V. Sopko

The evaporative cooling system for the ATLAS inner detector

This paper describes the evaporative system used to cool the silicon detector structures of the inner detector sub-detectors of the ATLAS experiment at the CERN Large Hadron Collider. The motivation for an evaporative system, its design and construction are discussed. In detail the particular requirements of the ATLAS inner detector, technical choices and the qualification and manufacture of final components are addressed. Finally results of initial operational tests are reported. Although the entire system described, the paper focuses on the on-detector aspects. Details of the evaporative cooling plant will be discussed elsewhere.

Measurement of energy levels in a silicon detector damaged by neutrons

The level of defects in a semiconductor silicon detector diode made of high resistivity N type material and exposed to neutrons in a research nuclear reactor was examined by measuring the thermally stimulated current (TSC). A modified TSC method was employed where the released charge was measured in the reverse direction on a diode with zero bias voltage. Electrons captured in cooled traps due to the photoelectric effect are released when the material is heated. The detector was irradiated with an integral neutron flux of 7.63 × 1015 n/cm2.

The use of contrast agent for imaging biological samples

The technique of X-ray transmission imaging has been available for over a century and is still among the fastest and easiest approaches to the studies of internal structure of biological samples. Recent advances in semiconductor technology have led to the development of new types of X-ray detectors with direct conversion of interacting X-ray photon to an electric signal. Semiconductor pixel detectors seem to be specially promising; compared to the film technique, they provide single-quantum and real-time digital information about the objects being studied. We describe the recently developed radiographic apparatus, equipped with Medipix2 semiconductor pixel detector. The detector is used as an imager that counts individual photons of ionizing radiation, emitted by an X-ray tube (micro- or nano-focus FeinFocus). Thanks to the wide dynamic range of the Medipix2 detector and its high spatial resolution better than 1μm, the setup is particularly suitable for radiographic imaging of small biological samples, including in-vivo observations with contrast agent (Optiray). Along with the description of the apparatus we provide examples of the use iodine contrast agent as a tracer in various insects as model organisms. The motivation of our work is to develop our imaging techniques as non-destructive and non-invasive. Microradiographic imaging helps detect organisms living in a not visible environment, visualize the internal biological processes and also to resolve the details of their body (morphology). Tiny live insects are an ideal object for our studies.

High resolution radiography of ambers with pixel detectors

Radiography serves as a powerful non-destructive technique for studying inner structure of biological samples and materials. In the last years X-ray imaging has taken advantage of the developments in instrumentation such as table-top micro-focus X-ray tubes and quantum counting pixel detectors. The imaging setups used for our measurements allow for the observation of tiny samples including fossils in amber. The main goal of the study was to apply microradiography as representative of non-destructive and non-invasive methods for imaging fossils in amber. Those fossils are generally not easy to visualize, especially their internal structures. We investigated a combination of sources and detectors: (a) an X-ray unit for mammography with tungsten anode, emissive spot of 100 μm and an amorphous selenium imager; (b) a micro-focus X-ray tube with tungsten anode, emissive spot of 5 μm and a flat panel imager; (c) a nano-focus X-ray tube with tungsten anode, with gauge of emissive spot of 1 μm and as imager the pixel semiconductor detector Medipix2. The study of fossils in amber can be for example not well visible because of the presence of organic detritus from various sources. The amber preserves various ancient biological objects which are fully or partly saturated with amber resin. These samples attenuate X-rays similarly, but the use of pixel detectors enables capturing these differences, without permanent destruction of the samples (cracking, slicing, etc.). Microradiographic studies are completed by the observation of amber fossils in scanning and transmission electron microscopes.

Study of PIN diode energy traps created by neutrons

Characterization of radiation defects is still ongoing and finds greater application in the increasing radiation doses on semiconductor detectors in experiments. Studying the changes of silicon PIN diode for high doses of radiation is the fundamental motivation for our measurements. In this article we describe the behavior of the PIN diode and development of the disorder caused by neutrons from a 252Cf and doses up to 8 Gy. The calibration curve for PIN diode shows the effect of disorders as the changes of the voltampere characteristics depending on the dose of neutron irradiation. The measured values for defects are in good agreement with created energy traps.

Microradiography of biological samples with Timepix

Microradiography is an imaging technique using X-rays in the study of internal structures of objects. This rapid and convenient imaging tool is based on differential X-ray attenuation by various tissues and structures within the biological sample. The non-absorbed radiation is detected with a suitable detector and creates a radiographic image. In order to detect the differential properties of X-rays passing through structures sample with different compositions, an adequate high-quality imaging detector is needed. We describe the recently developed radiographic apparatus, equipped with Timepix semiconductor pixel detector. The detector is used as an imager that counts individual photons of ionizing radiation, emitted by an X-ray tube FeinFocus with tungsten, copper or molybdenum anode. Thanks to the wide dynamic range, time over threshold mode — counter is used as Wilkinson type ADC allowing direct energy measurement in each pixel of Timepix detector and its high spatial resolution better than 1μm, the setup is particularly suitable for radiographic imaging of small biological samples. We are able to visualize some internal biological processes and also to resolve the details of insects (morphology) using different anodes. These anodes generate different energy spectra. These spectra depend on the anode material. The resulting radiographic images varies according to the selected anode. Tiny live insects are an ideal object for our studies.

Biological object recognition in μ-radiography images

This study presents an applicability of real-time microradiography to biological objects, namely to horse chestnut leafminer, Cameraria ohridella (Insecta: Lepidoptera, Gracillariidae) and following image processing focusing on image segmentation and object recognition. The microradiography of insects (such as horse chestnut leafminer) provides a non-invasive imaging that leaves the organisms alive. The imaging requires a high spatial resolution (micrometer scale) radiographic system. Our radiographic system consists of a micro-focus X-ray tube and two types of detectors. The first is a charge integrating detector (Hamamatsu flat panel), the second is a pixel semiconductor detector (Medipix2 detector). The latter allows detection of single quantum photon of ionizing radiation. We obtained numerous horse chestnuts leafminer pupae in several microradiography images easy recognizable in automatic mode using the image processing methods. We implemented an algorithm that is able to count a number of dead and alive pupae in images. The algorithm was based on two methods: 1) noise reduction using mathematical morphology filters, 2) Canny edge detection. The accuracy of the algorithm is higher for the Medipix2 (average recall for detection of alive pupae =0.99, average recall for detection of dead pupae =0.83), than for the flat panel (average recall for detection of alive pupae =0.99, average recall for detection of dead pupae =0.77). Therefore, we conclude that Medipix2 has lower noise and better displays contours (edges) of biological objects. Our method allows automatic selection and calculation of dead and alive chestnut leafminer pupae. It leads to faster monitoring of the population of one of the worlds important insect pest.

Study of the development of defects in Si PIN diodes exposed to 23 GeV/c protons

Studying the development of crystallographic defects in PIN diodes is the focus of the RD-50 (CERN) research project. The study was carried out on Si PIN diodes manufactured and used in the Czech Republic. The Si PIN diodes were irradiated with 23 GeV/c protons at doses ranging from 0.5 to 43 Gy. The Si PIN diodes were calibrated in a 23 GeV/c proton source [1], and the energy traps of the defects produced were measured by the DLTS method. The IV characteristics and the parameters of the defects were studied. The 23 GeV/c protons produce typical crystallographic defects and, at higher doses, bring about their regrouping thus giving rise to a new generation of defects. Defects are classified by their energy levels in the forbidden band from the conduction band. The mechanism influencing the parameters of the defects is discussed. The study of defects in silicon is important for silicon-based electronic elements used in cosmic research because of their effects on the operability and reliability of electronic equipment installed in satellites. Another application is the dosimetry measurements involving various types of particle accelerators.

Kinematic Analysis of Relativistic Hypernuclei Decays

Reliable model for three and four particle decays is still open problem in subnuclear physics. In several laboratories, such as JINR Dubna (Russia), KEK (Japan), COSY (Germany), INFN Frascati (Italy, Collaboration FINUDA), T.Jefferson National Laboratory, USA, experiments for study Λ hypernuclei are underway or upcoming. This contribution is devoted to kinematic analysis of weak decay of Λ hypernuclei produced on nuclei beams in Nuclotron experiments in Joint Institute for Nuclear research.