CERF Bonner Sphere Spectrometer response to charged hadrons

SINBAD ABSTRACT NEA-1552/24

Response of a Bonner Sphere Spectrometer to charged hadrons

1. Name of Experiment:
------------------
Response of a Bonner Sphere Spectrometer to charged hadrons measured at CERF
facility (2003).

2. Purpose and Phenomena Tested:
----------------------------
Bonner Sphere Spectrometers (BSS) are employed in neutron spectrometry and
dosimetry since many years. Recent developments have seen the addition to
a conventional BSS of one or more detectors (a �detector� is the moderator
plus thermal neutron counter) specifically designed to improve the overall
response of the spectrometer to neutrons above 10 MeV. These additional
detectors employ a shell of high-Z material (lead in the BSS used in the
present experiment) within the polyethylene moderator.

Under certain circumstances, the use of high-Z moderators may present a
problem, as they have a significant response to the charged hadron component
accompanying the neutrons emitted from a target. Conventional polyethylene
moderators show a similar behaviour but less pronounced. These secondary
hadrons interact with the moderator and generate neutrons, which are in turn
detected by the counter. To take this effect into account in the unfolding
procedure, the response function of the various detectors of the BSS to
charged hadrons must be known.

A real �calibration� of the BSS response to charged hadrons would require
the availability of broad beams of protons and charged pions of several
defined energies up to tens of GeV, which is not really feasible. Thus an
experimental verification of the response of the BSS to a monoenergetic beam
of high-energy hadrons was performed at the CERF facility at CERN as
described here.

3. Description of Source and Experimental Configuration:
----------------------------------------------------
Source:
120 GeV/c positive hadron beam (composed of 1/3 protons and 2/3 pions) of
Gaussian profile with FWHM (full width at half maximum) of 30.5 mm and
31.7 mm in the horizontal and vertical planes, respectively (determined with
a multi-wire proportional chamber), at the CERF facility at CERN [1].

4. Measurement System and Uncertainties:
------------------------------------
The BSS used in the present experiment employs a spherical Centronics SP9
3He proportional counter (3.2 cm active diameter, filled to a pressure of
202 kPa 3He and 101 kPa Kr, see he3.jpg) at the centre of moderators of
different diameters and composition: five polyethylene spheres of outer
diameter 81 mm, 108 mm, 133 mm, 178 mm and 233 mm (see bss233.jpg) and two
spheres made of a composite polyethylene/cadmium/lead moderator to detect
high-energy neutrons (called Stanlio and Ollio, see stanlio.pdf, stanlio2.jpg,
ollio.pdf and ollio.jpg). The density of the polyethylene (CH2)n is 0.963 g/cm3.
The material compositions (e.g. H and C contents in the polyethylene) are
given in the included FLUKA input files in the COMPOUND card as "atom
relative content". Note that the H to C ratio slightly different from 2,
according to the polyethylene specifications as given by the manufacturer.

For the simulations, the geometrical description of the spheres is the
following. At the centre of each sphere there is a spherical cavity housing the
3He counter. The counter being 3.2 cm in diameter, the cavity is 3.3 cm in
diameter so as to leave an air gap of 0.5 mm between counter and moderator. The
moderator has a cylindrical hole for housing the detector stem. The stem is a
hollow cylinder (filled with air) with inner radius of 5.1 mm and outer radius
of 6.5 mm. On the opposite side of the counter the stem extends by 8 mm into
the polyethylene. The stem and counter walls are made of aluminum. Five of the
spheres are made of pure polyethylene with outer diameter as given above.
Ollio is a sphere with outer diameter of 255 mm, consisting of moderator shells
of (from the central 3He proportional counter outwards) 3 cm polyethylene,
1 mm cadmium, 1 cm lead and 7 cm polyethylene thickness. Stanlio has outer
diameter of 118.5 mm and consists of moderator shells of 2 cm polyethylene,
1 mm cadmium and 2 cm lead thickness. The geometry of the seven detectors is
also described in the FLUKA input files (see section 5).

Each detector of the BSS was exposed to the 120 GeV/c hadron beam. The beam
impinged on each sphere at 25 mm from its centre.

The 10% uncertainty given on the experimental data is an estimate of the
total uncertainties, but it is mainly uncertainty on the beam monitoring
(which is done with an air-free ionisation chamber placed in the beam, the
standard monitor used at CERF). The statistical uncertainty on the number of
counts is negligible. The information on the uncertainties due to the size or
positioning of the sphere is not available.

5. Description of Results and Analysis:
-----------------------------------
Monte Carlo simulations with the FLUKA code were performed reproducing the
exact experimental conditions. The 14 input files are given (two per each of
the seven detectors of the BSS, one for 120 GeV/c proton beam and one for
120 GeV/c positive pion beam). The experimental data (counts per beam
particle) are compared with the results of the Monte Carlo simulations in
the table results.txt. The agreement is rather good except for the two smaller
spheres where the experimental figure is about of factor 2 higher than the
simulation value. This may be due to the fact that, given the dimension of
the sphere and the beam size, the sphere was possibly not fully intercepting
the beam. Even a small variation in the beam position and/or dimension would
cause a significant variation in the count-rate.

The present results served as experimental confirmation of the complete
response matrix of the BSS to charged hadrons calculated with FLUKA [2]. The
information on the response of the BSS to charged hadrons was used to correct
the results of an experiment performed at CERN aimed at determining the
neutron yield and spectral fluence at various angles from unshielded, semi-
thick copper, silver and lead targets, bombarded by a mixed proton/pion beam
with 40 GeV/c momentum [3].

6. Special Features:
----------------
None

7. Author/Organizer:
----------------
Experiment and analysis:
S. Agosteo (1), E. Dimovasili (2), A. Fass� (3) and M. Silari (2)
(1) Politecnico di Milano, CESNEF, Via Ponzio 34/3, 20133 Milano, Italy
(2) CERN, 1211 Geneva 23, Switzerland
(3) SLAC, P.O. Box 4349, Stanford, CA 94309, USA

Compiler of data for Sinbad:
M. Silari
CERN, 1211 Geneva 23, Switzerland

Reviewer of compiled data:
I. Kodeli
OECD/NEA, 12 bd des Iles, 92130 Issy les Moulineaux, France

8. Availability:
------------
Unrestricted

9. References:
----------
[1] A. Mitaroff and M. Silari. The CERN-EU high-energy Reference Field (CERF)
facility for dosimetry at commercial flight altitudes and in space,
Radiation Protection Dosimetry 102, 7-22, 2002.

[2] S. Agosteo, E. Dimovasili, A. Fass� and M. Silari. The response of a
Bonner Sphere Spectrometer to charged hadrons,
Radiation Protection Dosimetry 110, 161-168, 2004.

[3] S. Agosteo, C. Birattari, E. Dimovasili, A. Foglio Para, M. Silari, L.
Ulrici and H. Vincke. Neutron production from 40 GeV/c mixed proton/pion
beam on copper, silver and lead targets in the angular range 30-135�.
Nuclear Instruments and Methods B 229, 24-34, 2005.

10. Data and Format:
---------------

DETAILED FILE DESCRIPTIONS
--------------------------
Filename Size[bytes] Content
----------------- ----------- -------------
1 cerfbss-a.htm 12,476 This information file
2 CERF-RPD102_2002.pdf 403,616 Description of the CERF facility
3 Neudos9(DivRep).pdf 243,440 Reference paper with description of experiment, response
matrix to charged hadrons of the BSS.
4 he3.jpg 12,045 Figure of the FLUKA model of Centronics SP9 He-3
proportional counter
5 bss233.jpg 14,529 Figure of the FLUKA model of the 233 mm (outer diameter)
pure polyethylene sphere
6 stanlio.pdf 31,568 Cut through the moderator of the lead-modified Bonner Sphere
Stanlio
7 stanlio2.jpg 17,248 Figure of the FLUKA model of Stanlio
8 ollio.pdf 60,782 Cut through the moderator of the lead-modified Bonner Sphere
Ollio
9 ollio.jpg 20,358 Figure of the FLUKA model of Ollio
10 81_pions.inp 7,492 FLUKA input: 120 GeV/c pions on 81 mm polyethylene sphere
11 81_protons.inp 7,486 FLUKA input: 120 GeV/c protons on 81 mm polyethylene sphere
12 108_pions.inp 7,493 FLUKA input: 120 GeV/c pions on 108 mm polyethylene sphere
13 108_protons.inp 7,487 FLUKA input: 120 GeV/c protons on 108 mm polyethylene sphere
14 133_pions.inp 7,493 FLUKA input: 120 GeV/c pions on 133 mm polyethylene sphere
15 133_protons.inp 7,487 FLUKA input: 120 GeV/c protons on 133 mm polyethylene sphere
16 178_pions.inp 7,493 FLUKA input: 120 GeV/c pions on 178 mm polyethylene sphere
17 178_protons.inp 7,487 FLUKA input: 120 GeV/c protons on 178 mm polyethylene sphere
18 233_pions.inp 7,490 FLUKA input: 120 GeV/c pions on 233 mm polyethylene sphere
19 233_protons.inp 7,485 FLUKA input: 120 GeV/c protons on 233 mm polyethylene sphere
20 ollio_pions.inp 7,930 FLUKA input: 120 GeV/c pions on Ollio
21 ollio_protons.inp 7,928 FLUKA input: 120 GeV/c protons on Ollio
22 stanlio_pions.inp 7,843 FLUKA input: 120 GeV/c pions on Stanlio
23 stanlio_protons.inp 7,837 FLUKA input: 120 GeV/c protons on Stanlio
24 results.pdf 23,668 Comparison between experimental data and FLUKA predictions
25 results.txt 1,402 Comparison between experimental data and FLUKA predictions

Figures are included in the jpg and PDF formats.

SINBAD Benchmark Generation Date: 09/2006
SINBAD Benchmark Last Update: 09/2006