Izumi Ogawa

Experimental Investigation of Thermal Neutron-Induced Single Event Upset in Static Random Access Memories

Static random access memories (SRAMs) with a borophosphosilicate glass film (BPSG) and with a normal silicon oxide film (non-boron-doped silicon oxide film) were fabricated, and the influence of a BPSG film on the neutron-induced single event upsets (SEUs) was investigated. It was confirmed that the thermal neutron reactions on boron atoms in a BPSG film induce SEUs. The SEU rate depends strongly on cell charge and increases as the cell charge decreases. In future SRAMs with a small cell size and low operating voltage, the thermal neutron reaction on boron atoms in a BPSG film will play a significant role to the neutron-induced single event upsets.

Neutrino-less double-β decay of 48Ca studied by CaF2(Eu) scintillators

We searched for the neutrino-less double-β decay(0νββ) of 48 Ca by using CaF 2 (Eu) scintillators. Analysis of their pulse shapes was effective to reduce backgrounds. No events are observed in the Q ββ value region for the data of 3394 kg day. It gives a lower limit (90% confidence level) of T 0νββ ½ > 2.7 x 10 22 year for the half-life of 0νββ of 48 Ca. Combined with our previous data for 1553 kg day [I. Ogawa et al., Nucl. Phys. A730, 215 (2004)], we obtained a more stringent limit of T 0νββ ½ > 5.8 × 10 22 year.

Study of Double Beta Decay of {sup 48}Ca by CANDLES

CANDLES is the project to search for neutrino‐less double beta decay (0νββ) of 48Ca. The observation of 0νββ will prove existence of a massive Majorana neutrino. We have developed the new detector system CANDLES which features CaF2 (pure) scintillators. Here expected performances of the system for background rejection are presented. It is also described current status of development for the detector system.

CANDLES for double beta decay of 48Ca

CANDLES is the project to search for neutrinoless double beta(0νββ) decay of 48Ca by using CaF2 scintillators. The observation of 0νββ decay will prove existence of a massive Majorana neutrino. The expected performances of the CANDLES system for lightsignal detection and background rejection are presented here. The current status of development for the detector system is also described.

Influence of Elastic Scattering on the Neutron-Induced Single-Event Upsets in a Static Random Access Memory

Neutron single-event upsets (SEUs) induced by elastic scattering were investigated by an experiment using 2 MeV neutron beams and by a calculation based on scattering cross-section data and angular distribution data from the evaluated nuclear data file (ENDF). The SEU rates obtained by the calculation and experiment are in fairly good agreement if the region sensitive to the SEUs (sensitive volume) is properly defined. Adopting the calculation to atmospheric neutrons, the fraction of the SEU rates induced by elastic scattering accounted for approximately 26 to 32% of the total fast-neutron-induced SEUs in the atmospheric neutron environment. Reducing well depth will effectively reduce the number of SEUs.

Low radioactivity CaF{sub 2} scintillator crystals for CANDLES

CANDLES is the project to search for neutrinoless double beta (0νββ) decay of 48Ca by using CaF2 scintillators. The observation of 0νββ decay will prove the existence of massive Majorana neutrinos. Expected performances and current status of the CANDLES system are described.

Study of Ca-48 double beta decay with CANDLES

CANDLES is the project to search for double beta decay of 48Ca by using CaF2 scintillators. If neutrinos have Majorana mass they violate lepton number conservation and neutrino-less double beta decay (OvDBD) can then take place. Therefore the study of the 0 vDBD is one of the most fundamental researches to be carried out in a coming decade. We have been studying the DBD of 48Ca using CaF2 scintillators. The Q value of 48Ca is the highest (4.27 MeV) among potential DBD nuclei. It is far above energies of γ-rays from natural radio-activities (maximum 2.615 MeV from 208TI decay), therefore we can naturally expect small backgrounds in the energy region we are interested in. Required performances for the detector are radio-purity, good background rejection efficiency and good energy resolution. We have constructed CANDLES III detector in our laboratory at sea level, which consists of 60 CaF2 crystals with the total mass of 191 kg. We are studying the basic performances of the system, including the light collection, position reconstruction and background rejection. On the bases of experiences in CANDLES III, the CANDLES project will be scaled up to several tons of calcium to have the sensitivity to the mass region of interest.

Neutrino-less double-beta decay of Ca-48 studied by Ca F(2)(Eu) scintillators

We searched for the neutrino-less double-β decay(0νββ) of 48 Ca by using CaF 2 (Eu) scintillators. Analysis of their pulse shapes was effective to reduce backgrounds. No events are observed in the Q ββ value region for the data of 3394 kg day. It gives a lower limit (90% confidence level) of T 0νββ ½ > 2.7 x 10 22 year for the half-life of 0νββ of 48 Ca. Combined with our previous data for 1553 kg day [I. Ogawa et al., Nucl. Phys. A730, 215 (2004)], we obtained a more stringent limit of T 0νββ ½ > 5.8 × 10 22 year.

Evaluation of Fast Neutron Induced Single Event Upset in a Static Random Access Memory and Simulation by Monte Carlo N-Particle Code (MCNPX)

Neutron-induced single-event upsets (SEUs) in a 0.4 µm 4 Mbit CMOS SRAM (complimentary metal oxide semiconductor static random access memory) were investigated using high-energy neutron beams and Monte Carlo simulation by MCNPX (Monte Carlo N-Particle Code). The Monte Carlo simulation, based on the assumption that the primary cause of SEUs is alpha particles generated by nuclear fission, agreed with the experimental results within the accuracy of ±29% in the case of small cell charges (<10 fC). When the devices were exposed to fast neutrons in the front-surface direction, the SEU rates increased by a factor of 1.1 to 2 in comparison with the case of back-surface irradiation. According to the Monte Carlo simulation, the difference between the alpha particle production cross section of the carbon atom in package materials and that of the silicon atom caused this phenomenon.

Single Event Upset in Static Random Access Memories in Atmospheric Neutron Environments

Single-event upsets (SEUs) in a 0.4 ?m 4 Mbit complementary metal oxide semiconductor (CMOS) static random access memory (SRAM) were investigated in various atmospheric neutron environments at sea level, at an altitude of 2612 m mountain, at an altitude of commercial airplane, and at an underground depth of 476 m. Neutron-induced SEUs increase with the increase in altitude. For a device with a borophosphosilicate glass (BPSG) film, SEU rates induced by thermal neutrons increase with the decrease in the cell charge of a memory cell. A thermal neutron-induced SEU is significant in SRAMs with a small cell charge. With the conditions of small cell charge, thermal neutron-induced SEUs account for 60% or more of the total neutron-induced SEUs. The SEU rate induced by atmospheric thermal neutrons can be estimated by an acceleration test using 252Cf.