PROTECCION Y ADECUACION DE LOS SITIOS DE TRABAJO

This work utilised state of the art digital electronics manufactured by CAEN, allowing the pulse shapes of each channel to be recorded for analysis and digital triggering to be used. This is the first time these electronics have been used for a multichannel detector system used as a Compton camera. The digital electronics can fit into one standard VME crate, while the detectors and preamplifiers are powered from a standard NIM crate using appropriate modules. This makes the system more portable compared to using analogue electronics modules to create an appropriate trigger system which can use up to three NIM crates for triggering, another for HV and preamplifier power supply and either a VME or NIM crate to output signals to the DAQ.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 5 10 15 20 25 En e rg y re so lu ti on (FWHM i n k e V)

Strip number (1-13 , AC01-AC13 and 14-26, DC01-DC13)

FWHM for SiLi compared to Canberra values at 60keV

Canberra Source AC side Source DC side (a) 0 0.5 1 1.5 2 2.5 0 5 10 15 20 25 FWH M (k eV)

Strip number (1-13, AC01-AC13. 14-26, DC01-DC13)

FWHM for SiLi at 662keV

Source AC side Source DC side

(b)

Figure 4.2: Energy resolution measured for the SiLi detector. (a) shows the energy resolution at 60keV for the AC side and DC side compared to measurements taken by Canberra, with the source in different locations.(b) shows the energy resolution at 662keV for both sides of the detector.

4.4. The function of each unit will be introduced briefly, for further information please refer to the manufacturers handbooks for the appropriate modules.

(a)

(b)

Figure 4.3: Schematic diagram of the strip configuration of the HPGe detector used an absorber (a) along with a picture of the detector unit itself (b). Image (a) reproduced from [41]. Image (b) reproduced from [42].

4.2.1 High voltage (HV) and preamplifier power

To provide the bias voltages to the detectors two HV units were used. Two Ortec 671 amplifiers were used to provide the preamplifiers with power. The four units are NIM powered and placed into one high power crate, allowing for simple grounding and easier transportation.

The two detectors require different bias shutdown methods. A bias shutdown pre- vents HV being applied in the event that the detector warms up, preventing any damage that may occur due to contaminants being released into the vacuum from the molecular sieve. Typically this out-gassing will result in a spark across the crystal if the HV is still applied, leading to the FET on the preamplifiers blowing or in extreme cases the crystal itself cracking.

To provide the correct bias shutdown methods a Canberra 3106D HV supply was used with the SiLi detector, while an Ortec 659 HV supply was used with the HPGe detector.

4.2.2 GO Box

The dynamic range available on the CAEN V1724 card is 2.25V. This is very large com- pared to the output voltages per 1MeV of energy deposited that the detector pream- plifiers provide. To better match the dynamic range of the cards to the energy range

Si(Li) HPGe GO Box V1724 V1495 V2718 A2818 24 channels 24 channels

Accepted triggers, readout signal returned from V1495

Signals above V1724 threshold checked by V1495

Valid events output from V1724 via V2718 by optical link MIDAS

Figure 4.4: Flow diagram of the digital electronics used for this work. The GO Box provides an amplified signal from each detector output to the CAEN V1724 digitiser cards. Energy thresholds are set on the V1724’s and the signal from any channel which is above the appropriate threshold is passed to the CAEN V1495 trigger control card. The trigger logic programmed onto the V1495 is checked and if passed a signal is sent back to the V1724’s to read out the appropriate data to the MIDAS data acquisition software via an optical link using the V2718 and A2818.

of interest a fixed gain of a factor was put into place by using a gain offset box (GO Box). This GO Box allows a separate offset for each channel to be applied, as each pre amplifier has its own DC offset this allows them all to be set to a common level. The gain is fixed at a factor of 10, 5 or 2 and is set by changing an electronics board inside the unit, with the 5 and 10 times boards being used for the work discusses in Chapters 5 and 6 respectively. The GO Box was designed and built at the University of Liverpool.

4.2.3 CAEN V1724 Digitiser

The CAEN V1724 digitiser was used during this work. The digitiser has 8 channels on each card, running at a sampling rate of 100MHz with 14 bit resolution. Each channel is processed and controlled individually, with trigger levels, moving window deconvolution and other settings being set within the browser based control software. Six of these cards were used providing 48 channels of electronics. The cards are placed in a VME crate. Full information on the V1724 can be found at [44].

The 100Mhz clock of the first card is daisy chained across all the others allowing a common clock to be used for all the cards. This clock is used to check whether events are correlated correctly and allows us to be certain that events recorded occurred within a set time window.

Figure 4.5: Picture showing the gain/offset (GO) box used to provide additional am- plification to the preamplifier signals and also provide a common baseline level.

Figure 4.6: Picture showing the CAEN V1724 digitiser cards in a VME crate. 3 cards are shown with the clock and optical readout cables in place, along with the V2718 optical link bridge card which is at the left hand side of this image.

The cards can provide a data readout from either the VME databus or an optical link. The optical link provides a faster readout and was chosen for this work. Within the software it is possible to enable only certain channels, this makes checking individual channels trigger levels easier as the channel does not have to be disconnected from the electronics.

4.2.4 CAEN V1495 Trigger logic controller

The trigger outputs from each V1724 are fed to the V1495 trigger logic card, full details of this card can be found here [45]. This card is programmed with the trigger methodology used for a particular project. The trigger outputs from the V1724 for each event are checked against this logic and the card will either accept or reject the event. There is also an option to write out only the channels which trigger the system or all of the channels which are enabled or the channels which trigger along with their nearest neighbour.

For this project the trigger could be used to two ways, either singles mode used for calibration or in Compton mode for data collection. Singles mode will allow the system to record any triggering event, so if any channel activates the trigger the DAQ will record this event. Compton mode sets the requirement that an event must include at least one channel on each side of each detector activating the trigger within a given time window. The time window width is set within the software.

4.2.5 CAEN V2718 Optical link bridge

The outputs from the V1724’s are passed to a CAEN V2718 using optical fibres. This card acts as the VME controller, passing the configuration from the control PC to the cards via a A2818 PCi controller using optical fibre. The data is also transmitted through the bridge to the PC for analysis software, MIDAS, to process. The data trans- fer rate is up to 100MBytes/s using CONET2 (Chainable Optical Network) protocols.

4.2.6 MIDAS

MIDAS, Multi-Instance Data Acquisition System, is a data acquisition software package designed for the STFC at Daresbury. This package runs under a Linux, Windows or Sun OS environment, allowing full control of the system via a PC. An HTML link is used to control the settings on individual V1724 channels and also the V1495 settings.