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Optimization of Agata Physics – OASIS

OASIS

The project aims is an attempt to optimize the Advanced GAmma-ray<br />Tracking Array presently constructed in Europe. It is a project<br />addressing two aspects. The first is improving the performance of a<br />gamma-ray tracking array, i.e. the determination of the interaction<br />positions and the gamma-ray spectra. An important aspect of a new<br />scientific instrument is the understanding of its response<br />function. The second part of the OASIS consecrates on the task of<br />providing this knowledge to the community.

OASIS has as objectives to improve the precision of the determination of the interaction position of gamma-rays, the gamma-ray tracking and the exploitation of AGATA data.

A large number of scientific publications have been produced using<br />AGATA and more are coming, clearly showing the high performance of<br />AGATA which steadily improves as more crystals are added to the<br />array. However, it also stands clear that there is a need for improved<br />performance in the pulse-shape analysis and gamma-ray tracking<br />algorithms, especially as AGATA increases in size and profits more and<br />more from the gamma-ray tracking concept. Also, as the performance<br />grows the physics performed will increase the demand on our<br />understanding of the characteristics of the AGATA detector system. The<br />objective of this ANR is to remove these obstacles by<br />- Improving the performance of the pulse shape analysis and gamma-ray<br /> tracking. Here the novelty as compared to present techniques would<br /> be the feed back from gamma-ray tracking to the PSA in an<br /> interactive manner. The other novelty would be to use the full<br /> pulse shape to improve the quality of the input into the<br /> pulse-shape algorithms. The use of identical digitizer as used in<br /> AGATA for experimental characterization of AGATA detectors is a <br /> necessary ingredient to achieve this goal.<br />- Translating known gamma-ray spectroscopy techniques to gamma-ray<br /> tracking arrays. An example would be angular distributions of<br /> quasi-continuum structures in gamma-ray spectra, deconvolution of<br /> gamma-ray spectra to extract average fold and energy etc.<br />Both these goals will be achieved by combining the evaluation of the<br />progress on in-beam data from the AGATA campaigns as well as from<br />dedicated measurements on single AGATA crystals with pulse shape<br />calculations and Monte Carlo simulations of AGATA. The project will<br />also give valuable knowledge about segmented HPGe<br />detectors. Furthermore, we intend to extract as much scientific<br />results as possible by<br />- The application of the above results when analyzing experiments<br /> from the AGATA campaigns.

Objective 1 will be addressed using numerical simulations. A code
developed by the scientific coordinator during the last decade, called
AGATAGeFEM solves the electric field and weighting potentials using
Finite Element methods with tetrahedral cells. The AGATAGeFEM code
will be used to investigate what deficiencies in the pulse-shape data
base that are responsible for the problems encountered by the
PSA. This will be done by generating a set of interaction points in
AGATA using the AGATA Monte Carlo simulation package, and then
calculating the corresponding pulse shapes with varying
parameters. These pulses will then be analyzed using the same code as
used for data analysis in AGATA. It is hoped that this way the
discrepancies between the modeled and real HPGe detectors used in
AGATA can be revealed.

Objective 2 will be addressed by adding information to the PSA
algorithms. In general the idea is to add information to pulse-shape
decomposition step to allow a better determination of the number of
interactions inside single segments. One possibility is to let PSA
suggest several solutions varying the number of interaction points in
a segment. A second road to follow would be to combine the PSA and
gamma-tracking steps into one minimization problem. One could even
imagine calculating the pulse shapes used as a part of the
minimization this way allowing a correct modeling of the
charge-carrier cloud extension and diffusion.

Objective 3 will be addressed by the use of concatenation of
experimental data to create high multiplicity events combined with
geant4 Monte Carlo simulations+Pulse shape calculations. By assuring
that we can reproduce the response of AGATA using simulations, these
will be a good tool to deconvolute experimental spectra. The ambition
of OASIS is to push the performance of AGATA so that physical results
that were not accessible before OASIS will be after.

The investigation of the possibility to perform «boot strapping« to
extract good error estimates from the PSA has been performed. The
results are in clear agreement with estimations obtained
in previous similar investigations but it seems difficult to use them for an
event-by-event error estimate. At the IPHC infrastructure to perform
scanning of AGATA detectors that will allow investigation of charge
cloud size effects have been put in place.

It has been identified during the OASIS project that a key features
that is missing for improving gamma-ray tracking is an error estimate
for the position coming from pulse-shape analysis. This is where the
OASIS project has started and it is also where more effort will be
put. This will mostly be done using pulse-shape calculations where
different input to the calculations will be varied before applying the
pulse-shape analysis of the AGATA. This calculations should also be
combined with data taken at Strasbourg using their scanning table.

For the aspect «exploitation« of AGATA, the work is now focused on
correctly extracting angular distributions and correlations by using
data from in-beam experiments where it is possible to calculate the
expected angular distributions.

The results of the work of the two post-docs, Sylvain Leblond and
Marco Siciliano have been presnted by them at international workshops
AGATA week and AGATA-Gretina Workshop.

The OASIS project aims at optimizing the science production of the Advanced GAmma-ray Tracking Array (AGATA) gamma-ray spectrometer. Presently installed at the Grand Accélérateur National d'Ions Lourds (GANIL) at Caen, France, AGATA has passed the demonstrator phase of its early implementation (15 high-purity germanium detectors) and now contains 32 such detectors with infrastructure to accommodate 45 detectors
covering 1pi of solid angle.

AGATA is a new generation gamma-ray spectrometer designed to overcome the inherent limitation of the previous generation of Compton suppressed HPGe detector arrays. By replacing the Anti-Compton shields, which occupy a significant amount of solid angle, with HPGe detectors solid angle converage, and hence efficiency, can be increased. However, for this approach to produce high quality gamma-ray spectra an alternative Compton suppression technique has to be developed. This is gamma-ray tracking: The energy and position of individual gamma-ray interaction points inside the HPGe is determined using highly segmented detectors combined with digital electronics and pulse-shape analysis.
These interaction points are then tested for the hypotheses that they belong to a fully absorbed gamma ray. For the gamma-ray tracking to work the gamma-ray interaction points have to be located to within 5 mm inside the detectors. A very important additional increase in performance comes from the very high effective angular granulation of AGATA given by knowing the interaction positions giving very good Doppler Correction capabilities, something very important in modern experimental nuclear structure research. Because of the high performance of AGATA it is considered a very important detector for the future and present nuclear structure research facilities in Europe, such as FAIR, HIE-ISOLDE, SPES, and SPIRAL2.

Since the first physics campaign with AGATA started has showed its high performance in experimental situation where the sensitivity is dominated by the Doppler broadening of the gamma-ray peaks, for high-count rate situations, and when it is beneficial to have a very compact gamma-ray spectrometer - AGATA has proven the be a technical success in many ways.

During the work analyzing experimental data the AGATA collaboration, and the gamma-ray tracking community, has however seen that the performance of AGATA in terms of Compton suppression from the gamma-ray tracking is not what simulations suggests it should be. It is believed in the gamma-ray tracking community that cause for this is related to problems with the pulse-shape analysis. Although the nominal position resolution
from the pulse-shape analysis is within the required limits several indications points to that the pulse-shape analysis does not perform as good as is needed. The OASIS project aims at carefully investigating the reasons for this using computer simulations to try to reproduce and understand the deficiencies seen in experimental data. One particular problem that will be addressed within the OASIS project is that of correctly determining the number of actual interaction that a gamma-ray has had with the AGATA.Several novel ideas are to be investigated.

Finally, many aspects of analyzing -ray spectroscopy data have to be reviewed when using AGATA. This mainly comes from the fact that there is more detailed information to look at offering new possibilities. What was previously simple calibration procedures using source data, such as efficiency calibrations, now has complex dependencies on the experimental situation and choices made for the gamma-ray tracking algorithms. Other methods, e.g. to determine angular correlations and distributions, also need to be developed specifically for gamma-ray tracking. A part of OASIS is dedicated to this work, making sure that the gamma-ray tracking community will have thoroughly tested and quantified procedures.

Project coordinator

Madame Araceli Lopez-Martens (Centre de Sciences Nucléaires et de Sciences de la Matière)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IPHC Institut pluridisciplinaire Hubert Curien
GANIL Grand accélérateur national d'ions lourds
IRFU Institut de Recherche sur les lois Fondamentales de l'Univers
CSNSM Centre de Sciences Nucléaires et de Sciences de la Matière

Help of the ANR 386,437 euros
Beginning and duration of the scientific project: June 2018 - 36 Months

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