VATLY, a cosmic ray laboratory in Hanoi, progress report PN Diep, PN Dong, PTT Nhung, P Darriulat, NT Thao, DQ Thieu and VV Thuan VATLY, INST, Hanoi, Vietnam

VATLY, Vietnam Auger Training LaboratorY is a cosmic ray laboratory located in Hanoi aimed at creating a group contributing to the research activity of the Pierre Auger Observatory with which we are associated.

At the last Rencontres we reported about measurements of the atmospheric muon spectrum in Hanoi that was contributing useful data to the understanding of atmospheric neutrino oscillation experiments.  (degrees) Azimuthal distribution of cosmic muon flux in Hanoi

Since then our studies have been made in closer connection with Auger, both at the Institute (using equipment installed on the roof of the laboratory) and from data collected at the Pierre Auger Observatory. Summary 1. The Pierre Auger Observatory 2. Studies using laboratory equipment 2.1 Response to muons of a replica of an Auger tank 2.2 Multitank detection of air showers 2.3 Refurbishing of the Hanoi Auger tank 3. Studies using Auger data 3.1 Simulation of FD reconstruction 3.2 Study of FADC traces, muon to photon ratio 4. Teaching and Training 5. Summary and perspectives 6. Publications

PIERRE AUGER OBSERVATORY High energy cosmic rays are observed from the extensive air showers that they produce when entering the atmosphere. One method consists in sampling the particle density on ground, another method consists in detecting the fluorescence light produced on nitrogen molecules along the shower axis. In both cases timing gives the direction and intensity gives the energy but both methods suffer of very different systematic sources of errors. The Auger Observatory is the first large hybrid detector ever built: it combines the strengths of Surface Detector Array & Air Fluorescence Detectors

The main physics of the Pierre Auger Observatory Accurate measurement of the high end of the energy spectrum Settlement of the GZK question Identification of possible sources Nature of the primaries

Auger Surface Detector Giant detector arrays are made of scintillators or water Cherenkov counters. That of the Pierre Auger Observatory covers 3000 km 2 with a triangular grid having a 1.5 km mesh size (1600 detectors of 10m 2 area each). Around eV showers involve 15 to 20 detectors. Typical angular and energy resolutions are 1.5° and 20%.

Surface array view Lateral distribution function fit ENERGY MEASUREMENT IN THE SURFACE DETECTOR (SD ) The energy measurement obtained from the surface detector relies on the dependence of the measured signals on the distance to the shower axis (so-called lateral distribution function, LDF).

Studies using laboratory equipment They aim at getting familiar with the behavior of the Auger detectors 2.1 Response to muons of a replica of an Auger tank 2.2 Multitank detection of air showers 2.3 Refurbishing of the Hanoi Auger tank

RESPONSE TO MUONS OF A REPLICA OF AN AUGER TANK x We have constructed a replica of one of the 1600 Auger surface detectors on the roof of our laboratory in order to get familiar with its behaviour and performance. The detector is a water Cherenkov counter using three 8” photomultiplier tubes collecting, from the top of the tank, the Cherenkov light diffused on its walls. A study of its response to muons was performed using a movable scintillator hodoscope located below it in the laboratory. It was fragmented in such a way as to allow for 256 effective muon beams, impacting on the tank in different positions and at different incidence angles. cm

The pulse height distribution obtained for vertical traversing muons is wider and its average value lower than achieved in Auger, implying the need for a factor 4 increase of the collected light. The pulse heights measured in each phototube are observed to be essentially proportional to the length of the muon track in water. Smaller effects, such as the light attenuation in water depending on the proximity of the track to the PMT, have also been studied. We are currently improving the water purity and the quality of the wall surface in order to reach this goal.

AFTERPULSING We found that the over 30 year old phototubes that we were using suffered from an important afterpulsing: the gas in their glass envelopes gets ionized by the primary photoelectrons and the produced ions were accelerated toward the photocathodes. Autocorrelation spectra showed afterpulses after a few μs. New phototubes given to VATLY by the manufacturer of the Auger PMT’s (Photonis) have now been installed in the tank.

MULTITANK DETECTION OF AIR SHOWERS We have installed and are currently operating three additional smaller water tanks (3000 liters each), each seen by two photomultiplier tubes, around the Auger Cherenkov counter. The aim is to detect extensive air showers giving coincident signals in several of these. 325cm

Azimuthal distribution of air showers (projection on the horizontal plane of the unit vector directed along the reconstructed shower axis) After running in, the system has been operated during three months using a scintillator instead of water, providing a sample of more than 10 5 showers having a signal in each of the small tanks. These data are presently being compared to the prediction of a simulation. This trigger will next be used to study the response of the refurbished Auger counter.

Refurbishing the Hanoi Auger tank The Hanoi Auger tank has now been refurbished: new Photonis phototubes have been installed, the walls have been coated with aluminized mylar, a new water filtering system has been installed, the counter has been run in without water but with a scintillator plate laying on its ground plate.

Studies using Auger data 3.1 Simulation of FD reconstruction 3.2 Study of FADC traces, muon to photon ratio Theses will be made in cotutelle with Paris and Catania.

THE AUGER FLUORESCENCE DETECTOR

Four stations of six eyes each, each eye covering a field of view of 30°×28° with a mirror focusing on an array of 22×20 pixels (photomultiplier tubes), each having 1.5° aperture. They measure the induced fluorescence of nitrogen molecules (near UV). UV-Filter nm camera 440 PMTs 11 m 2 mirror

The pixel pattern defines the shower detector plane (accurately), the time distribution along the track locates the shower within this plane (not accurately) FD SIMULATION AND SHOWER RECONSTRUCTION

Using our simulation program, we have studied the detailed nature of the uncertainties attached to the location of the shower in the shower-detector plane (left plot). They are compared below to the real situation in Auger (right plot). A single eye alone is insufficient for an accurate location of the shower in the plane. Binocular detection or hybrid detection (FD and SD) are necessary. Both are available in Auger.  0  R p

Our simulation program has been used to understand in simple terms specific features of the detection of UHE showers in a fluorescence detector. The Pierre Auger Observatory was used as an illustration. In addition to the difficulty of positioning accurately the shower in the shower-detector plane in the case of monocular observation (pointing to the importance of making binocular, or better hybrid observations) other effects were considered: The compatibility between fluorescence and surface detections was studied and found to be very good in the PAO design; Considerations on the necessity to extrapolate the observed shower profile outside the field of view shed some light on the different requirements implied by a good energy measurement; Parameters such as the spans in time and in atmosphere thickness covered by the field of view were identified as good indicators of the quality of the measurements performed.

STUDY OF FADC TRACES, MUON TO PHOTON RATIO: REDUCING FADC TRACES TO A SUM OF PEAKS The surface detector signals are stored in flash analog to digital converters (FADC, 25ns bin size). The time distribution of the pulse height (averaged over the three photomultiplier tubes) is the sum of muon and electron- photon signals; the former cluster around a mean pulse height (or area) corresponding to muons traversing the whole water volume; the latter have a rapidly decreasing exponential distribution starting at very low values defined by the detection threshold (typically 1/30 of the muon signal). Disentangling the two and reducing the FADC traces to a sum of peaks is therefore an essential preliminary to a detailed understanding of the shower properties.

FADC spectrum of a big photon event (data & fit) Time in bins of 25 ns

FADC spectrum of a four muon event (data & fit) Time in bins of 25 ns

FADC spectrum of a complex event (data & fit) Time in bins of 25 ns

FADC spectrum of another complex event (data & fit) Time in bins of 25 ns

TEACHING AND TRAINING VATLY has been actively contributing to the teaching of particle physics and astrophysics in the Ha Noi universities (national and pedagogic) and Institute of Physics. Modern astrophysics lectures were delivered for the first time to Vietnamese university students in Increased support from the Ha Noi faculty will help improving this program in the future. VATLY is providing supervision for graduation (currently 2), master (currently 4) and PhD (currently 2) theses in collaboration with the Hanoi National University and Institute of Physics as well as with the Hanoi University of Technology. The PhD program is under joint supervision with western universities (currently Paris and Catania) yielding to a PhD degree in both Vietnam and the foreign country. The research group could still welcome a few aditional students.

CONTENTS OF THE COSMOLOGY AND ASTROPHYSICS COURSE - Introduction - Some elementary results of (mostly) Newtonian mechanics - Inflation - The content of the universe - Birth and life of stars - The death of stars, gravitational collapses - Violent events, accretion - Cosmic rays - Appendix (gravitation, particle physics, nuclear physics, plasmas, MHD, spectroscopy, some useful numbers, some names & dates, some pictures)

In 2006, we supervised the first graduation diplom written on Astrophysics at the Hanoi National University, a study of Sagittarius A*, the black hole at the center of the Milky Way. Radio (VLA) X-ray (Chandra) Infrared (NAOS-CONICA )

Summary Cosmic ray studies have been performed by VATLY, including muon flux measurements and studies of the response of an Auger Cherenkov counter to muons. They are currently being extended to the detection of extensive air showers using three additional counters in coincidence. The future of the VATLY Laboratory is with the Pierre Auger Observatory, this is the aim of most of our present activities. Simulation studies of the surface and fluorescence detectors have been performed with particular emphasis on the understanding of systematic uncertainties. Studies are underway to learn about the nature of the primaries by reducing the FADC traces to a sum of muon and electron-photon peaks. Contribution is also being given to the reduction of the fluorescence detector data. Our association with the PAO is an endless source of research subjects in collaboration with prestigious universities from all over the world. VATLY is actively contributing to University training and teaching. We are indebted to CERN, RIKEN, the World Laboratory, the French CNRS, the Rencontres du Vietnam and the Vietnam Academy of Science and Technology for invaluable support. We are particularly grateful to the Pierre Auger Collaboration for their constant interest and support in the development of our group.

VATLY PUBLICATIONS Measurement of the Vertical Cosmic Muon Flux in a Region of Large Rigidity Cutoff, P.N. Dinh et al., Nucl. Phys., 627B (2002) 29 Measurement of the Zenith Angle Distribution of the Cosmic Muon Flux in Hanoi, P.N. Dinh et al., Nucl. Phys. 661B (2003) 3 Dependence of the cosmic muon flux on atmospheric pressure and temperature, P.N. Diep et al., Com. Phys. Vietnam 14 (2003) 57 The cosmic ray research in Hanoi: The Auger experiment and measurements made at home, P.N. Dinh, Nucl. Phys.722A (2003) 439 Measurement of the east-west asymmetry of the cosmic muon flux in Hanoi, P.N. Diep et al., Nucl. Phys. 678B (2004) 3 Properties and performance of the prototype instrument for the Pierre Auger Observatory, Auger Collaboration, J. Abraham et al., Nucl. Inst. Meth A523 (2004) 50 Atmospheric muons in Hanoi, P.N. Diep et al., Com. Phys. Vietnam 15 (2005) 55 On the detection of ultra-high energy cosmic rays: Fluorescence detection, P.N. Diep et al., Com. Phys. Vietnam, 2006, to be published. RECENT PRESENTATIONS TO CONFERENCES Atmospheric muons in Hanoi, D.Q.Thieu at the Vth Rencontres du Vietnam, Hanoi, August 2004 and by P.N.Diep at the IXth APPC conference, Hanoi, October Cosmic ray studies at VATLY, P.T.T.Nhung at the Hanoi-Osaka Forum, Hanoi, September In addition, on various occasions, the Auger Collaboration has been giving numerous presentations and progress reports with which some VATLY members are associated. THESES Ph D theses: Dang Quang Thieu, 2005 Master theses: Nguyen Hai Duong, 2004; Pham Thi Tuyet Nhung, three master theses currently underway Diplom works: Pham Ngoc Diep and Pham Thi Tuyet Nhung, 2003; Dinh Lam Anh Huyen, 2004; Nguyen Viet Phuong and Kim Thi Phuong, 2006