical signals in response to interactions with gamma-rays. Each detector module also contains a liquid nitrogen dewar with volume (~2L) sufficient to keep the detection elements cold for hours.

Interface to Gammasphere Detectors

The signals from detectors are processed through external analog interface modules before connection to the digitizer. In the case of Digital Gammasphere ‘pickoff cards’ differentiate the single-ended input Ge Center signal to remove the long ramp and reset associated with the transistor-reset preamplifiers. As of 2021, the Gammasphere hardware is being upgraded. The original control/monitoring/power system developed in the 1990s, and the ‘pickoff cards’ designed in the early 2000s, plus all cabling are being replaced. The Slope Box Extension is a new system that provides all the features originally envisioned for the ‘pickoff’ cards that were not implemented in the original design, plus completely replaces the old control/monitoring/power system. As of August 2021, approximately 17% of Gammasphere has been converted to Slope Box Extension hardware.

Irrespective of old system or new, the signals are all driven differentially into the digitizer with a variable DC offset. The signal applied to all digitizer channels is unipolar with the dominant time constant defined by the RC timing of the channel. The time constant of the pickoff card for the Ge Center channel is nominally 50us; detectors with Slope Box Extension systems have programmable time constants. The BGO and Ge Side signals are natively unipolar as driven by the detector and so are simply amplified and converted to differential; the time constants of the BGO and Ge Side signals are similar to those of the Ge Center channel. Typically the differential signals are offset by approximately 1.5V so that the un-driven signal rests near but not at the one rail of the digitizer module’s input range (i.e. around ADC channel 2000 for Ge Center signals). This allows the maximum dynamic range for the unipolar signals. The fourth signal from each detector is called the BGO pattern. This last signal can be used to monitor the individual BGO tubes that form the BGO sum signal and can also monitor the Ge Center at a different gain for calibration purposes. It also has a mode in which the signal is driven by seven comparators, each with a fixed and binary-weighted analog swing, to provide a mix of tube activity indicator and time-over-threshold energy information from all 7 BGO channels simultaneously. Please refer to The Slope Box Extension documentation for additional information on how these signals are formed.

For more information on digitizer firmware see Advanced User Guides

Go back to Digital Gammasphere Upgrade Project