Novel approach to imaging by cosmic-ray muons

Cosmic-ray muons can be used for imaging of large structures, or high-density objects with high atomic number. The first task can be performed by measurement of muon absorption within very thick material layers, while the second approach is based on muon multiple scattering. However, the muon imaging of small structures with low atomic number and density was not yet solved appropriately. Here we show the first results of cosmic-ray muon imaging of small objects made of elements of low atomic number. This novel approach includes detection of secondary particles produced by muons, which were not used at all in previous muon imaging methods. Thus, the list of elements, as well as the range of dimensions of objects which can be imaged are significantly expanded. editor’s choice Copyright c © EPLA, 2016 The interaction of energetic cosmic particles with the Earth atmosphere leads to a shower of subatomic particles in a cascade process. To the Earth surface mostly highenergy (2GeV in average) muons arrive with a flux on the order of 10000m−2min−1. It was recognized a long time ago that these highly penetrating particles can be used for inspecting large-volume rock structures [1–3]. After these applications based on muon absorption, a muon tomography method based on multiple muon scattering was developed [4–6]. These methods have been exploited for the inspection of large industrial structures [7,8] and large vessels [9], for the detection of nuclear threats [10] and proposed for exploration of Mars geology [11]. It was predicted that with large-area muon detectors, useful images can be derived in minutes [12]. In the present paper we want to demonstrate the completely new approach in which muon-induced secondaries (mostly the bremsstrahlung from electrons and positrons created by muons) can be used for imaging purposes. New imaging methods by other incident particles could be also established by the method we describe. In our previous experiments [13] using a plastic muon detector and a high-purity germanium (HPGe) gamma (a)E-mail: [email protected] detector in coincidence, we measured the muon secondary production in various materials. The present experimental setup (sketched in fig. 1(a)) is the combination of the HPGe gamma spectrometer with a muon tracker [14,15] consisting of four Close Cathode Chamber (CCC) [16,17] detection layers with dimensions of 25 cm× 25 cm and segmentation of 4mm× 4mm. The data on the muon trajectories have been stored in the tracker’s data acquisition system, together with a time stamp of each event. The tracker system has been designed to optimally match the HPGe geometry. The position resolution of the tracker without the HPGe detector is better than 2.5mm (RMS) for each layer, shown in fig. 2(a), leading to extrapolated trajectory precision better than 2.5mm. Figure 2(a) has been prepared such that the position difference between the fitted 4-point track and the actual position is shown, without excluding the detector under test. The tracking efficiency for each layer, shown in fig. 2(b) is around 96%. The combined tracking efficiency, assuming three collinear track points, is above 99%. For the events with at least two coincident hits, the tracker generates also logic output signals. These signals have been stored in the CAEN four-input fast-digitizer unit within the list containing the time stamp of the acquired signal. In addition to the tracker’s signals, the