COSMIC RAY STUDIES AT VATLY

PN Diep, PN Dong, PTT Nhung, P Darriulat,

NT Thao, DQ Thieu and VV Thuan

VATLY, INST, Hanoi, Vietnam

1. Presentation of VATLY (a recent photograph of the group)

The VATLY group on the roof of the laboratory

The Vietnam Auger Training Laboratory (VATLY, meaning “physics” in Vietnamese) is a cosmic ray laboratory installed in the premises of the Institute for Nuclear Science and Technology in Hanoi. Its research is centered on the Pierre Auger Observatory, located in Argentina, that studies ultra high energy cosmic rays with unprecedented accuracy. VATLY is also equipped with detectors of its own (scintillator hodoscopes and water Cherenkov counters).

CONTENT

• MEASUREMENTS MADE AT VATLY

1. Measurement of the muon flux in Hanoi

2. Response of a water Cherenkov counter to muons

3. Stopping muons, afterpulsing

4. Extensive air showers

• ANALYSES AND SIMULATIONS RELATED TO AUGER

1. General presentation, the observatory and its physics

2. The Auger surface detector

3. The Auger fluorescence detector

SUMMARY

LIST OF PUBLICATIONS

PART I

MEASUREMENTS MADE AT VATLY

& from cosmic ray

proton or nucleus strong interaction with atmosphere

+ - pairsnegligible

weak decays

real photons &

electrons only muons

negligible

mesons & hyperons

electromagnetic decays

Measuring the muon flux is of interest to atmospheric neutrino oscillation experiments such as SuperKamiokande that need a good knowledge of the neutrino flux in order to analyse their data. To a good approximation each muon produced in air showers initiated by a primary cosmic nucleus (mostly proton) is accompanied by a muon neutrino.

p1

2

(oscillates over 13000km)

SK

Earth

proton 2

proton 1 1

2

The rigidity cutoff that describes the effect of the earth magnetic field on primary cosmic rays is maximal in the Vietnam region: 17GV compared to 11GV in Japan and typically 4GV in most European and north American laboratories where muon fluxes have been measured. This makes the present data of particular interest.

Longitude

Lati

tud

e

Iron

Scintillator

Lucite

1.9m

80cm

40cm40cm

axis

b)a)

A scintillator telescope was used for the measurement of the muon flux. Its acceptance was 2.22 10-2m2sr and its momentum cutoff 120MeV/c. A lead absorber and an iron converter were used to disentangle the electron-photon and hadron backgrounds from the muon component.Point to point uncertainties were at the 1.3% level while an overall 2.2% uncertainty applied globally to all data.

ZENITH ANGLE DISTRIBUTION

(degrees)

The dependence of the muon flux on zenith angle is essentially of the form 0 = (1 - .108 sin2) cos2.

Here, for convenience, we plot J = /0 . Fluxes measured toward the north (open circles) or averaged between east and west (full circles) are shown separately. Good agreement is found with Honda model’s prediction (dash and full lines respectively).

The east-west asymmetry expected from the effect of the earth magnetic field on cosmic primaries (positively charged) has been measured as a function of zenith angle and the azimuthal distribution of the muon flux has been measured at zenith angles of 500 and 650 in the region of maximal asymmetry. The data are essentially consistent with the prediction of the Honda model.

(degrees)

(degrees)

RESPONSE OF THE HANOI CHERENKOV COUNTER TO MUONS

120

162

203

120

40

20x20

1

2

3

4 5 6

7

8

9

10 11 12

360

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. Small corrections allowing for the light attenuation in water and depending on the proximity of the track to the PMT have been worked out and applied.

We are currently improving the water purity and the quality of the wall surface in order to reach this goal.

AFTERPULSING

Muons stopping in the water and subsequently decaying into an electron and two neutrinos should give two light pulses separated by a laps of time of 2.2 μs on average (the muon lifetime) and having an exponential distribution. An attempt at detecting such events has revealed the presence of an overwhelming autocorrelation signal that prevented the experiment to be reliably performed.

A lab study of the phenomenon unambiguously identified afterpulsing as the source of this autocorrelation: the gas in the glass envelope of old PMT’s (ours are 30 years old) gets ionized by the primary photoelectrons and the produced ions are accelerated toward the photocathode (or an earlier dynode).

This time autocorrelation spectrum is typical and shows afterpulses at a few μs , making the muon lifetime measurement impossible. New phototubes given to VATLY by the manufacturer of the Auger PMT’s (Photonis) are currently being installed on our tank.

MUONS STOPPING IN THE HODOSCOPE

Out of an initial sample of over one million triggers we were able to select 72 candidates from their energy loss distribution in the upper layers (Bragg curve) and from their time of flight between the upper and lower counters (2m apart). A detailed comparison with a sub-sample of 1569 well identified crossing muons obeying the same cuts concluded at the presence of a genuine 3.8 standard deviation signal (33.1± 8.6 events), in agreement with expectation.

The time of flight distribution of the 72 candidates is shown together with the background expected from crossing muons.

As an illustration of the good performance of our trigger scintillation hodoscope we have searched for a signal of muons stopping in its lowest layer (3cm thick), namely having a momentum of the order of 140 MeV/c.

EXTENSIVE AIR SHOWERS

We are currently installing 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

217

112

185

140

115

100

392

305

40

PART 2

ANALYSES AND SIMULATIONS RELATED TO AUGER

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 PHYSICS OF THE PIERRE AUGER THE PHYSICS OF THE PIERRE AUGER OBSERVATORYOBSERVATORY

ULTRA HIGH ENERGY COSMIC RAYS (UHECR’s)ULTRA HIGH ENERGY COSMIC RAYS (UHECR’s)

• Accurate measurement of the high end of the energy spectrum

• Settlement of the GZK controversy

• Identification of possible sources

• Nature of the primaries

x = 2.7

x = 3.0

x = 2.7

Cosmic rays are ionized nuclei that travel in space up to extremely high energies,

~1020 eV=16 Joules! There are very few of them but they carry as much energy as the CMB (nearly 1 billion photons per proton) or the visible light or the magnetic fields ~1eV/cm3

Their composition is similar to that of matter in the universe except that the rare nuclei of the latter are more common in cosmic rays: spallation reactions in ~7g/cm2 of interstellar matter They have a power law spectrum over 32 decades (12 decades in energy), ~ E-2.7. The ankle and the knee are not really understood.

Above 1020 eV or so one expects the spectrum to be cut off (the Greisen-Zatsepin-Kuzmin, “GZK” cutoff, photoproduction of pions on the CMB photons) unless sources are close to us (ie are not AGN's). This issue is currently controversial.

There is now strong evidence from X-ray and gamma-ray astronomy that galactic supernova remnants are sources of cosmic rays and that acceleration takes place on the shock front.

CANGAROO RXJ1713.7-3946

RA

1km 1Gm 1pc 1Mpc

1G

1G

1MG

1TG 1020eV proton

s

RG LOBESSNR

SUN SPOTS

AGNWD

NS

L

B

UHECR’s may have their origin in the same mechanism of diffusive shock acceleration, at play in larger extragalactic structures such as active galactic nuclei and the radio lobes of radiogalaxies.

THE AUGER SURFACE DETECTORTHE AUGER SURFACE DETECTOR

Giant detector arrays are made of scintillators or water Cherenkov counters. That of the Pierre Auger Observatory covers 3000 km2 with a triangular grid having a 1.5 km mesh size (1600 detectors of 10m2 area each). One >1020eV shower detected every week, involving 15 to 20 detectors. Typical angular and energy resolutions are 1.5° and 20%.

A developing shower

SHOWER RECONSTRUCTION

4 unknowns: event time t0,

shower front curvature C,

shower axis direction cosines u

For each detector hit:

σ = u.a

t = t0+σ-C[t2+a2-2σt]

The figure is drawn in the shower detector plane when the shower front hits the detector

σ

ρ

t

a RImpact

on ground

Detector

Shower front

Surface array viewLateral 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).

Our simulation code is currently being used to study the systematic uncertainties attached to this measurement.

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.

We are currently developing such a program, preliminary results are presented.

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

Both the pulse height and pulse area distributions show a clear muon peak (arrow) standing above a background of electron-photon signals. Muon identification cannot be made on an event-to-event basis but only on a statistical basis.

Decimal logarithm of the pulse height Decimal logarithm of the pulse area

In order to study the performance of the program we analyse traces obtained by adding a typical muon peak to real traces.

We distinguish cases where the additional peak is added in a populated region of the initial trace (in) from cases where it is added in a deserted region (out)

Deviation of the number of peaks Deviation of the number of peaks

In the case where the muon peak is added in a populated region the program finds only 0.3 additional peak instead of 1 on average.

This number increases to 0.7 when only peaks higher than 2/3 of a muon peak are considered. The reason is that smaller peaks can easily be obscured or artificially produced. This limitation is inherent to the problem rather than to the procedure used here.

Deviation of the number of peaks Deviation of the number of peaks

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 300-400nm camera

440 PMTs

11 m2 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

TEACHING

• VATLY has been actively contributing to the teaching of particle physics and astrophysics in the Ha Noi universities (national and pedagogic) as well as in the Institute of Physics through lectures and supervision of master and graduation theses.

• Thanks to VATLY, modern astrophysics lectures were delivered for the first time to Vietnamese university students in 2005. Increased support from the Ha Noi faculty should help increasing their audience.

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. A program has been written to reduce the surface detector FADC traces to a sum of muon and electron-photon peaks and will now be used to extract physics information.

We are indebted to CERN, RIKEN, the French CNRS, the Rencontres du Vietnam and the Natural Science Council of Vietnam 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

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 2004 •Over 20 papers have been presented by the Auger Collaboration at the 29th International Cosmic Ray Conference, Pune, India, 2005

THESES

•Ph D theses: Dang Quang Thieu, 2005•Master theses: Nguyen Hai Duong, 2004 + four master theses currently underway•Diplom works: Pham Ngoc Diep and Pham Thi Tuyet Nhung, 2003

Dinh Lam Anh Huyen, 2004 + two diplom works currently underway