CERN Accelerating science

Image of the beam spot captured from the MCP+phosphor screen setup captured by a CCD camera. The dashed ellipse indicates the beam size at the full width at half maximum level. The dimensions of this ellipse are $1.5\,\mathrm{mm}\times 2.7$\,mm. The $x$ and $y$ axes correspond to the horizontal and vertical beam position, respectively. Axis tick labels are rounded to the nearest decimal in mm.

The $\alpha$-decay spectrum of the CaF$_2$ 350 sample (see Tab. \ref{tab:crystal-specs} for details of the crystal), the $\alpha$-lines from the $A=229$ (square), $A=228$ (triangle), and $A=227$ (circle) decay chains are indicated.

(Color online) Cherenkov spectra from $A=230$ implantations for CaF$_2$ (circles), MgF$_2$ (triangles), LiSrAlF$_6$ (x's), AlN (squares) and SiO$_2$ (diamonds) normalized by the activity deduced from the $\gamma$-data recording during implantation. Broad radioluminescence peaks are present in CaF$_2$ bulk at approximately 142\,nm and 177\,nm. All spectra were taken with a measurement time of 10 seconds per data point and a step size of 100 motor positions between each point.

Typical spectra taken with a 2\,mm (a-g) and 250\,$\mu$m (h,i) entrance slit demonstrating the fitting method for the radiative decay of $^{229m}$Th. Measurements (a-g) were taken with a measurement time of 10 seconds per point and a step size of 100 motor positions between each point. Measurements (h,i) were taken with a 10 second measurement time per point and a step size of 15 motor positions between each point. The dashed curve is a Gaussian peak in (a to e) and a Lorentzian peak in (h and i). The dot-dashed curve is a polynomial representing the Cherenkov background radiation and the dotted curve is the radioluminescence peak possibly due to a color center. The solid black line is the dark count rate from the PMT. The reduced Cherenkov background in case of spectra taken with the thin film crystals (a, d, h) did not allow to distinguish clearly between the Cherenkov background from the dark countrate. (a) CaF$_2$ 350 thin film, (b) MgF$_2$ bulk, (c) CaF$_2$ bulk, (d) CaF$_2$ 850, (e) LiSrAlF$_6$, (f) AlN, and (g) SiO$_2$, (h) CaF$_2$ 350 and (i) LiSrAlF$_6$.

Radiative decay fractions for each crystal relative to CaF$_2$ bulk. An upper limit is reported for AlN and SiO$_2$ due to the absence of a signal, therefore a downward-pointing arrow is shown for both of these crystals.

(a): DFT structure 1 for the Th defect in SiO$_2$. Th is shown in green, Si in blue, and O in red. (b): DFT structure 2 for the Th defect in SiO2. (c) Thorium projected density of states (PDOS) in Th:SiO2 defect structure 1. Energies are relative to the highest occupied band, and the black dashed line at 8.355 eV marks the $^{229}$Th isomer transition energy (note that since the nuclear excited state is not an electronic state, marking it relative to the Fermi energy is an arbitrary choice). (d) Thorium PDOS in structure 2.

An example spectrum of the nitrogen line at 141\,nm used for the offset determination. The solid line is the fit to the data.