Investigation and development of thin diamond detectors

Iznimna električna, optička, toplinska i mehanička svojstva čine dijamant materijalom koji bi u skoroj budućnosti mogao zamijeniti siliciji u standardnoj upotrebi za izradu raznih elektroničkih sklopova, ali i detektora zračenja. Ključnu ulogu pritom ima napredak u proizvodnji umjetnih dijamantnih kristala postupkom kemijske depozicije ugljika iz parne faze (CVD). U ovom je radu razvijen transmisijski dijamantni detektor koji istovremeno služi i kao vakuumski prozor za izlaz ubzanih iona iz vakuumskog sustava ionskog ubrzivača u atmosferu. Uređaj se temelji na ultra-tankoj dijamantnoj membrani proizvedenoj jetkanjem debljeg CVD dijamantnog uzorka Ar-O2 plazmom. Zahvaljujući visokoj tvrdoći dijamanta, 6 mikrometara debela membrana u stanju je podnijeti znatno naprezanje uslijed razlike u tlakovima od 1 bar. Metalizacijom nasuprotnih strana dijamantne membrane i uspostavom električnog polja unutar detektora dolazi do stvaranja mjerljivog električnog signala pri upadu nabijenih čestica. Ostvarena, gotovo 100%-tna efikasnost detekcije čak i za ubrzane protone, vrlo je bitna kad se uzorci postavljeni izvan vakuumske komore žele ozračiti točnim brojem čestica, kao što je slučaj kod npr. ozračivanja živih stanica pojedinačnim ionima. Također, testovi otpornosti na zračenje pokazali su očekivano bolje ponašanje 6 mikormetarskog dijamantnog detektora u odnosu na 50 mikrometarski dijamantni i silicijski detektor, zbog čega je njegova primjena moguća i u uvjetima velikih struja čestica. Ipak, uočeni efekt polarizacije dijamantnih detektora predstavlja za sada nepremostivi problem pri detekciji teških iona kratkog dosega. Tada dolazi do djelomičnog poništenja narinutog električnog polja pa signal iz detektora opada.Because of the outstanding electrical, optical, thermal and mechanical properties, diamond is considered as a potential substitute that could replace silicon in production of standard electronic devices. A large success in growth of high quality synthetic diamonds by the chemical vapour deposition method has initiated research directed towards the application of this material for construction of charged particle detectors. In the present work, the development and analysis of a novel transmission diamond detector, based on the ultra-thin diamond membrane, are described. The 6 μm thick membrane was prepared by Ar-O2 plasma etching and serves simultaneously as a vacuum window which enables the extraction of the ion beam from the vacuum system of the accelerator into the atmosphere. Metallization of the opposite surfaces on the membrane allows for the electric field to be applied to the bulk of diamond plate. An impinging ion produces now a measurable electrical signal that can be recorded by standard electronic chain. A hit detection efficiency of close to 100%, even for energetic protons, makes the device ideal for applications that require accurate and controlled dose delivery to a sample that is placed outside of the vacuum chamber. Moreover, the results of the radiation-hardness test indicate a wider possible range of device applications, including those that involve high currents of charged particles or long exposure to radiation. However, the detection of short range ions is disturbed by the polarization effect which occurs due to deep level traps that are present in even the best quality diamond crystals

Creation and characterization of He-related color centers in diamond

Diamond is a promising material for the development of emerging applications in quantum optics, quantum information and quantum sensing. The fabrication and characterization of novel luminescent defects with suitable opto-physical properties is therefore of primary importance for further advances in these research fields. In this work we report on the investigation in the formation of photoluminescent (PL) defects upon MeV He implantation in diamond. Such color centers, previously reported only in electroluminescence and cathodoluminescence regime, exhibited two sharp emission lines at 536.5 nm and 560.5 nm, without significant phonon sidebands. A strong correlation between the PL intensities of the above-mentioned emission lines and the He implantation fluence was found in the 10^15-10^17 cm^{-2} fluence range. The PL emission features were not detected in control samples, i.e. samples that were either unirradiated or irradiated with different ion species (H, C). Moreover, the PL emission lines disappeared in samples that were He-implanted above the graphitization threshold. Therefore, the PL features are attributed to optically active defects in the diamond matrix associated with He impurities. The intensity of the 536.5 nm and 560.5 nm emission lines was investigated as a function of the annealing temperature of the diamond substrate. The emission was observed upon annealing at temperatures higher than 500{\deg}C, at the expenses of the concurrently decreasing neutral-vacancy-related GR1 emission intensity. Therefore, our findings indicate that the luminescence originates from the formation of a stable lattice defect. Finally, the emission was investigated under different laser excitations wavelengths (i.e. 532 nm and 405 nm) with the purpose of gaining a preliminary insight about the position of the related levels in the energy gap of diamond

The evaluation of radiation damage parameter for CVD diamond

There are a few different phenomenological approaches that aim to track the dependence of signal height in irradiated solid state detectors on the fluence of damaging particles. However, none of them are capable to provide a unique radiation hardness parameter that would reflect solely the material capability to withstand high radiation environment. To extract such a parameter for chemical vapor deposited (CVD) diamond, two different diamond detectors were irradiated with proton beams in MeV energy range and subjected afterwards to ion beam induced charge (IBIC) analysis. The change in charge collection efficiency (CCE) due to defects produced was investigated in context of a theoretical model that was developed on the basis of the adjoint method for linearization of the continuity equations of electrons and holes. Detailed modeling of measured data resulted with the first known value of the kr product for diamond, where k represents the number of charge carriers’ traps created per one simulated primary lattice vacancy and r represents the charge carriers’ capture cross section. As discussed in the text, this product could be considered as a true radiation damage parameter

Dose- and Volume-Limiting Late Toxicity of FLASH Radiotherapy in Cats with Squamous Cell Carcinoma of the Nasal Planum and in Mini Pigs

Purpose: The FLASH effect is characterized by normal tissue sparing without compromising tumor control. Although demonstrated in various preclinical models, safe translation of FLASH-radiotherapy stands to benefit from larger vertebrate animal models. Based on prior results, we designed a randomized phase III trial to investigate the FLASH effect in cat patients with spontaneous tumors. In parallel, the sparing capacity of FLASH-radiotherapy was studied on mini pigs by using large field irradiation. Experimental Design: Cats with T1-T2, N0 carcinomas of the nasal planum were randomly assigned to two arms of electron irradiation: arm 1 was the standard of care (SoC) and used 10 × 4.8 Gy (90% isodose); arm 2 used 1 × 30 Gy (90% isodose) FLASH. Mini pigs were irradiated using applicators of increasing size and a single surface dose of 31 Gy FLASH. Results: In cats, acute side effects were mild and similar in both arms. The trial was prematurely interrupted due to maxillary bone necrosis, which occurred 9 to 15 months after radiotherapy in 3 of 7 cats treated with FLASH-radiotherapy (43%), as compared with 0 of 9 cats treated with SoC. All cats were tumor-free at 1 year in both arms, with one cat progressing later in each arm. In pigs, no acute toxicity was recorded, but severe late skin necrosis occurred in a volume-dependent manner (7–9 months), which later resolved. Conclusions: The reported outcomes point to the caveats of translating single-high-dose FLASH-radiotherapy and emphasizes the need for caution and further investigations. See related commentary by Maity and Koumenis, p. 363

Electrical stimulation of non-classical photon emission from diamond color centers by means of sub-superficial graphitic electrodes

Focused MeV ion beams with micrometric resolution are suitable tools for the direct writing of conductive graphitic channels buried in an insulating diamond bulk, as already demonstrated for different device applications. In this work we apply this fabrication method to the electrical excitation of color centers in diamond, demonstrating the potential of electrical stimulation in diamond-based single-photon sources. Differently from optically-stimulated light emission from color centers in diamond, electroluminescence (EL) requires a high current flowing in the diamond subgap states between the electrodes. With this purpose, buried graphitic electrode pairs, 10 μm spaced, were fabricated in the bulk of a single-crystal diamond sample using a 6 MeV C microbeam. The electrical characterization of the structure showed a significant current injection above an effective voltage threshold of 150 V, which enabled the stimulation of a stable EL emission. The EL imaging allowed to identify the electroluminescent regions and the residual vacancy distribution associated with the fabrication technique. Measurements evidenced isolated electroluminescent spots where non-classical light emission in the 560-700 nm spectral range was observed. The spectral and auto-correlation features of the EL emission were investigated to qualify the non-classical properties of the color centers