Optical and THz investigations of mid-IR materials exposed to alpha particle irradiation

The paper is the first comprehensive study on alpha particle irradiation effects on four mid-IR materials: CaF2, BaF2, Al2O3 (sapphire) and ZnSe. The measurements of the optical spectral transmittance, spectral diffuse reflectance, radioluminescent emission, terahertz (THz) spectral response, transmittance, absorbance, refractive index, real and imaginary parts of the dielectric constant and THz imaging are used as complementary investigations to evaluate these effects. The simulations were run to estimate: (i) the penetration depth, (ii) the scattering of alpha particle beam, (iii) the amount of material affected by this interaction, and (iv) the number of vacancies produced by the radiation exposure for each type of material. The simulation results are compared to the off-line measurement outcomes. The delay and spectral composition change of the reflected THz signal highlight the modification induced in the tested materials by the irradiation process.


Supplementary information Additional results
Supplementary Fig. 1 reproduces the simulation of alpha particle penetration depths and scattering patterns in BaF 2 , Al 2 O 3 , ZnSe. In Supplementary Fig. 2 are given the penetration depth and the sample mass affected by alpha particle irradiation as resulted from the simulations.
Supplementary Figure 2. Penetration depth and the optical material mass affected by alpha particle irradiation for the four windows, simulations done for a beam charge of 100 µC.
The results of the simulations regarding the number of vacancies produce by alpha particle incidence on each tested sample are provided in Supplementary Fig. 3. irradiated mid-IR optical materials: a -CaF 2 before irradiation; b -CaF 2 after irradiation (total dose 17.52 MGy); c -BaF 2 before irradiation; d -BaF 2 after irradiation (total dose 109.2 MGy); e -sapphire before irradiation (total dose 23.16 MGy); f -sapphire after irradiation; g -ZnSe before irradiation; h -ZnSe after irradiation (total dose 12 MGy).
The variation of the optical spectral transmittance of the samples before and after irradiation is illustrated in Supplementary Fig. 7. The degradation of samples' surface quality under alpha particle irradiation can be estimated from the optical spectral diffused reflectance measurements ( Supplementary Fig. 9). The reflected THz signal variation in frequency domain as it is changed by alpha particle irradiation is illustrated in Supplementary Fig. 11. The ration of the two signals is given in the inset. For readers' convenience, the water related absorption bands were removed.   h -ZnSe sample irradiated at the dose of 6 MGy.

Investigated samples
Considering the incidence of some mid-IR materials used in spectroscopy applications we studied: CaF 2 , BaF 2 , ZnSe and sapphire (Supplementary Table 1) as they were exposed to alpha particle irradiation. For reader's convenience we included some data sheets characteristics typical for these materials. The irradiations were run in several subsequent steps, and, before and after each irradiation, their characteristics were measured in the optical and THz spectral range.
We included in the Table S1 general data concerning optical, mechanical and thermal specifications, as well as information about the density and molar mass of the investigated specimen, as they are of interest in evaluating each material's behaviour during the interaction with alpha particles, considering that: (i) the irradiation is performed in vacuum without assisted cooling; (ii) the absorbed dose, the penetration depth and the degradation of the sample surface quality are depended on the characteristics of exposed material.   Table 1. Characteristics of the investigated mid-IR optical materials.

Alpha particle irradiation and simulations
Four series of irradiation with 3 MeV alpha particles were performed at the U-120 Cyclotron accelerator, in operation at the Horia Hulubei" National Institute of Physics and Nuclear Engineering, having individual doses corresponding to a charge of: 100 µC, 200 µC, 300 µC, 300 µC. The equivalent accumulated total charges for each sample were in this case: 100 µC, 300 µC, 600 µC and 900 µC, respectively. Special mounts were designed to hold the windows during the irradiation as these were mounted in a XYZ micro positioning stage (travel range = 10 mm, travel resolution = 0.001 mm), which made possible the precise repositioning of the sample irradiated area, for subsequent irradiations. The alpha particle beam cross section (A), as measured by exposing a sensitive film to the beam, was appreciatively 6 mm 2 . During the irradiations, the beam current was kept at 100 nA, while the focusing system current was 20 nA, in order to assure reproductively the beam diameter at the sample surface.
In Supplementary Table 2 are given the irradiation conditions for each type of mid-IR optical material and the corresponding irradiation doses. The volume (V) of optical material affected by the alpha particles is: where A is alpha particle beam cross section, and d denotes the penetration depth.
The mass (m) of the mid-IR optical particle affected by the interaction is: where ρ designates the optical material density.
The equivalent dose (H) received by the sample upon an irradiation is computed as follows: For on-line radioluminescence measurements, a UV-visible 400 µm core diameter multimode optical fiber was installed in the experimental vacuum chamber (Supplementary Fig. 20) and was coupled by a vacuum feedthrough to a high sensitive QE65000 Ocean Optics optical fiber spectrometer. Radioluminescence data were collected using the spectrometer native Spectral Suite software. The box car and the average values were 1, while the integration time was 10 s (ZnSe), and 20 s (CaF 2 , BaF 2 , Al 2 O 3 ). The spectra were acquired over the spectral range from 200 nm to 1 µm.
Supplementary Figure 20. The optical fiber-based setup for on-line, under vacuum, radioluminescence monitoring during alpha particle irradiation.
The SRIM application was used to estimate the alpha particle beam penetration in each mid-IR

Optical investigations
The optical investigations were performed prior to irradiation and after each irradiation step and consist of: -visual and microscopic inspection of the samples for irradiation induced color change and radiation induced window's surface roughness modification; -spectral measurements of the optical transmittance and optical diffused reflectance, over a wide spectral range, appropriate to each investigated optical material.

THz spectral investigations THz imaging
The TeraView TPS Spectra 3000 spectrometer was used with the Transmittance and the Reflectance Imaging Module. Supplementary Figure 3 provide information on the THz reflectance based imaging geometry, indicating the plans or axis used to represent the THz detected signal prior and post irradiation.
Supplementary Figure 21. The detection geometry used in THz reflectance imaging. The change of material properties upon the irradiation produces a delay in the detected pulse as referred to the interrogation pulse.