Variations in the Neutron Time Delay Distribution at thePrincess Sirindhorn Neutron Monitor
1,2 , D. Ruolo1,2 , N. Kamyan1,2 , T. Nutaro2,3 , S. Sumran3 ,333
4
C. Chaiwattana , N. Gasiprong , C. Channok , M. Rujiwarodom ,2,445
55
P. Tooprakai, B. Asavapibhop , J. W. Bieber , J. M. Clem , P. Evenson ,6and K. Munakata
A. Sáiz

1 Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
2 Thailand Center of Excellence in Physics, CHE, Ministry of Education, Bangkok 10400, Thailand
3 Department of Physics, Ubon Ratchathani University, Ubon Ratchathani 34190, Thailand
4 Department of Physics, Chulalongkorn University, Bangkok 10400, Thailand
5 Bartol Research Institute and Department of Physics and Astronomy, University of Delaware,

Newark, DE 19716, USA

6 Physics Department, Shinshu University, Matsumoto, Nagano 390-8621, Japan
August 16, 2012

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 2012

1 / 20Outline

Introduction
Cosmic Rays
The Princess Sirindhorn Neutron Monitor
Time delay distributions
Chance coincidences
Following counts
Leader fraction

LCorrections for dead time and overow
Analysis of temporal variations ofL

Results
Conclusions

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 20122 / 20Introduction

Cosmic Rays

Cosmic Rays and Space Weather

I Galactic cosmic rays withE ∼ tens of GeV

IHave high speeds, long mean free paths

IAre aected by interplanetary structures related to solar activity

ICan be used for remotely probing near-Earth space =⇒ space weather
forecasting

I For example:IEects of solar wind structures (high speed solar wind streams, CIRs) at ornear Earth seen in cosmic ray anisotropies [Yeeram et al, in preparation]

I

Loss cones prior to Forbush decreases [Leerungnavarat et al 2003]

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 20123 / 20Introduction

Cosmic Rays

Cosmic Ray Detection

I Cosmic rays produce
cascades of secondary
particles on Earth's
atmosphere

I If energetic enough,secondary particles reachground level

I Neutron monitors measureatmospheric neutrons fromcosmic ray cascades
ISensitive to air mass:

P correction

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 2012

4 / 20Introduction

Cosmic Rays

Neutron Monitors

I Neutron monitors are designed tomonitor number of cosmic rays

I Originally not designed to measurecosmic ray energy

I Multi-station measurements cangive spectral information

I What can be done with a single
station?

Figure: Neutron monitor locations andvertical cuto rigidity contours. Credit:
R. PyleAlejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 2012

5 / 20Introduction

The Princess Sirindhorn Neutron Monitor

The Princess Sirindhorn Neutron Monitor (PSNM)I PSNM operates since late 2007
I Location: Doi Inthanon, Chiang Maiprovince

I Altitude: 2565 m over sea level
I World's highest cosmic ray cuto rigidity

pc /q ) for a xed station, 16.8 GV(Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 2012

6 / 20Introduction

The Princess Sirindhorn Neutron Monitor

PSNM is a standard 18-NM64:

I 18 proportional counter tubes

BF3 gas)I 30 tons of Pb as neutron(10producerI Polyethylene neutron reectorand moderators

Oneatmosphericwith aneutroninteracts

Pb nucleus producing moreneutrons, which are moderated by thepolyethylene and nally detected in
the proportional counters throughn + 10B −−→ 4He + 7Li ∗ .
Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 20127 / 20Time delay distributions

Time delays

The electronics in PSNM's data acquisition system records time delays between
successive neutron counts

I Long time delays: counts from independent atmospheric neutrons
I Short time delays: mostly from neutrons produced from the same=⇒ information about cosmic ray energy

Pb nucleus

ICosmic rays with higher energy produce higher energy atmospheric neutrons

INeutrons with higher energy produce larger numbers of neutrons in theIMultiple neutrons produced together in thePb show short time delays

Pb

I But some are just chance coincidences!

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 20128 / 20Time delay distributions

Chance coincidences

Chance coincidences

R (t ) is the probability of time delay ≥ t for a neutron count in one counter tube
I n(t ) is the probability density function: n(t ) ≡ −dR /dtI

α is the probability per unit time of having a new countIf all the counts were independent:dR = −αR ,dt
andord ln R = −α,dtso

R = e−αtn = α e−αt=⇒ a straight line in a log-linear plot of n(t )Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 20129 / 20Time delay distributions

Chance coincidences(a)

1000

100
1-s / ) t(nn(t ) at one counteriExample of10tube recorded during one hour

I Long time delays (a) show1

the exponential distribution
typical of unrelated events

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

t / siI Short time delays (b)

(b)

1000

deviate substantially from64

the exponential (blue line).

21is / ) t(n10064dead time

210

0.0

0.5

1.0

1.5

2.0

t / msi

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201210 / 20

Time delay distributions

Following counts

Following counts

Short time delays are dominated by counts of neutrons produced from the same

Pb nucleus =⇒ following counts

I Total distribution is not the sum of follower distribution and chancecoincidences: distributions are not independent!

I Distribution of chance coincidences gets aected by followers, and vice versa
The conventional way to estimate following counts:

I Number of counts during a short time window: multiplicity
I Contaminated by chance coincidences

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201211 / 20

Time delay distributions

Leader fraction L

Leader fractiontIf β( ) is the probability per unit time of a following count with delayt since theprevious count from the same production event, thendR = −R (α + β(t )) ,dt

d ln R = − (α + β(t )) ,dt

orWe dene the nuclear part ofRt

For long time delays,Rt

R = e−αt e− 0 β(t ) dt
00

R (t ) as:Rn (t ) ≡ e− 0 β(t ) dt

Thenso0n = α e−αt Rn − e−αt0dRn
dtdRn /dt ' 0 and Rn ' constant: Rn ' Rn (∞) ≡ L=⇒ leader fraction, andAlejandro Sáiz (Mahidol University)n ' α L e−αt
Neutron Time Delay Distribution at the PSNM

August 16, 201212 / 20

Time delay distributions

Leader fraction L(a)1000

L is the probability for a neutron

1001-s / ) t(ncount to be the leader in a mul-Iitiple count

L is larger when multiplicity
is smaller

101

I It may contain information
about cosmic ray energy

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

t / siI Free from eects of chance(b)1000coincidences

64

Fitting the exponential tail in the21-100is / ) t(nn ' α L e−αt we canestimate both α and L.
data (a) to64

dead time

2100.0

0.5

1.0

1.5

2.0t / msi

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201213 / 20

Time delay distributions

Corrections for dead time and overow

Corrections to the equationt

I The electronics have a dead time dead ' 0.1

tdead are not recorded)

ms (time delays shorter thant
toverow are recorded modulo toverow )I There is an electronics overow at overow ' 142ms (time delays longer thanBoth eects vary with α. We include the corrections in order to estimate

L moreaccurately:n ' α L eαtdead 1 + αtoverowe

Alejandro Sáiz (Mahidol University)

1−1

Neutron Time Delay Distribution at the PSNM

e−α .
t

August 16, 201214 / 20

Analysis of temporal variations of L

Analysis of temporal variations of L

We estimate α and then

L from t to exponential tail of time delay distributions

at PSNMI Hourly-averaged distributions
I 18 months of data (July 2009January 2011)
I Individual counter tubes

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201215 / 20

Analysis of temporal variations of L

Results

Variations of αImiddle tube

α related to24

uncorrectedpressures( ahpla1-=⇒ atm.

)

count rate

23

22
end tube
21

20

34.0

2.20
2.1533.0)h( stnuoc1-)

35.0

2.25

s( 81/stnuoc

36.0

6

2.30x101-19)gHmm( P568
566
564
562
560
558
7/1/2009

1/1/2010

7/1/2010
UT

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNMAugust 16, 201216 / 20

Analysis of temporal variations of L

Results

Variations of L
0.8290

middle tube

0.8280IL should be

end tube
0.8140coincidences

0.8130

L is quite

0.8120constant foreach tube

I

0.8260eect of chance)gHmm(PIL

free from the0.8270

L is lower for560

0.82800.82700.8260

Lrroc

the sides

562

0.8290the end tubes:
reection at

566
564

0.81400.81300.81207/1/2009

1/1/2010

7/1/2010
UT

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201217 / 20

Analysis of temporal variations of L

Results

Variations of L

I Small but clear
correlation withatm. pressure

I At highP (larger0.835

air mass)

0.830

multiplicity issmaller so

L is

I Correcting for thistrend still leaves aseasonal variation

Llarger

0.825

0.820

0.815

0.810

0.805

I Maybe related toatmospheric

560562

564

566

568

P (mmHg)

structure?

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201218 / 20

Analysis of temporal variations of L

Results

Variations of L

0.81100.8105

conrmed using
daily histograms for
3 years

Lcorr

I Seasonal variations

0.81000.8095

1/1/2008

1/1/2009

1/1/2010

1/1/2011

UT

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201219 / 20

Conclusions

Conclusions

I We analyze neutron time delay distributions for the PSNMI We nd time variations in the leader fraction
I

Related to true neutron multiplicity

I

Not contaminated by chance coincidencesL

I In the present study, variations may be atmosphericThank you for your attention

Alejandro Sáiz (Mahidol University)

Neutron Time Delay Distribution at the PSNM

August 16, 201220 / 20