K. Morishima

Development of a new automatic nuclear emulsion scanning system, S-UTS, with continuous 3D tomographic image read-out

The previous version of the automatic nuclear emulsion scanning system had a limit on read-out speed of several microscope views per second (views/s). This was due to unavoidable mechanical vibration when microscope stage was stopped to acquire tomographic images along the optical axis in emulsion. To overcome this limit, we succeeded in developing optics synchronized to stage movement so that tomographic images can be acquired without stopping a stage. This new system, S-UTS, is now operative with scanning speed of 50 views/s, or 72 cm2/h, with high efficiency and sub-μm precision. It plays an essential role in the OPERA experiment at CERN.

Discovery of a big void in Khufu’s Pyramid by observation of cosmic-ray muons

The Great Pyramid, or Khufu’s Pyramid, was built on the Giza plateau in Egypt during the fourth dynasty by the pharaoh Khufu (Cheops), who reigned from 2509 bc to 2483 bc. Despite being one of the oldest and largest monuments on Earth, there is no consensus about how it was built. To understand its internal structure better, we imaged the pyramid using muons, which are by-products of cosmic rays that are only partially absorbed by stone. The resulting cosmic-ray muon radiography allows us to visualize the known and any unknown voids in the pyramid in a non-invasive way. Here we report the discovery of a large void (with a cross-section similar to that of the Grand Gallery and a minimum length of 30 metres) situated above the Grand Gallery. This constitutes the first major inner structure found in the Great Pyramid since the nineteenth century. The void, named ScanPyramids’ Big Void, was first observed with nuclear emulsion films installed in the Queen’s chamber, then confirmed with scintillator hodoscopes set up in the same chamber and finally re-confirmed with gas detectors outside the pyramid. This large void has therefore been detected with high confidence by three different muon detection technologies and three independent analyses. These results constitute a breakthrough for the understanding of the internal structure of Khufu’s Pyramid. Although there is currently no information about the intended purpose of this void, these findings show how modern particle physics can shed new light on the world’s archaeological heritage.

Comprehensive track reconstruction tool "NETSCAN 2.0'' for the analysis of the OPERA Emulsion Cloud Chamber

NETSCAN is a track reconstruction algorithm used in Emulsion Cloud Chambers (ECC). NETSCAN and Emulsion Cloud Chambers were used in the DONUT experiment to detect Tau-neutrinos interactions. The algorithm has been revised in order to cope with the OPERA event analysis performed in Japan. A new version of NETSCAN was necessary to effectively analyze the most massive emulsion experiment in history with limited resources.

The analysis of interface emulsion detector for the OPERA experiment in JAPAN Scanning facility

The OPERA experiment, designed to search for neutrino oscillations, started its physics run in July 2008. It is the most recent emulsion-counter hybrid experiment. In this experiment two different types of emulsion detectors are used. One is called Emulsion Cloud Chamber (ECC), and is used as a target. The other is called Changeable Sheet (CS), which is attached to the downstream side of each ECC to identify the ECC in which a tagged neutrino interaction occurred. The CS interfaces rough tracking information obtained in the electronic detectors to the ECC and therefore is a key element of the emulsion-counter hybrid experiment. As a first step, a CS selected by electronic detectors is scanned and analyzed to decide if the ECC is to be developed and analyzed in detail; a few CS are usually to be analyzed to identify an ECC for each tagged event due to the inaccuracy of the electronic detectors. The CS has a large scanning area and therefore Japan and Europe (Gran Sasso) are sharing in the CS analysis load to handle the scanning job. In this paper, the CS analysis method developed for the Japan scanning facility is described in detail. 100 million tracks are read out on each CS by an automatic emulsion read-out system ( S-UTS ), most of them are so-called fake tracks mainly due to low momentum Compton electrons or random noise and a few real tracks from a tagged neutrino interaction have to be picked up among a huge background. A dedicated method to reject this background without losing real tracks has been developed on the basis of analyzing track data obtained by S-UTS and combining these data with a final selection using additional information on multiple Coulomb scattering of candidate tracks obtained by human eye check to eliminate a remaining background. Performance of this method is shown to be sufficient for the OPERA experiment.

Extra-large crystal emulsion detectors for future large-scale experiments

Photographic emulsion is a particle tracking device which features the best spatial resolution among particle detectors. For certain applications, for example muon radiography, large-scale detectors are required. Therefore, a huge surface has to be analyzed by means of automated optical microscopes. An improvement of the readout speed is then a crucial point to make these applications possible and the availability of a new type of photographic emulsions featuring crystals of larger size is a way to pursue this program. This would allow a lower magnification for the microscopes, a consequent larger field of view resulting in a faster data analysis. In this framework, we developed new kinds of emulsion detectors with a crystal size of 600-1000 nm, namely 3-5 times larger than conventional ones, allowing a 25 times faster data readout. The new photographic emulsions have shown a sufficient sensitivity and a good signal to noise ratio. The proposed development opens the way to future large-scale applications of the technology, e.g. 3D imaging of glacier bedrocks or future neutrino experiments.

Measurement of low-energy neutrino cross-sections with the PEANUT experiment

The PEANUT experiment was designed to study the NuMi neutrino beam at Fermilab. The detector uses a hybrid technique, being made of nuclear emulsions and scintillator trackers. Emulsion films act as a micrometric tracking device and are interleaved with lead plates used as passive material. The detector is designed to precisely reconstruct the topology of neutrino interactions and hence to measure the different contributions to the cross section. We present here the full reconstruction and analysis of 147 neutrino interactions and the measurement of the quasi-elastic, resonance and deep-inelastic contributions to the total charged current cross section at the energies of the NuMi neutrino beam. This technique could be applied for beam monitoring in future neutrino facilities, and this paper shows its proof-of-principle.

Progress in development of neutron energy spectrometer for deuterium plasma operation in KSTAR.

Two types of DD neutron energy spectrometer (NES) are under development for deuterium plasma operation in KSTAR to understand behavior of beam ions in the plasma. One is based on the state-of-the-art nuclear emulsion technique. The other is based on a coincidence detection of a recoiled proton and a scattered neutron caused by an elastic scattering of an incident DD neutron, which is called an associated particle coincidence counting-NES. The prototype NES systems were installed at J-port in KSTAR in 2012. During the 2012 and 2013 experimental campaigns, multiple shots-integrated neutron spectra were preliminarily obtained by the nuclear emulsion-based NES system.

Development and utilization of ``Plate Changer'' system for neutrino interaction locations in OPERA emulsion target

In the OPERA experiment, so-called Scan Back method is used to locate neutrino interaction vertices in the emulsion target named ECC (Emulsion Cloud Chamber). In Scan Back method, tracks detected in the most downstream emulsion plate in ECCs are followed to upstream plate by plate until it reaches to the interaction points. In order to treat a number of neutrino interactions recorded in OPERA, dedicated systems called Plate Changer has been developed and utilized to neutrino event location in Japan. The details of the system have been described in this report. Until the end of November 2012, 6223 Scan Back trials have been performed using this system, and strongly contributed to the detection of tau neutrino appearance.

Development of Neutron Measurement in Intense Gamma Field Using New Type of Nuclear Emulsion

To precisely measure the neutron emissions from a spent fuel assembly of a fast breeder reactor, we formed nuclear emulsions based on a non-sensitized Oscillation Project with Emulsion tRacking Apparatus (OPERA) film with AgBr grain sizes of 60, 90, and 160 nm. The efficiency for 252Cf neutron detection of the new emulsion was calculated to be 0.7 × 10−4, which corresponded to an energy range from 0.3 to 2 MeV and was consistent with a preliminary estimate based on experimental results. The sensitivity of the new emulsion was also experimentally estimated by irradiating with 565 keV and 14 MeV neutrons. The emulsion with an AgBr grain size of 60 nm had the lowest sensitivity among the above three emulsions but was still sensitive enough to detect protons. Furthermore, the experimental data suggested that there was a threshold linear energy transfer of 15 keV/μm for the new emulsion, below which no silver clusters developed. Further development of nuclear emulsion with an AgBr grain size of a few tens of nanometers will be the next stage of the present study.

First demonstration of gamma-ray imaging using a balloon-borne emulsion telescope

We promote the precise gamma-ray observation project Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE), which uses balloon-borne emulsion gamma-ray telescopes. The emulsion telescope realizes observations with high angular resolution, polarization sensitivity, and large aperture area in the 0.01--100 GeV energy region. Herein, we report the data analysis of emulsion tracks and the first demonstration of gamma-ray imaging via an emulsion telescope by using the flight data from the balloon experiment performed in 2015 (GRAINE 2015). The emulsion films were scanned by the latest read-out system for a total area of 41 m