2024/2/27, AndreaM

We analyse roughly one month of data acquired with the GAGG underground, from 2024/1/15 to 2024/2/12. The dataset is 29 days long, the total number of acquired events is 38193181, which corresponds to a rate of ∼15 Hz.

We consider the discrimination parameter normamplitude η=Amplitude/Integral and we define the β/γ and α bands as follow:

• β/γ : 0.03 < η <0.1
• α : 0.01 < η < 0.03

The 2D plot in Figure 1 shows η vs Integral for the events.

Figure 1
Total 2D plot η vs Integral	Zoom-in for Integral<82000

The large majority of the events (96%) is falling in the β/γ band. Beyond ²⁰⁸Tl peak there is a first population of order 900 events probably due to the overlap of the 2.6 MeV γ with another lower energy γ (583 keV, 511 keV,…). Then there is another population (only 8 events) around Integral = 150-200*10³ which maybe is due to other γ overlaps or are we seeing some first γ from Gd-n capture?
NOTE: The range 150-200*10³ in ADC counts corresponds to 5.8-7.7 MeV (as shown later in the note).

The β/γ band.

Figure 2

We recognize two clear peak in this spectrum: one around 40*10³ which is due to the 1.5 MeV ⁴⁰K decay and one at 70*10³ from the 2.6 MeV ²⁰⁸Tl decay. Even if very preliminary, we can try a simple gaussian fit of these peaks.

Figure 3
40K fit	208Tl fit

Just to have a feeling, the following table reports the fit result and the first estimation of the energy resolution.

The α band.

Figure 4

For the α events we have to consider a quenching effect. We can try to have a first estimation looking at some characteristic peaks of the α spectrum.

• The narrow peak below 10⁴ is due to the α-decay of ¹⁵²Gd at 2.2 MeV. This represents the dominant feature in the α spectrum.
• The peak slightly below 2*10⁴ is probably a mixture of the ²³⁸U α-decays at 4.2 and 4.15 MeV. In the left tail some other α-decay is probably present, maybe the ∼4 MeV decays from ²³²Th.
• The peak between 2-2.5*10⁴ can be tentatively attributed to ²³⁴U decay at 4.78 and 4.72 MeV.
• Finally with the delayed coincidence analysis we have identified the peak between 3.5-4*10⁴ as due to ²¹⁶Po α-decay at 6.8 MeV.
  

We can try again a simple gaussian fit of these 4 peaks.

Figure 5
152Gd [2.2 MeV]	238U [4.2 MeV]
234U [4.8 MeV]	216Po [6.8 MeV]

The following table summarises the results of the fit, with a tentive estimation of the quenching effect with respect to the energy scale derived from the β/γ band (∼26000 ADC/MeV).

Next steps

1. Detailed study of α-α and α-γ delayed coincidences
2. Conversion of ADC counts in number of PE
3. Improve the discrimination parameter η
4. AmBe calibration to see neutron captures
5. Calibration with γ sources for detailed energy scale and resolution studies (?)