Recoil decay tagging (RDT) is a very powerful method for the spectroscopy of exotic nuclei. RDT is a delayed coincidence technique between detectors usually at the target position and at the focal plane of a spectrometer. Such measurements are often limited by dead time. This paper describes a novel triggerless data acquisition method, which is being developed for the Gamma Recoil Electron Alpha Tagging (GREAT) spectrometer, that overcomes this limitation by virtually eliminating dead time. Our solution is a total data readout (TDR) method where all channels run independently and are associated in software to reconstruct events. The TDR method allows all the data from both target position and focal plane to be collected with practically no dead-time losses. Each data word is associated with a timestamp generated from a global 100-MHz clock. Events are then reconstructed in real time in the event builder using temporal and spatial associations defined by the physics of the experimen

Appelbe, D E

Butler, P A

Coleman-Smith, P J

Cresswell, J R

Freeman, S J

Herzberg, R D

Hibbert, I

Joss, D T

Lazarus, I H

Letts, S C

Page, R D

Pucknell, V F E

Regan, P H

Sampson, J

Simpson, J

Thornhill, J

Wadsworth, R

English

Lazarus IH

Appelbe DE

Butler PA

Coleman-Smith PJ

Cresswell JR

Freeman SJ

Herzberg RD

Hibbert I

Joss DT

Letts SC

Page RD

Pucknell VFE

Regan PH

Sampson J

Simpson J

Thornhill J

Wadsworth R

ePubs: the open archive for STFC research publications

The GREAT triggerless total data readout method

Lazarus, IH

Appelbe, DE

Butler, PA

Coleman-Smith, PJ

Cresswell, JR

Freeman, SJ

Herzberg, RD

Hibbert, I

Joss, DT

Letts, SC

Page, RD

Pucknell, VFE

Regan, PH

Sampson, J

Simpson, J

Thornhill, J

Wadsworth, R

University of Surrey

IEEE Transactions on Nuclear Science

White Rose Research Online

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Article:
Lazarus, I H, Appelbe, D E, Butler, P A et al. (14 more authors) (2001) The GREAT 
triggerless total data readout method. IEEE Transactions on Nuclear Science. pp. 567-
569. ISSN 0018-9499 
https://doi.org/10.1109/23.940120
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 48, NO. 3, JUNE 2001 567
The GREAT Triggerless Total Data Readout Method
I. H. Lazarus, Member, IEEE, D. E. Appelbe, P. A. Butler, P. J. Coleman-Smith, Member, IEEE, J. R. Cresswell,
S. J. Freeman, R. D. Herzberg, I. Hibbert, D. T. Joss, S. C. Letts, R. D. Page, V. F. E. Pucknell, P. H. Regan,
J. Sampson, J. Simpson, J. Thornhill, and R. Wadsworth
Abstract—Recoil decay tagging (RDT) is a very powerful
method for the spectroscopy of exotic nuclei. RDT is a delayed
coincidence technique between detectors usually at the target
position and at the focal plane of a spectrometer. Such measure-
ments are often limited by dead time. This paper describes a novel
triggerless data acquisition method, which is being developed for
the gamma recoil electron alpha tagging (GREAT) spectrometer,
that overcomes this limitation by virtually eliminating dead time.
Our solution is a total data readout (TDR) method where all
channels run independently and are associated in software to re-
construct events. The TDR method allows all the data from both
target position and focal plane to be collected with practically no
dead-time losses. Each data word is associated with a timestamp
generated from a global 100-MHz clock. Events are then recon-
structed in real time in the event builder using temporal and spa-
tial associations defined by the physics of the experiment.
Index Terms—Alpha-particle spectroscopy, beta-ray spec-
troscopy, coincidence circuits, data acquisition, gamma-ray
spectroscopy, nuclear physics, triggering.
I. INTRODUCTION
T
HE gamma recoil electron alpha tagging (GREAT) spec-
trometer [1] will be used in nuclear structure physics ex-
periments to analyze the decay particles and gamma rays of re-
coiling nuclei produced in heavy-ion fusion reactions. Initially,
it will be used at the focal plane of RITU [2], the recoil separator
at Jyväskylä, Finland.
Normally, the spectrometer will be used in conjunction with
a target position detector array to collect prompt gamma rays
or conversion electrons emitted during the initial stages of the
decay of the recoiling nucleus. Examples of target position de-
tector arrays to be used with GREAT are VEGA [3] or Juro-
sphere [4] for gamma rays and SACRED [5] for conversion elec-
trons. Fig. 1 shows a generic experimental setup for GREAT
Manuscript received November 1, 2000; revised January 18, 2001. This
work was supported by the U.K. Engineering and Physical Science Council
(EPSRC) under Grant GR/M79998/01 and by the European Community—Ac-
cess to Research Infrastructure action of the Improving Human Potential
Programme—under Contract HPRI-1999-CT-50017.
I. H. Lazarus, D. E. Appelbe, P. J. Coleman-Smith, S. C. Letts, V. F. E. Puck-
nell, and J. Simpson are with Daresbury Laboratory, Warrington WA4 4AD,
U.K. (e-mail: I.H.Lazarus@dl.ac.uk).
P. A. Butler, J. R. Cresswell, R. D. Herzberg, I. Hibbert, R. D. Page, J.
Sampson, and J. Thornhill are with Oliver Lodge Laboratory, University of
Liverpool, Liverpool L69 7ZE, U.K.
S. J. Freeman is with Department of Physics and Astronomy, University of
Manchester, Manchester M13 9PL, U.K.
D. T. Joss is with the School of Sciences, Staffordshire University, Stoke on
Trent ST4 2DE, U.K.
P. H. Regan is with Department of Physics, University of Surrey, Guildford
GU2 7XH, U.K.
R. Wadsworth is with Department of Physics, University of York, York YO10
5DD, U.K.
Publisher Item Identifier S 0018-9499(01)04902-4.
Fig. 1. Typical GREAT experiment configuration at Jyväskylä with a target
array detecting either gamma rays or conversion electrons and a recoil separator.
at Jyväskylä, where an incoming heavy ion beam hits a target
in the center of a detector array sensitive either to conversion
electrons or to gamma rays. The resulting fusion products pass
through the magnets, which act as a recoil separator to select cer-
tain mass-charge-energy combinations for transmission to the
GREAT detectors at the focal plane.
The GREAT spectrometer uses five focal plane detector
types:
1) 28 silicon p-i-n diodes for conversion electrons;,
2) silicon (Si) strip detectors (two-dimensional, 60 40) for
recoils and for alpha and beta decays;
3) multiwire proportional chamber (MWPC) for recoil de-
tection and identification;
4) planar germanium (Ge) strip detector (two-dimensional,
24 12) to detect betas and low-energy photons;
5) segmented large volume Ge detector for higher energy
gammas.
Recoil decay tagging (RDT) [6] experiments collect data at
three distinct stages: first prompting radiation at the target po-
sition, secondly recoiling nuclei as they emerge from the recoil
separator into GREAT, and thirdly emissions from decays of im-
planted nuclei in the GREAT spectrometer’s detectors. Each of
these three stages contributes to the triggering.
The delay between the prompt radiation and detection of re-
coils in GREAT depends on the mass and energy of the recoiling
nucleus and also the characteristics of the spectrometer. Times
of flight range from 0.5 to 5 s. Emissions of gamma rays, al-
phas, betas, or electrons from implanted nuclei in the Si strip
detectors can take place over a wide range of times from a few
nanoseconds to hundreds of seconds. The correlation between
the decaying implanted nuclei and the other data is made posi-
tionally and relies on having only one recoil per pixel in the Si
0018–9499/01$10.00 © 2001 IEEE
568 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 48, NO. 3, JUNE 2001
strip detectors within the correlation interval. The correlation of
recoils with their subsequent decays is at the heart of the RDT
technique.
II. CONVENTIONAL RDT TRIGGERING
Up to now, RDT experiments have normally used a common
dead-time data-acquisition system where a trigger condition
might typically be that any focal plane detector is active. This
occurs some microseconds after the radiation is emitted at the
target position because of the recoil’s time of flight through
the separator. The time of flight delay is overcome by using
delayed coincidence mode in the analog-to-digital converters
(ADCs) instrumenting the target array. This means that the
target position ADCs are gated whenever a recoil is detected
at the focal plane at a rate of up to 10 kHz (assuming 10%
efficiency for transmission to focal plane and 100-kHz target
rate). Singles data at the target position are collected separately
for monitoring purposes but not written to tape. The gate width
depends on the decay lifetime of the implanted nuclei. Typically
gate widths of up to 5 s are used. This period is sufficient to
include recoil implantations at the focal plane and decay of
short lived isomers to the ground state as well as gammas at
the target. (Focal plane alpha and beta ground state decays take
place much later and are recorded in separate events.) During
the gate period, all ADCs that fire are converted and written
to tape along with a single timestamp based on the start of the
gate. Both good data and randoms (rubbish) are written to tape
at a rate of up to 10 kHz (rate depends on the type of reaction
under study).
Dead time can be calculated assuming a rate of 5 kHz at the
focal plane in the silicon detector and allowing 35 s for the gate
and for readout of the ADCs. In this case, we have a dead time
of 18%, rising to 35% if the rate in the Silicon detector increases
to 10 kHz. Obviously, longer gates increase the dead time still
further.
Longer gates are desirable for studying isomeric decays
where, for example, an isomer with a half-life of 10 s requires
a gate of 40–50 s. However, a 50- s gate, with readout time
added, and a rate of 10 kHz leads to a dead time of 75%, and
a 50- s gate incorporates a lot of random gammas from the
target position’s delayed coincidence ADCs.
Another problem related to dead time is that during readout,
the focal plane electronics is dead, which means that any further
alpha or beta ground state emissions are missed. These are rare
events, sometimes only a few per day, so missing even one is a
serious drawback. In some experiments, the fixed “blind” period
at the end of the gate during readout can be a problem, although
normally gates are adjusted to include all interesting lifetimes.
To summarize, the problems with the normal RDT triggering
method are as follows.
1) Wide gates (over 10 s) are desirable from the physics
perspective, but are precluded by dead time, causing loss
of rare ground state emission events from implanted nu-
clei at the focal plane.
2) The data written to tape include both good and random
data, which must later be separated. This problem gets
worse with wider gates.
The extra random data extend the readout time and also increase
the data rate to tape by increasing the data volume.
To overcome the dead-time problem and to select only good
data to be written to tape the GREAT electronics will use a trig-
gerless data collection method called total data readout.
III. TOTAL DATA READOUT (TDR)
The TDR method overcomes the dead-time limitations by
reading all the data, timestamping them, and then collecting the
event fragments together in software in the event builder using
spatial and temporal correlations.
The correlation in the event builder can be very complex
or quite simple. A simple example would eliminate random
gamma rays by starting with recoils, and then looking back in
time using a window centered on the time of flight ( some jitter
period). All gamma rays detected during that window would be
considered to be correlated with the recoil. At the same time, the
coordinate of the implanted nucleus would be recorded, and
later emissions from that location can be correlated and treated
as implant decays.
A more complex example would require that both the
recoil-gamma condition and the implant decay condition are
true within some defined time (based on the implanted nucleus’
decay half-life) before data are recorded.
Software triggering like that described here clearly requires a
large amount of memory and processing power in the event pro-
cessing system. Our estimates indicate that TDR has recently
become feasible as a result of the increasing power commer-
cially available in affordable CPUs.
IV. TDR ELECTRONICS AND DATA ACQUISITION
Fig. 2 shows a schematic overview of the GREAT electronics
and data acquisition system. In this section, the various parts of
the system are described.
The front-end electronics [constant fraction discrimina-
tors (CFDs) and shaping amplifiers] comprises commercial
NIM/CAMAC units. The timestamping ADC card is a new
design in VXI-D format offering 32 independent channels (14
bits with sliding-scale correction and zero suppression). The
ADC conversion and readout time is less than the shaping
amplifier’s busy period, so the only counting losses are those
due to pileup. A gate is generated for each input either from
its associated external CFD or using a software controlled
gate matrix, which permits a single input to start a group of
gates, for example, one wire in the MWPC to gate itself and
its immediate neighbors. The timestamp is recorded for each
channel at the start of its gate. In the event of pileup, a second
timestamp is recorded to indicate the second firing of the CFD
within the original gate period. Qualifiers mark data that is
affected by pileup or coincident with a front panel veto input.
The timestamping requires the distribution and synchroniza-
tion of a 100-MHz clock. The clock distribution takes advantage
of the facilities built into D-sized VXI, i.e., a 100-MHz clock,
ClockSynch signal (to select one edge of the clock), and two
STAR lines, which arrive at each slot within a crate with less
LAZARUS et al.: GREAT TRIGGERLESS TOTAL DATA READOUT METHOD 569
Fig. 2. Schematic diagram of the GREAT TDR system’s electronics and data
acquisition.
than 2-ns skew. A VME module known as the Metronome con-
trols the clock distribution and maintains synchronization of all
the ADCs.
A hit pattern VME module provides a method to include pat-
terns in the data stream along with a timestamp. Hit patterns
may be either made up of all the bits that are active at some
time during the gate input (gated) or transitional. Transitional
patterns generate a timestamp whenever an input bit becomes
active. A transitional pattern mode may be either gated or free
running. Conventional clocked hit patterns are also possible by
using a very narrow gate to sample the inputs in gated mode.
The operation of the pattern modules is controlled from soft-
ware with individual enable/disable mask bits for each input.
The timestamped data words from the ADCs and the pattern
units are each tagged with an address to identify which detector
they come from and which signal they represent within that de-
tector. The data items from both ADCs and pattern cards are
then sent in time-order via point-to-point data links (SHARC
digital signal-processor (DSP) links [7]) into VME cards, where
SHARC DSPs receive and buffer the information ready for use
by the event builder. Currently, the DSPs are used only for data
moving, but there is spare processing capacity that could be used
later for real-time diagnostic data histogramming. Every ADC
card and pattern module has its own SHARC data link. Each
link can cope with up to 50 k gates/s on all 32 of its associated
ADC inputs. The ADC cards have a diagnostic scaler for missed
counts, which indicates whether the SHARC data links and their
buffers ever filled to the point of blocking data collection.
Data are transferred from the buffers to the event builder over
a gigabit Ethernet. Output data from the event reconstruction
processors in the event builder are sent to tape for storage using
one or more data streams where each data stream corresponds
to one class of reconstructed event. In a typical RDT experiment
there could, for example, be three streams: one for the prompt
recoil-gamma data (gamma ray spectroscopy), one for the emis-
sion from the implanted nuclei (decay spectroscopy), and one
for events where items from the first two streams have been suc-
cessfully correlated (recoil decay tagging).
V. STATUS
The GREAT project is funded and under construction in the
United Kingdom. The system design is complete and reported
here. The event builder design has started and the electronics
design is taking place during 2000/1. Detectors are on order and
it is anticipated that the system will be assembled and tested
during 2001 for experiments in 2002.
VI. CONCLUSION
The TDR method opens new possibilities for the study of
exotic nuclei by allowing the collection of very rare events in
a system that simultaneously collects high rate data without
dead-time problems. The only losses are due to pileup in the
shaping amplifiers, which limits the rate on any one channel. If
GREAT works as expected, then in the future TDR will become
the architecture of choice for large nuclear structure projects and
hardware trigger systems will become obsolete.
REFERENCES
[1] P. A. Butler, “New developments in electron and recoil decay spec-
troscopy for studies of exotic and heavy nuclei,” Acta Phys. Polonica
B, vol. 31, no. 1, pp. 9–19, 2000.
[2] M. Leino, J. Äystö, T. Enqvist, P. Heikkinen, A. Jokinen, and M. Nurmia
et al., “Gas-filled recoil separator for studies of heavy elements,” Nucl.
Instrum. Meth. B, vol. 99, p. 653, 1995.
[3] “VEGA, a proposal for Versatile and Efficient GAmma-detectors,” GSI,
Darmstadt, Internal Rep., 1998.
[4] K. Helariutta, J. F. C. Cocks, T. Enqvist, P. T. Greenlees, P. Jones, and
R. Julin et al., “Gamma-spectroscopy of 192-195 Po,” Eur. Phys. J., vol.
A6, p. 289, 1999.
[5] P. A. Butler, P. M. Jones, K. J. Cann, J. F. C. Cocks, G. D. Jones, and R.
Julin et al., “Electron spectroscopy using a multi-detector array,” Nucl.
Instrum. Meth. A, vol. 381, p. 433, 1996.
[6] E. S. Paul, P. J. Woods, T. Davinson, R. D. Page, P. J. Sellin, and C. W.
Beausang et al., “In-beam Gamma-ray spectroscopy above 100Sn using
the new technique of recoil delay tagging,” Phys. Rev. C, vol. 51, no. 1,
pp. 78–87, Jan. 1995.
[7] Analog Devices, ADSP-2106x SHARC User’s Manual, Mar. 1995, ch.
9.


Lazarus, IH

Appelbe, DE

Butler, PA

Coleman-Smith, PJ

Cresswell, JR

Freeman, SJ

Herzberg, RD

Joss, DT

Letts, SC

Page, RD

Pucknell, VFE

Regan, PH

Surrey Research Insight

Crossref

Recoil Decay Tagging (RDT) is a very powerful method for the spectroscopy of exotic nuclei. RDT is a delayed coincidence technique between detectors usually at the target position and at the focal plane of a spectrometer. Such measurements are often limited by dead time. This paper describes a novel triggerless data acquisition method which is being developed for the Gamma Recoil Electron Alpha Tagging (GREAT) spectrometer that overcomes this limitation by virtually eliminating dead time. Our solution is a Total Data Readout (TDR) method where all channels run independently and are associated in software to reconstruct events. The TDR method allows all the data from both target position and focal plane to be collected with practically no dead time losses. Each data word is associated with a timestamp generated from a global 100MHz clock. Events are then reconstructed in real time in the event builder using temporal and spatial associations defined by the physics of the experiment

Lazarus, I. H.

Appelbe, D. E.

Butler, P. A.

Coleman-Smith, P. J.

Cresswell, J. R.

Freeman, S. J.

Herzberg, R. D.

Hibbert, I.

Joss, D. T.

Letts, S. C.

Page, R. D.

Pucknell, V. F E

Regan, P. H.

Sampson, J.

Simpson, J.

Thornhill, J.

Wadsworth, R.

The University of Manchester - Institutional Repository

This is a repository copy of The GREAT triggerless total data readout method.White Rose Research Online URL for this paper:https://eprints.whiterose.ac.uk/969/Article:Lazarus, I H, Appelbe, D E, Butler, P A et al. (14 more authors) (2001) The GREAT triggerless total data readout method. IEEE Transactions on Nuclear Science. pp. 567-569. ISSN 0018-9499 https://doi.org/10.1109/23.940120eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 48, NO. 3, JUNE 2001 567The GREAT Triggerless Total Data Readout MethodI. H. Lazarus, Member, IEEE, D. E. Appelbe, P. A. Butler, P. J. Coleman-Smith, Member, IEEE, J. R. Cresswell,S. J. Freeman, R. D. Herzberg, I. Hibbert, D. T. Joss, S. C. Letts, R. D. Page, V. F. E. Pucknell, P. H. Regan,J. Sampson, J. Simpson, J. Thornhill, and R. WadsworthAbstract—Recoil decay tagging (RDT) is a very powerfulmethod for the spectroscopy of exotic nuclei. RDT is a delayedcoincidence technique between detectors usually at the targetposition and at the focal plane of a spectrometer. Such measure-ments are often limited by dead time. This paper describes a noveltriggerless data acquisition method, which is being developed forthe gamma recoil electron alpha tagging (GREAT) spectrometer,that overcomes this limitation by virtually eliminating dead time.Our solution is a total data readout (TDR) method where allchannels run independently and are associated in software to re-construct events. The TDR method allows all the data from bothtarget position and focal plane to be collected with practically nodead-time losses. Each data word is associated with a timestampgenerated from a global 100-MHz clock. Events are then recon-structed in real time in the event builder using temporal and spa-tial associations defined by the physics of the experiment.Index Terms—Alpha-particle spectroscopy, beta-ray spec-troscopy, coincidence circuits, data acquisition, gamma-rayspectroscopy, nuclear physics, triggering.I. INTRODUCTIONTHE gamma recoil electron alpha tagging (GREAT) spec-trometer [1] will be used in nuclear structure physics ex-periments to analyze the decay particles and gamma rays of re-coiling nuclei produced in heavy-ion fusion reactions. Initially,it will be used at the focal plane of RITU [2], the recoil separatorat Jyväskylä, Finland.Normally, the spectrometer will be used in conjunction witha target position detector array to collect prompt gamma raysor conversion electrons emitted during the initial stages of thedecay of the recoiling nucleus. Examples of target position de-tector arrays to be used with GREAT are VEGA [3] or Juro-sphere [4] for gamma rays and SACRED [5] for conversion elec-trons. Fig. 1 shows a generic experimental setup for GREATManuscript received November 1, 2000; revised January 18, 2001. Thiswork was supported by the U.K. Engineering and Physical Science Council(EPSRC) under Grant GR/M79998/01 and by the European Community—Ac-cess to Research Infrastructure action of the Improving Human PotentialProgramme—under Contract HPRI-1999-CT-50017.I. H. Lazarus, D. E. Appelbe, P. J. Coleman-Smith, S. C. Letts, V. F. E. Puck-nell, and J. Simpson are with Daresbury Laboratory, Warrington WA4 4AD,U.K. (e-mail: I.H.Lazarus@dl.ac.uk).P. A. Butler, J. R. Cresswell, R. D. Herzberg, I. Hibbert, R. D. Page, J.Sampson, and J. Thornhill are with Oliver Lodge Laboratory, University ofLiverpool, Liverpool L69 7ZE, U.K.S. J. Freeman is with Department of Physics and Astronomy, University ofManchester, Manchester M13 9PL, U.K.D. T. Joss is with the School of Sciences, Staffordshire University, Stoke onTrent ST4 2DE, U.K.P. H. Regan is with Department of Physics, University of Surrey, GuildfordGU2 7XH, U.K.R. Wadsworth is with Department of Physics, University of York, York YO105DD, U.K.Publisher Item Identifier S 0018-9499(01)04902-4.Fig. 1. Typical GREAT experiment configuration at Jyväskylä with a targetarray detecting either gamma rays or conversion electrons and a recoil separator.at Jyväskylä, where an incoming heavy ion beam hits a targetin the center of a detector array sensitive either to conversionelectrons or to gamma rays. The resulting fusion products passthrough the magnets, which act as a recoil separator to select cer-tain mass-charge-energy combinations for transmission to theGREAT detectors at the focal plane.The GREAT spectrometer uses five focal plane detectortypes:1) 28 silicon p-i-n diodes for conversion electrons;,2) silicon (Si) strip detectors (two-dimensional, 60 40) forrecoils and for alpha and beta decays;3) multiwire proportional chamber (MWPC) for recoil de-tection and identification;4) planar germanium (Ge) strip detector (two-dimensional,24 12) to detect betas and low-energy photons;5) segmented large volume Ge detector for higher energygammas.Recoil decay tagging (RDT) [6] experiments collect data atthree distinct stages: first prompting radiation at the target po-sition, secondly recoiling nuclei as they emerge from the recoilseparator into GREAT, and thirdly emissions from decays of im-planted nuclei in the GREAT spectrometer’s detectors. Each ofthese three stages contributes to the triggering.The delay between the prompt radiation and detection of re-coils in GREAT depends on the mass and energy of the recoilingnucleus and also the characteristics of the spectrometer. Timesof flight range from 0.5 to 5 s. Emissions of gamma rays, al-phas, betas, or electrons from implanted nuclei in the Si stripdetectors can take place over a wide range of times from a fewnanoseconds to hundreds of seconds. The correlation betweenthe decaying implanted nuclei and the other data is made posi-tionally and relies on having only one recoil per pixel in the Si0018–9499/01$10.00 © 2001 IEEE568 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 48, NO. 3, JUNE 2001strip detectors within the correlation interval. The correlation ofrecoils with their subsequent decays is at the heart of the RDTtechnique.II. CONVENTIONAL RDT TRIGGERINGUp to now, RDT experiments have normally used a commondead-time data-acquisition system where a trigger conditionmight typically be that any focal plane detector is active. Thisoccurs some microseconds after the radiation is emitted at thetarget position because of the recoil’s time of flight throughthe separator. The time of flight delay is overcome by usingdelayed coincidence mode in the analog-to-digital converters(ADCs) instrumenting the target array. This means that thetarget position ADCs are gated whenever a recoil is detectedat the focal plane at a rate of up to 10 kHz (assuming 10%efficiency for transmission to focal plane and 100-kHz targetrate). Singles data at the target position are collected separatelyfor monitoring purposes but not written to tape. The gate widthdepends on the decay lifetime of the implanted nuclei. Typicallygate widths of up to 5 s are used. This period is sufficient toinclude recoil implantations at the focal plane and decay ofshort lived isomers to the ground state as well as gammas atthe target. (Focal plane alpha and beta ground state decays takeplace much later and are recorded in separate events.) Duringthe gate period, all ADCs that fire are converted and writtento tape along with a single timestamp based on the start of thegate. Both good data and randoms (rubbish) are written to tapeat a rate of up to 10 kHz (rate depends on the type of reactionunder study).Dead time can be calculated assuming a rate of 5 kHz at thefocal plane in the silicon detector and allowing 35 s for the gateand for readout of the ADCs. In this case, we have a dead timeof 18%, rising to 35% if the rate in the Silicon detector increasesto 10 kHz. Obviously, longer gates increase the dead time stillfurther.Longer gates are desirable for studying isomeric decayswhere, for example, an isomer with a half-life of 10 s requiresa gate of 40–50 s. However, a 50- s gate, with readout timeadded, and a rate of 10 kHz leads to a dead time of 75%, anda 50- s gate incorporates a lot of random gammas from thetarget position’s delayed coincidence ADCs.Another problem related to dead time is that during readout,the focal plane electronics is dead, which means that any furtheralpha or beta ground state emissions are missed. These are rareevents, sometimes only a few per day, so missing even one is aserious drawback. In some experiments, the fixed “blind” periodat the end of the gate during readout can be a problem, althoughnormally gates are adjusted to include all interesting lifetimes.To summarize, the problems with the normal RDT triggeringmethod are as follows.1) Wide gates (over 10 s) are desirable from the physicsperspective, but are precluded by dead time, causing lossof rare ground state emission events from implanted nu-clei at the focal plane.2) The data written to tape include both good and randomdata, which must later be separated. This problem getsworse with wider gates.The extra random data extend the readout time and also increasethe data rate to tape by increasing the data volume.To overcome the dead-time problem and to select only gooddata to be written to tape the GREAT electronics will use a trig-gerless data collection method called total data readout.III. TOTAL DATA READOUT (TDR)The TDR method overcomes the dead-time limitations byreading all the data, timestamping them, and then collecting theevent fragments together in software in the event builder usingspatial and temporal correlations.The correlation in the event builder can be very complexor quite simple. A simple example would eliminate randomgamma rays by starting with recoils, and then looking back intime using a window centered on the time of flight ( some jitterperiod). All gamma rays detected during that window would beconsidered to be correlated with the recoil. At the same time, thecoordinate of the implanted nucleus would be recorded, andlater emissions from that location can be correlated and treatedas implant decays.A more complex example would require that both therecoil-gamma condition and the implant decay condition aretrue within some defined time (based on the implanted nucleus’decay half-life) before data are recorded.Software triggering like that described here clearly requires alarge amount of memory and processing power in the event pro-cessing system. Our estimates indicate that TDR has recentlybecome feasible as a result of the increasing power commer-cially available in affordable CPUs.IV. TDR ELECTRONICS AND DATA ACQUISITIONFig. 2 shows a schematic overview of the GREAT electronicsand data acquisition system. In this section, the various parts ofthe system are described.The front-end electronics [constant fraction discrimina-tors (CFDs) and shaping amplifiers] comprises commercialNIM/CAMAC units. The timestamping ADC card is a newdesign in VXI-D format offering 32 independent channels (14bits with sliding-scale correction and zero suppression). TheADC conversion and readout time is less than the shapingamplifier’s busy period, so the only counting losses are thosedue to pileup. A gate is generated for each input either fromits associated external CFD or using a software controlledgate matrix, which permits a single input to start a group ofgates, for example, one wire in the MWPC to gate itself andits immediate neighbors. The timestamp is recorded for eachchannel at the start of its gate. In the event of pileup, a secondtimestamp is recorded to indicate the second firing of the CFDwithin the original gate period. Qualifiers mark data that isaffected by pileup or coincident with a front panel veto input.The timestamping requires the distribution and synchroniza-tion of a 100-MHz clock. The clock distribution takes advantageof the facilities built into D-sized VXI, i.e., a 100-MHz clock,ClockSynch signal (to select one edge of the clock), and twoSTAR lines, which arrive at each slot within a crate with lessLAZARUS et al.: GREAT TRIGGERLESS TOTAL DATA READOUT METHOD 569Fig. 2. Schematic diagram of the GREAT TDR system’s electronics and dataacquisition.than 2-ns skew. A VME module known as the Metronome con-trols the clock distribution and maintains synchronization of allthe ADCs.A hit pattern VME module provides a method to include pat-terns in the data stream along with a timestamp. Hit patternsmay be either made up of all the bits that are active at sometime during the gate input (gated) or transitional. Transitionalpatterns generate a timestamp whenever an input bit becomesactive. A transitional pattern mode may be either gated or freerunning. Conventional clocked hit patterns are also possible byusing a very narrow gate to sample the inputs in gated mode.The operation of the pattern modules is controlled from soft-ware with individual enable/disable mask bits for each input.The timestamped data words from the ADCs and the patternunits are each tagged with an address to identify which detectorthey come from and which signal they represent within that de-tector. The data items from both ADCs and pattern cards arethen sent in time-order via point-to-point data links (SHARCdigital signal-processor (DSP) links [7]) into VME cards, whereSHARC DSPs receive and buffer the information ready for useby the event builder. Currently, the DSPs are used only for datamoving, but there is spare processing capacity that could be usedlater for real-time diagnostic data histogramming. Every ADCcard and pattern module has its own SHARC data link. Eachlink can cope with up to 50 k gates/s on all 32 of its associatedADC inputs. The ADC cards have a diagnostic scaler for missedcounts, which indicates whether the SHARC data links and theirbuffers ever filled to the point of blocking data collection.Data are transferred from the buffers to the event builder overa gigabit Ethernet. Output data from the event reconstructionprocessors in the event builder are sent to tape for storage usingone or more data streams where each data stream correspondsto one class of reconstructed event. In a typical RDT experimentthere could, for example, be three streams: one for the promptrecoil-gamma data (gamma ray spectroscopy), one for the emis-sion from the implanted nuclei (decay spectroscopy), and onefor events where items from the first two streams have been suc-cessfully correlated (recoil decay tagging).V. STATUSThe GREAT project is funded and under construction in theUnited Kingdom. The system design is complete and reportedhere. The event builder design has started and the electronicsdesign is taking place during 2000/1. Detectors are on order andit is anticipated that the system will be assembled and testedduring 2001 for experiments in 2002.VI. CONCLUSIONThe TDR method opens new possibilities for the study ofexotic nuclei by allowing the collection of very rare events ina system that simultaneously collects high rate data withoutdead-time problems. The only losses are due to pileup in theshaping amplifiers, which limits the rate on any one channel. IfGREAT works as expected, then in the future TDR will becomethe architecture of choice for large nuclear structure projects andhardware trigger systems will become obsolete.REFERENCES[1] P. A. Butler, “New developments in electron and recoil decay spec-troscopy for studies of exotic and heavy nuclei,” Acta Phys. PolonicaB, vol. 31, no. 1, pp. 9–19, 2000.[2] M. Leino, J. Äystö, T. Enqvist, P. Heikkinen, A. Jokinen, and M. Nurmiaet al., “Gas-filled recoil separator for studies of heavy elements,” Nucl.Instrum. Meth. B, vol. 99, p. 653, 1995.[3] “VEGA, a proposal for Versatile and Efficient GAmma-detectors,” GSI,Darmstadt, Internal Rep., 1998.[4] K. Helariutta, J. F. C. Cocks, T. Enqvist, P. T. Greenlees, P. Jones, andR. Julin et al., “Gamma-spectroscopy of 192-195 Po,” Eur. Phys. J., vol.A6, p. 289, 1999.[5] P. A. Butler, P. M. Jones, K. J. Cann, J. F. C. Cocks, G. D. Jones, and R.Julin et al., “Electron spectroscopy using a multi-detector array,” Nucl.Instrum. Meth. A, vol. 381, p. 433, 1996.[6] E. S. Paul, P. J. Woods, T. Davinson, R. D. Page, P. J. Sellin, and C. W.Beausang et al., “In-beam Gamma-ray spectroscopy above 100Sn usingthe new technique of recoil delay tagging,” Phys. Rev. C, vol. 51, no. 1,pp. 78–87, Jan. 1995.[7] Analog Devices, ADSP-2106x SHARC User’s Manual, Mar. 1995, ch.9.

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The GREAT triggerless total data readout method

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