Research at VATLY : main themes and recent results. PN Diep , PN Dong, PTT Nhung, P Darriulat, NT Thao and VV Thuan VATLY, INST, Hanoi, Vietnam. Hoi An, 2007.
PN Diep, PN Dong, PTT Nhung, P Darriulat,
NT Thao and VV Thuan
VATLY, INST, Hanoi, Vietnam
Hoi An, 2007
Azimuthal distribution of cosmic muon flux in Hanoi
Our first measurements were of the atmospheric muon spectrum in Hanoi. They contributed useful data to the understanding of atmospheric neutrino oscillation experiments. They show a strong east-west asymmetry due to the effect of the Earth magnetic field which is particularly important in Hanoi (rigidity cutoff of 17GeV).
Since then our studies have been made in closer connection with the Pierre Auger Observatory (PAO), both at the Institute (using equipment installed on the roof of the laboratory) and from data collected there.
1. The Pierre Auger Observatory
2. Studies using laboratory equipment
Muon rates/ Response to muons of a replica of an Auger tank
and refurbishment/ Multitank detection of air showers/
3. Studies using Auger data
Simulation of FD reconstruction/Study of FADC traces,
muon to photon ratio/ The PMT asymmetry at early times
4. Teaching and Training
5. Summary and perspectives
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 Pierre Auger Observatory is the first large hybrid detector ever built: it combines the strengths of
Surface Detector Array
Air Fluorescence Detectors
The giant detector array of the PAO is made of water Cherenkov counters and covers 3000 km2 with a triangular grid having a 1.5 km mesh size (1600 detectors of 10m2 area each). Around 1020eV showers involve 15 to 20 detectors. Typical angular and energy resolutions are 1.5° and 20%.
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).
Lateral distribution function fit
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).
11 m2 mirror
They aim at getting familiar with the behavior of the Auger detectors
2.1Response to muons of a replica of an Auger tank
2.2Multitank detection of air showers
2.3Refurbishing of the Hanoi Auger tank
2.4 Photocathode efficiency measurements
RESPONSE TO MUONS OF A REPLICA OF AN AUGER TANK
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.
A study of its response to muons was performed using a movable scintillator hodoscope located below it in the laboratory.
The pulse heights measured in each phototube are observed to be essentially proportional to the length of the muon track in water.
The pulse height distribution obtained for vertical traversing muons was wider and its average value lower than achieved in Auger, implying the need for a factor 4 increase of the collected light.
We have now improved the water purity and the quality of the wall surface in order to reach this goal.
The Hanoi Auger tank has now been refurbished: new Photonis phototubes have been installed, the walls have been coated with aluminized mylar, a newwater filtering system has been installed, the counter has now been run in.
Data have been collected with an independent trigger on extensive air showers (see later). - First results show major improvement of the optical quality of the counter. In particular strong correlations between the signals recorded in each of the 3 PMTs are now observed.- A precise measurement of the light yield remains to be done.
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.
- A factor of two reduction of the data sample gives a 15% improvement on the timing measurement and increases the main tank charge by 38%.
Photocathode efficiency measurements
- We measured the response of the photocathode to single photons at IPN Orsay using a scintillator exposed to Americium as a light source.
- The fine structure of the time distribution and its dependence on incidence angle have been studied.
- Evidence for a dependence of the PMT gain on geometry is calling for additional measurement as a function of impact point which are currently undertaken in Hanoi.
3.1Simulation of FD reconstruction
3.2Study of FADC traces, muon to photon ratio
Theses under joint supervision with foreign universities.
The pixel pattern defines the shower detector plane (accurately), the time distribution along the track locates the shower within this plane (not accurately)
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.
Time in bins of 25 ns
Time in bins of 25 ns
Time in bins of 25 ns
Time in bins of 25 ns
Moreover, the light reaching the PMT’s in the first 25ns or so is not equally shared between the three PMTs. It displays a strong asymmetry that is correlated with the azimuth of the incoming particles and with their impact points.
Cosmic ray studies have been performed by VATLY, including muon flux measurements, studies of the response of an Auger Cherenkov counter to muons and to extensive air showers using three additional trigger 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 EU-Asia link program, 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.
Information and documentation
concerning VATLY is available on our site