2024/7/18
Andrea & Giovanni

This page is meant to summarise the general plan, goals and list of steps for the GAGG project roughly to the end of this year. A first important deadline is the Low Radioactivity Techniques 2024 conference (1-4 October, Krakow, Poland). We want to give a presentation of the first results of GAGG project. Deadline for abstract submission: August 18^th.

The outcome of the work in the medium term will be the subject of our first paper, which we outline at the end of this note.

Delayed coincidences.

• Identification of the events belonging to the triplet ²²⁴Ra (5.7 MeV α) → ²²⁰Rn (6.2 MeV α, t_1/2= 55 s) → ²¹⁶Po (6.8 MeV α, t_1/2=0.14 s). The strategy is to start from the last couple (the one with t_1/2=0.14 s) and then go backward by about 3*55 s = 165 s (or maybe 5*55 s = 275 s) to tag the first couple. We want to check the time distribution and the energies of the different signals in the decay (2D plot). We can also look at the delay in inverted order to check the background.
• Identification of the doublet ²²²Rn (5.5 MeV α) → ²¹⁸Po (6.0 MeV α, t_1/2=3.1 minutes).
• Identification of the doublet ²¹⁹Rn (6.8 MeV α) → ²¹⁵Po (7.4 MeV α, t_1/2=1.78 ms), belonging to ²³⁵U chain.
• With the correct identification of the events from ²³⁵U chain, we can clean the α region with energy between 6.5 and 7.5 MeV approximately. We can then proceed and look for the doublet ²¹⁴Bi (β/γ decays with different energies) → ²¹⁴Po (7.7 MeV α, t_1/2= 164 μs).
• BONUS: We can also try to look for ²¹²Bi (β/γ up to 2.25 MeV) → ²¹²Po (8.8 MeV α, t_1/2 = 299 ns). Given the very short half-life, we need to look for the α decays in the same waveform as the β/γ decay.

α energy scale.
Excluding the BONUS α peak at 8.8 MeV, in the most optimistic case we could count on a total of 9 peaks to build the α energy scale: ¹⁵²Gd (2.2 MeV), ²²²Rn (5.5 MeV), ²²⁴Ra (5.7 MeV), ²¹⁸Po (6.0 MeV), ²²⁰Rn (6.2 MeV), ²¹⁶Po (6.8 MeV), ²¹⁹Rn (6.8 MeV), ²¹⁵Po (7.4 MeV) and ²¹⁴Po (7.7 MeV).

Cleaning of the region above ²⁰⁸Tl peak.
We want to identify ad exclude those events which populate the region above the energy of ²⁰⁸Tl peak (2.6 MeV), which constitute a background for the γ from neutron captures. The idea is to exploit the coincident decay ²¹²Bi (6 MeV α, BR=36%) → ²⁰⁸Tl (γ decays only when above 2.6 MeV, t_1/2 = 3.1 minutes).

Here two points are relevant:

• A good α energy scale to properly select the correct energy range for the α from ²¹²Bi decay.
• The ability to identify as much events from other delayed coincidences as possible, to clean up the α population and identify more easily the α from ²¹²Bi decay.

We need to assess the lifetime loss we get when cleaning the data with this method. If it stays around or less than 20%, it would be feasible to apply this quality cuts. Higher value of lifetime loss would question this selection. Of course this has also to be compared with the background reduction we get in the region of interest.

Analysis of thermal neutrons.
We took thermal neutron data for about 30 days (to be checked). We can use these data for a measurement of the thermal neutrons in our detector. A rough calculation shows that we may expect >5 events on top of ≤10 background events in 30 days.

• Implementing resampling for the acquisition of LiI data. → Done (Gio, 29.07.2024)
• Implementing multi-channel DAQ.
• Installing and reading temperature sensor.

• We need to fix the timestamp variable.
• Write a module in octopus that determines the position of ⁴⁰K peak run-by-run and correct the energy scale accordingly.
• Write a module in octopus which provides both the α and β energy scale for each event.
• Improve on the discrimination of pile-up events (Giovanni has some ideas).

LiI

Next week we will take AmBe calibration with LiI crystal. In view of the validation of simulations, we also plan to acquire γ-n data (with the Ra-Be source). We want to check particle discrimination via PSD.

GAGG

We want to check if the use of the filtered amplifier will improve the PSD. We also plan to take some data with a thin copper foil around the crystal. Some data can be taken with Pb and Cu shield. An idea for the future is to take data with the GAGG above ground and check if we can discriminate muons, protons and neutrons.

BrLa

Next week we should be able to acquire data with the BrLa crystal. We need to measure its background taking some data with Cu and Pb shield. We will check the possibility of PSD. We will study the region above ²⁰⁸Tl. We plan to couple the BrLa crystal to some Gd sulfate to detect γ from neutron capture.

This topic is currently much less developed than the others. Giovanni wants to start setting up the general infrastructure for the simulations, but this will require some time. After this framework is set we can start sharing tasks for the development of simulations.

In the meanwhile, it's possible to start playing with MCNP, which deals with neutron propagation. It is available on gagg-nd computer, currently in benato folder. The documentation is also available there.

The following is the general structure we imagine for the first paper.

1. Description of the detector
2. Description of the setup for the measurements
3. Processing, reconstruction and PSD
4. Background characterization: delayed coincidences and contaminations
5. Results: intrinsic background, estimation of neutron detection efficiency, thermal neutrons
6. Outlook: planned work on simulations towards the final setup