Modeste Tchakoua Tchouaso

Energy response of diamond sensor to beta radiation

This paper demonstrates the ability of diamond sensors to respond to beta radiation. A Chemical Vapor Deposition (CVD) single crystal diamond was used in this work. The diamond crystal has a dimension of 4.5×4.5 by 0.5 mm thick. Metal contacts were fabricated on both sides of the diamond using titanium and palladium metals with thicknesses of 50 nm and 150 nm, respectively. The energy response of the diamond sensor was experimentally measured using three beta isotopes that cover the entire range of beta energy: 147Pm, a weak beta radiation with a maximum energy of 0.225 MeV, 2°4Tl, a medium energy beta radiation with a maximum energy of 0.763 MeV, and 9°Sr/9°Y, with both a medium energy beta radiation with a maximum energy of 0.546 MeV, and a high energy beta radiation with a maximum energy of 2.274 MeV. The beta measurements indicate that diamond sensors are sensitive to beta radiation and are suitable for beta spectroscopy. This is important in estimating dose since diamond is tissue equivalent, and the absorbed dose is easily determined from the energy and the mass of the active volume. The high energy betas from 2°4Tl and 90Sr/90Y penetrates the sensor without depositing sufficient energy in the active area because their range is larger than the thickness of sensor. The sensitivity of the detector is limited because of its small volume and can be improved by combining smaller area sensors since growing large size diamond is currently a challenge.

Introduction to Nuclear Batteries and Radioisotopes

This chapter provides the reader with background and fundamental information on the subject of nuclear batteries. The approach used in this chapter is to describe the characteristics of a nuclear battery relying on easy to understand physical properties. For example, a commonly used descriptive parameter is the maximum power density. However, a more intuitive parameter is to use the inverse of maximum power density and define a quantity of minimum volume per Watt for a radioisotope volume source or minimum surface area per Watt for a radioisotope surface source. This approach gives the reader a feel for actual dimensional limitations of the technology as well as other constraints.

Ti:Pt:Au:Ni thin-film CVD diamond sensor ability for charged particle detection

This work demonstrates the development of diamond sensors with reliable contacts using a new metallization formula, which can operate under high-pressure gas environment. The metallization was created using thin film layers of titanium, platinum, gold and nickel deposited on a single crystal electronic grade CVD diamond chip. The contacts were 2 mm in diameter with thickness of 50/5/20/150 nm of Ti:Pt:Au:Ni. The optimum operating voltage of the sensor was determined from the current-voltage measurements. The sensor was calibrated with 239Pu and 241Am alpha radiation sources at 300 V. The energy resolution of the Ti:Pt:Au:Ni diamond sensor was determined to be 7.6% at 5.2 MeV of 239Pu and 2.2% at 5.48 MeV of 241Am. The high-pressure gas loading environment under which this sensor was used is discussed. Specifically, experimental observations are described using hydrogen loading of nickel as a means of initiating low energy nuclear reactions. No neutrons, electrons, ions or other ionizing radiations were observed in these experiments.

High efficiency Dual-Cycle Conversion System using Kr-85

This paper discusses the use of one of the safest isotopes known isotopes, Kr-85, as a candidate fuel source for deep space missions. This isotope comes from 0.286% of fission events. There is a vast quantity of Kr-85 stored in spent fuel and it is continually being produced by nuclear reactors. In using Kr-85 with a novel Dual Cycle Conversion System (DCCS) it is feasible to boost the system efficiency from 26% to 45% over a single cycle device while only increasing the system mass by less than 1%. The Kr-85 isotope is the ideal fuel for a Photon Intermediate Direct Energy Conversion (PIDEC) system. PIDEC is an excellent choice for the top cycle in a DCCS. In the top cycle, ionization and excitation of the Kr-85:Cl gas mixture (99% Kr and 1% Cl) from beta particles creates KrCl* excimer photons which are efficiently absorbed by diamond photovoltaic cells on the walls of the pressure vessels. The benefit of using the DCCS is that Kr-85 is capable of operating at high temperatures in the primary cycle and the residual heat can then be converted into electrical power in the bottom cycle which uses a Stirling Engine. The design of the DCCS begins with a spherical pressure vessel of radius 13.7 cm with 3.7 cm thick walls and is filled with a Kr-85:Cl gas mixture. The inner wall has diamond photovoltaic cells attached to it and there is a sapphire window between the diamond photovoltaic cells and the Kr-85:Cl gas mixture which shields the photovoltaic cells from beta particles. The DCCS without a gamma ray shield has specific power of 6.49 W/kg. A removable 6 cm thick tungsten shield is used to safely limit the radiation exposure levels of personnel. A shadow shield remains in the payload to protect the radiation sensitive components in the flight package. The estimated specific power of the unoptimized system design in this paper is about 2.33 W/kg. The specific power of an optimized system should be higher. The Kr-85 isotope is relatively safe because it will disperse quickly in case of an accident and if it enters the lungs there is no significant biological half-life.

Beta Skin Dosimetry using Passivated Planar Silicon Detector

Accurate measurement of beta skin dose remains a challenge. This dose is defined as the dose to the basil layer at 7 mg/cm2 (approximately 70 µm) below the surface of the skin and averaged over an area of 1 cm2. This dose is dependent upon the energy of the beta contamination on the surface of the skin, the area of contamination and the attenuation of this radiation through the 7 mg/cm2 epidermal layer. Ideally, knowing the energy spectra of betas at this level below the surface of the skin would allow accurate prediction of dose. In this work, a Passivated Planar Silicon (PIPS) detector was tested by measuring beta spectra in a geometry simulating skin and, from that, estimating dose. Three calibrated beta sources were used, a low energy beta source, (147Pm), a medium energy source, (204Tl), and a high energy beta source, (90Sr/90Y) to cover the range of beta energies typically found in skin contamination events. Modelling utilized the MCNPX and VARSKIN 4.0 computer codes to calculate dose in skin and were found to be in good agreement with each other. Experimental measurements using a 300 µm thick, 3 cm2 PIPS and the three sources identified above showed good agreement with MCNPX results (and thus, also with VARSKIN). Finally, MCNPX modelling compared the dose rates from a commercially available, 100 µm thick, 1.5 cm2 PIPS detector and skin, and found that the beta dose could be accurately predicted within 17% over the range of beta energies tested. This result can be obtained with a single measurement and without the need for post data collection analysis.

Potential Applications for Nuclear Batteries

The obvious attributes of radioisotopes are a large amount of energy stored per unit mass and long life-time. These properties have been the primary motivation in the development of nuclear batteries driven with radioisotopes. The applications have changed over time based on the evolution of technology. In this chapter, successful nuclear battery designs are discussed along with the applications that they were designed for. In addition, the technologies which have evolved over time requiring significant energy storage and long shelf lives are discussed.

Interactions of Ionizing Radiation with Matter and Direct Energy Conversion

Ionizing radiation is a broad term which refers to the fact that different types of radiation will create ion pairs in matter. Ionizing radiation includes ions (e.g., fission fragments and alpha particles), beta particles, gamma rays, x-rays, and neutrons. Radioisotopes emit ionizing radiation and are viewed as the primary power source for nuclear batteries. This chapter will explore various radioisotope sources and their properties. The transducers which can be used in concert with radioisotope sources will be discussed.

Power Density Dilution Due to the Interface of the Isotope with the Transducer

In chapter 4, definitions for various types of dilution factors for a nuclear battery are discussed. The average atomic density of the radioisotope in a nuclear battery cell is described and a relationship between the average atomic density and dilution factor are derived. The dilution factor will impact the minimum scale and the power density of the battery. It is an important parameter that is used in the assessment of nuclear battery designs.

Efficiency Limitations for Various Nuclear Battery Configurations

A nuclear battery can be viewed as a radiation source embedded in various layers of materials with one of the layers being a transducer. The goal of nuclear battery design is to deposit as much of the power produced by the source into the transducer. Nuclear battery designs vary depending on the source, the transducer and the method by which the source and transducer are interfaced. It is this variability in design which can obfuscate the simple nature of the design. This chapter’s focus is on the fundamental vision of nuclear battery design.