Measurement and simulation of neutron detection efficiency in lead-scintillating fiber calorimeter

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Nov 26, 2013 (3 years and 11 months ago)

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Measurement and simulation of neutron detection

efficiency in lead
-
scintillating fiber calorimeter


Active

material
:




1
.
0

mm

diameter

scintillating

fiber

(Kuraray

SCSF
-
81
,

Pol
.
Hi
.
Tech

0046
),


emitting

in

the

blue
-
green

region
:

l
Peak

~

4
60

nm



Core
:

polystyrene,

r
=
1
.
050

g/cm
3
,

n=
1
.
6



High

sampling

structure
:



200

layers

of

0
.
5

mm

grooved

lead

foils

(
95
%

Pb

and

5
%

Bi)



Glue
:

Bicron

BC
-
600
ML,

72
%

epoxy

resin,

28
%

hardener



Lead
:
Fiber
:
Glue

volume

ratio

=

42
:
48
:
10


Good

time

resolution,

energy

response

and

high

photon

efficiency



s
E
/E

=

5
.
7

%

/

√E(GeV)

s
T
=

54

ps

/

√E(GeV)



The neutron beam line at TSL


Blue Hall

3
m

KLOE calorimeter module

E
KIN

(MeV)



A quasi
-
monoenergetic neutron beam from protons on


7
Li target (
7
Li(p,n)
7
Be
),
~
50% of neutrons at max energy



Three different energies used : 174, 46.5 and 21.8 MeV



Round collimator of 2cm Ø



Calorimeter from 5 to 6 m from target



Absolute neutron flux in the peak measured after the


last collimator by beam intensity monitor



Cyclotron RF period from 45 to 78 ns, depending on


energy

ABSTRACT

The

overall

detection

efficiency

to

neutrons

of

a

small

prototype

of

the

KLOE

Pb
-
scintillating

fiber

calorimeter

has

been

measured

at

the

neutron

beam

facility

of

The

Svedberg

Laboratory,

TSL,

Uppsala,

in

the

kinetic

energy

range

[
5
,
175
]

MeV
.

The

measurement

of

the

neutron

detection

efficiency

of

a

NE
110

scintillator

provided

a

reference

calibration
.

At

the

lowest

trigger

threshold,

the

overall

calorimeter

efficiency

ranges

from

30

%

to

50
%
.

This

value

largely

exceeds

the

estimated

8

%

expected

if

the

response

were

proportional

only

to

the

scintillator

equivalent

thickness
.

A

detailed

simulation

of

the

calorimeter

and

of

the

TSL

beam

line

has

been

performed

with

the

FLUKA

Monte

Carlo

code
.

The

simulated

response

of

the

detector

to

neutrons

is

presented,

as

well

as

a

first

data
-
Monte

Carlo

comparison
.

The

reasons

of

such

an

efficiency

enhancement,

in

comparison

with

the

typical

scintillator
-
based

neutron

counters,

are

explained,

opening

the

road

to

a

novel

neutron

detector
.

M. Anelli
a
, S. Bertolucci
a
, C. Bini
b
, P. Branchini
c
, C. Curcenau
a
, G. De Zorzi
b
, A. Di Domenico
b
, B. Di Micco
c
, A. Ferrari
d
, S. Fiore
b
, P. Gauzzi
b
,
S. Giovannella
a
, F. Happacher
a
, M. Iliescu
a
, M. Martini
a
, S. Miscetti
a
, F. Nguyen
c
, A. Passeri
c
, B. Sciascia
a
, F. Sirghi
a


a
Laboratori Nazionali di Frascati, INFN, Italy
b

Universita’ degli Studi “La Sapienza” e Sezione INFN di Roma, Italy
c
Universita’ degli Studi “ Roma Tre” e Sezione INFN di Roma3, Italy
d
Fondazione CNAO, Milano, Italy

Small

prototype

of

the

KLOE

calorimeter
:

60

cm

long,

3

x

5

cells

(
4
.
2

x

4
.
2

cm
2
),

read

out

at

both

ends

by

PMTs


Reference

NE
110

scintillator

counter
,

10
×
20

cm
2
,

5

cm

thick

read

out

at

both

sides

with

PMT’s


Rotating

frame

allows

for

detector

positioning

(data

taking

with

n

beam

-

calibration

with

cosmic

rays)


Low

beam

intensity

(
3
-
10

kHc/cm
2
)

at

collimator

exit

provides

negligible

contribution

of

double

neutron

counting

per

event



Trigger

built

by

the

coincidence

of

the

discriminated

signals

of

the

two

sides

for

each

detector
.

For

the

calorimeter

the

analog

sum

of

the

first

four

(out

fo

five)

planes

is

used
.

A

phase

locking

with

RF

signal

defines

a

precise

start

for

the

event

and

allows

time

of

flight

measurement
.


Typical

runs

consists

of

1

Mevents

acquired

at

~

2

kHz

rate,

thus

allowing

to

perform

scans

at

different

trigger

thresholds

The experimental set up and data sets


Measurement of overall neutron detection efficiency


α
R
)
1
(
R
α
R
R
ε
neutron
trigger
neutron
signal





H
F

R
neutron
: = Rate(ICM)





p

r
2
/ f
peak

ICM: Ionization Chamber Monitor



online rate determination

TFBC: Thin Film Breakdown Counter


absolute flux calibration of peak neutrons (K)

R
trigger

: Detector trigger rates from scalers

F
H
: fraction of halo neutron events [ H/(H+S) ]

a
㨠††††⁤整捴潲c慣数瑡e捥c㴱Ⱐ晲潭=䵃

Neutron fluence


Proton fluence



The

measurement

of

the

scintillator

efficiency

gives

a



cross

calibration

of

the

measurement

method

and

of

the



beam

monitor

accuracy
,

with

small

corrections

due

to



the

live

time

fraction




The

energy

scale

is

calibrated

with

a

90
Sr

b

獯s牣r
.


††

%

慣畲慣

景f

桯物h潮瑡l

獣s汥

⡴桲敳桯汤)

慮

瑨t


††
癥牴楣慬

潮

(
e
)





Results agree with “thumb rule” (
1%/cm
):


5%

for
5 cm thick scintillator

(at a threshold of

㈮2⁍敖




Agreement, within errors, with previous


published measurements in the same energy


range, after rescaling them to the used


thickness



Similar agreement also for low energy


measurements



Scintillator efficiency



The KLOE Pb
-
scintillating fiber calorimeter

Calorimeter efficiency





Energy

scale

set

using

MIP

calibration

of

all

channels,

and

using

the

MIP/MeV

scale

factor

of

the

KLOE

experiment




Energy

cut
-
off

introduced

by

the

trigger

evaluated

by

fitting

with

a

Fermi
-
Dirac

function

the

ratio

of

total/cluster

energy


at

different

thresholds




Systematic

errors

on

vertical

scale

dominated

by

halo

subtraction

and

absolute

neutron

flux




Systematics

on

horizontal

scale

conservatively

assigned

by

the

difference

between

cut
-
off

determined

with

an



independent

method

(cosmics

and

neutron

data

triggered

with

an

.
OR
.

Between

scintillators

and

calorimeter)




Stability

w
.
r
.
t
.

very

different

run

conditions
:

a

factor

4

variations

of

live

time

fraction

(f
LIVE
=
0
.
2



0
.
8
)

慮d

扥慭


††
楮瑥湳楴i

(

3




†
歈k⽣/
2

)


Very high efficiency
,

about 4 times larger than what expected

if only the amount of scintillator is taken into account

(
~
8% for 8 cm of scintillating fibers)

FLUKA simulation of beam
-
line and calorimeter

An efficiency enhancement w.r.t. bare scintillator counters is related to the

huge inelastic production of neutrons on the lead planes
:


-

produced isotropically and with a non negligible fraction of e.m. energy and protons which are detected in the nearby fibers


-

lower energy secondaries( E
≤ 19.6 MeV
)


larger probability of interaction in the calorimeter with further n/p/
γ

production (62/7/27%)



The measurement of the detection efficiency of a high sampling lead
-
scifi calorimeter to neutrons, in the energy range


[5,174] MeV, has been performed at TSL



The efficiency ranges between 30% and 50%, depending on the energy, at the lowest trigger threshold used, resulting


four times larger than what expected for an equivalent scintillator thickness

Conclusions and plans

Response on calorimeter module

Beam line simulation

Example of a neutron interaction

1.2 mm

1.35 mm

1.0 mm


Target


P
el
(%)



P
inel
(%)


Pb


32.6



31.4


Fibers


10.4


7.0


Glue


2.3


2.2

High probability to have

interactions in lead

174 MeV nutrons

For each beam energy, the overall efficiency is defined as the average over the full neutron energy spectrum

Neutron flux known with an accuracy of



㄰┠⠱㜴⁍敖⤠Ⱐ,〥
0潷敲灥慫敮敲杹)

Beam halo evaluation

Three

evidences

of

a

sizeable

beam

halo

contribution
:

1
.

Single

cell

clusters

show

enhanced

rate

on

lateral/central

cells

w
.
r
.
t
.

MC

2
.

Special

runs

@

22

MeV

with

calorimeter

out
-
of
-
beam

3
.

Horizontal

scan

with

TFBC

close

to

the

collimator

exit

at

low

energy



= Central cell



= Lateral cell

174 MeV: Signal from MC


Halo shape from lateral cells

21.8 MeV: Signal from MC


Halo shape from out
-
of
-
beam runs

The

KLONE

(
KLO
e

N
eutron

E
fficiency) group has measured the neutron detection

efficiency of a KLOE calorimeter prototype, at The Svedberg Laboratory (TSL), Uppsala,

Oct 2006


Jun 2007, performing also the whole simulation of the experiment.







Motivations:




Detection of neutrons of few to few hundreds of MeV is traditionally performed with


organic scintillators (elastic neutrons scattering on H atoms


production of protons


detected by the scintillator itself)


efficiency scales with thickness


~
1%/cm




Preliminary measurement at KLOE (neutron from K


beam pipe interactions) showed


an efficiency of

40% for
E
kin



20 MeV
. An efficiency of

10% would be expected


if the response were only due to the equivalent amount of scintillator in the calorimeter




Enhancement of neutron detection efficiency for fast neutron is observed in presence


of medium
-
high Z materials, particularly lead, as in the extended range rem counters


for radiation protection




The KLOE e.m. calorimeter has an excellent time resolution, good energy resolution,


and high efficiency for photons. If a high neutron detection efficiency were observed,


this could also be the first of a novel kind of neutron detectors


Neutron detection is important for the DAFNE
-
2 program @ LNF:



AMADEUS: study of deeply bounded kaonic nuclei



DANTE: measurement of nucleon time

汩步lr敧楯渠n⹭⸠景牭晡捴潲c

Shielding

(concrete / steel)

Calorimeter

7
Li Target

Y (cm)

Z (cm)

Z (cm)

X (cm)

beam

n

X (cm)

Z (cm)

p

n
1

n
2

n
3

n
4

primary
vertex

E
n

(p) = 126 MeV

Z (cm)

X (cm)

n

Single cell

clusters

Multiple cell

clusters

174 MeV neutrons

Scintillator efficiency measurement
, scaled by the scintillator ratio factor 8/5

Scintillator efficiency measurement
, scaled by the scintillator ratio factor 8/5

Threshold (MeV equiv. e


敮敲杹e

174 MeV neutrons

e

(%)



A full simulation with FLUKA is in progress. First data
-
MC comparisons are encouraging and allowed to disentangle a


neutron halo component in the beam



Further test are planned in two weeks at TSL @ 174 MeV with additional detectors: a KLOE prototype with high readout


granularity, a calorimeter with higher lead
-
scintillating fiber ratio and a beam position monitor