digibase 0.3.4

Interface to ORTEC/AMETEK digital PMT base / Multichannel Scaler

digiBase

Python interface to ORTEC/AMETEK digital MCA PMT base, the digiBASE.

The digiBASE plugs into JEDEC B14-38 sockets for 10-dynode-stage, 14-pin photomultiplier tubes. It provides the high voltage bias (50 - 1200V) and digitizes phototube pulses with a 10-bit ADC in one of two modes of operation:

• Pulseheight analysis (PHA) mode, where PMT pulses are binned into a 1024 element histogram, much like classic multichannel analyzers (MCAs);
• In list mode acquisition PMT pulses are available as an event-by-event list, each with a coarse 1 us timestamp

This is a 100% Python rewrite of C library interfaces to the digiBASE:

• libdbaserh
• libdbase

If you don't want to use the AMETEK Connections library, or can't, you may find this useful. I wrote this interface because I needed to create a Raspberry Pi data acquisition system for use in the field, outside the laboratory environment. The functionality is basic but currently supports:

• HV programming
• HV readback (version 0.3+)
• ADC fine gain setting
• Livetime / realtime setting and reading
• Lower-level discriminator set
• EXT gate OFF, ENABLED, COINCIDENCE modes
• PHA mode
• List-mode acquisition

It connects to the digiBASE over the USB bus and supports devices with USB vendor ID = 0x0a2d and product ID = 0x000f (digiBASE) or 0x001f (digiBASE-RH), which present slightly different communication interfaces.

Dependencies

Python 3.8+ is required. Additionally, it depends on the following packages:

• pyusb
• NumPy
• pytest (optional, to run unit tests)

Installation

Digibase is now a Python package on PyPI and is most easily installed using pip:

$ pip install digibase

which should install the project and all dependencies.

DigiBase Firmware

Device firmware must be loaded at power up. ORTEC distributes this firmware with their MAESTRO and Connection software. You may find this in the distribution media or in the install directories of the aforementioned software, in \WINDOWS, or in \WINDOWS\SYSTEM32 with filename digiBase.rbf or digiBaseRH.rbf. Place this file in the current working directory or set the environment variable DBASE_FIRMWARE to point to the file.

USB Device Permission (Linux)

Under Linux the module may fail to open the USB device due to lack of permission. While it's always possible to sudo chown 666 /dev/bus/usb/xxx/yyy it will likely be reset across power off or even after unplugging and plugging the device back in. The better solution in this case is to add a custom udev rule:

sudo echo <<EOF > /etc/udev/rules.d/50-usb-perms.rules
SUBSYSTEM=="usb", ATTRS{idVendor}=="0a2d", GROUP="users", MODE="0666"
EOF

Basic Usage

This project is primarily intended to be used as a library module that can be combined with other Python frameworks such as NumPy, SciPy, and matplotlib to realize data analysis and visualization pipelines for scintillator-based radiation detectors. The module may also be invoked from the command line to perform a limited set of functions: collection of MCA spectra and list-mode PMT hits to a data file and background-subtracted gamma detection.

$ python -m digibase --help

will provide help on the various commands and options it supports.

Python Module

As of version 0.3.1, multiple devices are supported and can be readout simultanesously. If you don't specify a serial number as argument to instance creation, it will connect to the first one found according to the pyusb documentation. This may be non deterministic behavior:

from digibase import digiBase, ExtGateMode
from time import sleep
base = digiBase()
print('Opened digiBase, serial number', base.serial)

should open the device and provide a serial number. If you provide a serial number as a string or an integer, it will search for and connect only to the device with that serial number:

base_a = digiBase(12)
base_b = digiBase(1830)

Configuration

At this point I don't specify the configuration of the device at power-up, and as far as I can tell the configuration does not persist across power down / power up. Therefore users must ensure that the device is properly configured the first time it is connected.

PMT Bias

High voltage bias for the PMT dynodes is programmed and enabled/disabled in separate steps:

base.hv = 800
base.hv_enabled = True

will set the HV to 800 V and turn it on if it is not already on. To turn off the HV, set the hv_enabled property to False:

base.hv_enabled = False

Electro-optical gain for photomultipliers typically behaves as $G = G_0 \left(\frac{V}{V_0}\right)^p$ where $p\sim 5-8$. It will depend on what's plugged into the socket.

Livetime and Realtime Counters and Presetting Acquisition Time

The digiBASE contains counters to separately track realtime and livetime. Furthermore, acquisition can be preset to stop when these counters reach a certain value. The following will setup the acquisition to stop either after 75 s of wall time or after one minute of livetime, whichever occurs first:

base.realtime_preset = 75.0
base.livetime_preset = 60.0 
base.set_presets(livetime=True, realtime=True)

Using the External Gate

Hit collection can be suppressed using a TTL-level signal connected to the SMA input on the base. This suppression occurs for both PHA and list mode acquisitions. The external gate mode determines how the base responds to this external gate:

• When base.ext_gate = ExtGateMode.OFF, the external gate is ignored, i.e. acquisition continues.
• When base.ext_gate = ExtGateMode.ENABLED, a TTL low on the external gate will suspend the data acquisition. As far as I can tell, this also stops the livetime counter as well as the microsecond timestamp counter in list acquisition mode.
• When base.ext_gate = ExtGateMode.COINCIDENCE, TTL low will stop hit collection (i.e. no hits will fill the MCA channels in PHA mode and hits will not appear in list mode), however the livetime and timestamp counters continue to tick.

Gain

Electronic amplifier gain is set and read by the fine_gain property of the digiBase class:

base.fine_gain = 1.0

The digiBASE can apparently adjust this parameter internally in order to track gain and offset drifts, as documented in the device manual, by tracking a peak in the acquired spectrum. This feature is enabled by passing triplets of (high_channel, center_channel, low_channel) to either of the gain or zero named parameters of the auto_stabilize method:

base.auto_stabilize(gain=(80, 75, 70), zero=(80, 75, 70))

will turn on both gain and offset stabilization using a peak that should be centered at 75 ADC counts inside an ROI of 70 to 80 ADC counts. Note: this feature is not tested; I recommend turning off auto stabilization which can be effected by calling with arguments set to None:

base.auto_stabilize(gain=None, zero=None)

or just

base.auto_stabilize()

PHA Mode Acquisition

A 15 second run in PHA mode with PMT set to 800 V, the lower-level discriminator set to 24 (ADC counts), and suppressing hits that are not coincident with a TTL high on the external gate signal might be configured like this:

base.hv = 800
base.hv_enabled = True
base.lld = 24
base.ext_gate = ExtGateMode.COINCIDENCE
base.livetime_preset = 15.0 
base.set_acq_mode_pha()

sleep(5)        # Sleep 5 seconds to allow HV to stabilize 
base.start()    # Start the acquisition

The run should automatically stop after 15 sec of livetime have elapsed. Note that the device does not block so you can stop the acquisition early, if desired.

sleep(5)
base.stop()

In any case you need to turn off the acquistion before starting again.

To access the pulseheight spectrum / MCA channels:

spectrum = base.spectrum

which returns a python list of integers of length 1024.

List Mode Acquisition

The list mode acquisition is a powerful feature of the digiBASE. Instead of having logic on the base fill histogram bins with the ADC values you get the individual PMT hits themselves along with microsecond-level timestamps. To invoke list mode instead of PHA:

base.set_acq_mode_list()
sleep(5) # If needed - only first time after changing HV
base.start()

Now don't dilly-dally before reading out the list buffer - it's only 128k elements (or maybe bytes in which case it's only 32k elements!) deep. If the internal buffer fills the acquisition stops (instead of acting like a queue and dropping early hits).

hits = []
while some_condition_is_true:
    while len(new_hits := base.hits) > 0: hits += new_hits

The device reads are limited to 4096 bytes so you need to ensure that there are not hits left in the buffer, hence that inner read loop. The hits themselves are 32-bit integers which encode time and charge. There are actually two kinds of data that are encountered in the hit list readout:

• PMT hits have bit 31 clear. In this case bits 30-21 are 10-bit ADC / charge and bits 20-0 are time in units of microseconds (according to the not very precise local oscillator on the digiBase);
• Time rollover words have bit 31 set. In this case bits 30-0 are the time epoch in microseconds. Because the PMT hits only have 21 bit these rollover markers allow for rollover correction.

Here's an example of how to use a hit list populated with PMT hits and rollover words:

t20 = 0
hit_times = []
hit_q = []
for h in hits:
    if h & 0x8000_0000:
        t20 = h & 0x7fff_ffff
    else:
        t = t20 + (h & 0x001f_ffff)
        hit_times.append(t)
        hit_q.append((h >> 21) & 0x3ff)