Understanding the intrinsic radioactivity energy spectrum from 176Lu in LYSO/LSO scintillation crystals

Abstract

Lutetium oxyorthosilicate (LSO) or lutetium yttrium oxyorthosilicate (LYSO) are the scintillator materials most widely used today in PET detectors due to their convenient physical properties for the detection of 511 keV annihilation photons. Natural lutetium contains 2.6% of ¹⁷⁶Lu which decays beta to excited states of ¹⁷⁶Hf producing a constant background signal. Although previous works have studied the background activity from LSO/LYSO, the shape of the spectrum, resulting from β-particle and γ radiation self-detection, has not been fully explained. The present work examines the contribution of the different β-particle and γ-ray interactions to provide a fuller comprehension of this background spectrum and to explain the differences observed when using crystals of different sizes. To this purpose we have shifted the continuous β-particle energy spectrum of ¹⁷⁶Lu from zero to the corresponding energy value for all combinations of the isomeric transitions of ¹⁷⁶Hf (γ-rays/internal conversion). The area of each shifted β-spectrum was normalized to reflect the probability of occurrence. To account for the probability of the γ-rays escaping from the crystal, Monte Carlo simulations using PENELOPE were performed in which point-like sources of monoenergetic photons were generated, inside LYSO square base prisms (all 1 cm thick) of different sizes: 1.0 cm to 5.74 cm. The analytic distributions were convolved using a varying Gaussian function to account for the measured energy resolution. The calculated spectra were compared to those obtained experimentally using monolithic crystals of the same dimensions coupled to SiPM arrays. Our results are in very good agreement with the experiment, and even explain the differences observed due to crystal size. This work may prove useful to calibrate and assess detector performance, and to measure energy resolution at different energy values.

Figure 1

(a) Simplified ¹⁷⁶Lu decay scheme and (b) β-particle energy spectrum corresponding to the β₁ transition.

Figure 2

Energy spectra of the different β + γ-ray/internal conversion combinations with (a) higher and (b) lower probability values. (c) Energy spectra obtained from the sum of the individual contributions. Results are shown for two crystal sizes: small cube 1 cm on the left and large 1 cm thick square base prism on the right.

Figure 3

Analytical (solid line), convolved with a varying Gaussian kernel, and experimental (dashed line) LYSO normalized energy spectra.

Figure 4

(a) Photon absorption probabilities and (b) Energy deposition probabilities as a function of crystal square base size. All crystals are square prisms 1.0 cm thick. Values calculated from Monte Carlo simulations with PENELOPE as described in the text.

Figure 5

Calculated LYSO energy spectra for 1.0 cm thick square base prisms of different sizes.