Summary of the meeting
Meeting on Monte Carlo simulation
inside the nTOF collaboration
List of participants:
"Jose Luis Tain"
"Mario De Poli"
A. Structure of the simulation group
The main purpose of the meeting was to define
the structure, objectives, tasks and priorities of the nTOF Monte
Carlo simulation group (TOFMC). E. Gonzalez proposed that the
simulation work can be done in a distributed way at the home institute
of every TOFMC participant. However, all the relevant information
should be concentrated in one place of easy access (email, WEB).
The group at CIEMAT offered to perform the co-ordination of the
simulation activities and also to provide the concentration, distribution
and archiving of the TOFMC data including:
- Centralisation all the information relevant
to the simulation group: geometry of the facility, properties
of the neutron beam, description and features of the detectors,
information about the samples to be measured and others.
- Collection/distribution of the simulation
- Attend the simulation requests and transmit
them to the simulation group.
B. Simulation strategy
- Different types of simulations are involved
in the MC for the nTOF:
- Neutron beam.
- Neutron background from the facility.
- Neutron background produced by the samples.
- Photons from the lead target.
- Photons from the samples.
- Detector responses.
- Global simulation is a problem to be solved
in several steps: Usually, the output of one step becomes the
input for the next one. Several codes can (should if possible)
be applied at each step. The transport of high energy neutrons
(En > 20 MeV) can be done for example with Fluka
(one of the best available codes) down to En = 20
MeV. The tracking of such neutrons is then continued either by
MCNP or EAMC (Energy Amplifier Monte Carlo). The tracking of
photons (produced by capture, bremsstrahlung or other processes)
can be done with MCNP, Fluka, GEANT or EGS. The simulations of
can be done independently with any of the codes well suited for
it (Fluka, GEANT, EGS). In addition, GEANT has been used for
some neutron transport problems. Extra codes will be needed in
order to compute the input of one phase from the output of the
previous one (for example, the cascades of g following the capture
processes computed with MCNP which can be simulated with the
same or a different code like GEANT). A remark was made on the
need to converge to the same versions of the codes. Additionally,
any modification of them should be properly documented.
- The strategy of using more than one point
of view for each simulation is recommended. In this sense, some
limited duplication with more than one group studying the same
problem will be encouraged.
The list of specific topics concerning simulations
were divided into five blocks:
- Simulation of the nTOF facility:
- Selection of the collimator shape, composition
- Evaluation of the beam intensity, profile
and energy spectrum of the main neutron beam at the target.
- Selection of the size and shape of the beam
pipe in the experimental station.
- Selection of the shapes and positions of
the shielding walls along the pipe.
- Evaluation of the neutron background at the
- End of the tube after the experimental station
- Detector responses.
- Simulation of the TOF detectors:
- Selection of the detector positions.
- Estimation of the detector efficiencies.
- Estimation of the detector sensitivity to
- Detector responses.
- Simulation of the specific run 2000 experiments:
- Simulation of the complete (simplified or
- Beam-target interactions: generation of g-cascades, charged
- Response of the detectors.
- Counting rates vs. Time/energy: beam time
- Background from the facility.
- Background from the different targets.
- Interaction between the detectors.
- Detector calibration needs?
- New phases of the facility:
- Needs for a near station (NS): type of measurements,
- Collimator requirements at the NS.
- Detector positions.
- Expected performances/counting rates (relevant
also to DAQ).
- Expected backgrounds and shielding needs.
- Design of future detectors and measurements:
- BaF2 4p detector, liq. noble gas detectors (Ar, Xe), advanced
- New targets for the experiments.
C. New layout of the facility
The new layout of the facility was presented
by E. Radermacher. With respect to the previous design, the lead
target has been moved 30 m inside the tunnel. This makes possible
(but delicate due to activation) to have access to it if needed.
The tube length is now 200 m, and the pipe has been divided in
three sections: an approx. 70 m long tube of 80 cm diameter, an
approx. 70 m long tube of 60 cm diameter and finally an approx.
60 m tube of 40 cm diameter. The geometry of the tube at the end
of the pipe as well as the shielding elements along the neutron
flight path have to be defined. The most urgent thing is to specify
the first shielding element at about 70 from the lead target (at
the 1st reduction of the tube diameter), since the
pipe will be finished up to this point by middle of November.
An important point is that the shielding can not be a wall: safety
reasons force to allow a free path through all the parts of the
tunnel. The drawings of the facility can be found in the copy
of the presented transparencies attached to this document. The
shielding after the 2nd reduction of the tube diameter
should be done before middle of December. Several participants
expressed their concerns about the chicane design of the walls
at the pipe diameter reduction locations and its effect on the
A comment was also made on the possibility
of having a counting station at 30 meters. There exists the possibility
of installing a device which permits to insert/remove fission
detectors inside the vacuum. However, the reduced space inside
the pipe (80 cm diameter) makes difficult to insert g-detectors for
capture measurements. For the second phase of facility, the installation
of a second pipe of 30 m length coming directly from the lead
target was discussed as a possible near capture measurements station.
Another difficulty is coming from the activation
of the target. Some calculations done by the CERN group indicate
will be too high there. Even at the end of the tube at 200 m,
it will be necessary to shield the g's coming from the target whenever some work is needed
there. A report of this calculation was requested.
Another topic of discussion was the carbon
window at the beginning of the pipe (80 cm diameter). It was stated
that calculations made at CERN reveal no influence of the window
on the time/energy relation of the neutrons. A report on the results
of these simulations was also requested.
Since the design of the facility has been modified,
a new overall simulation is required in order to have the proper
definition of the neutron source (intensity spatial profile and
angular and energy distributions) for the new conditions. This
task will be done by the CERN group: a DST (data summary tape)
containing all the important parameters related to the neutron
source at a certain position will be provided in the next weeks.
The details of the DST have still to be specified, and this must
be done very soon.
D. Descriptions of the facility and the detectors
The inputs needed for the simulations were
separated in three groups, even if those are strongly linked.
- The facility:
full geometrical description of the facility. Technical drawings
including dimensions and detailed descriptions of the material
compositions of all the elements are required. The contact person
- The detectors:
the complete description of the geometry, composition and surrounding
materials have to be provided. Additionally, the people constructing
a detector should be involved in the Monte Carlo simulation group.
The contact persons for detectors:
- The samples:
in order to define the beam parameters, some initial information
on the samples to be used is urgently needed. It was noticed
that the beam parameters also influence the geometry of the samples.
- Samples for capture measurements
Two different approaches seem to be possible.
The Saclay group pointed out that the samples used in their experiments
at Geel have the same dimensions as the beam pipe (8 cm) and
variable thickness (0.1 mm for Au and 1g/cm2 for 237Np).
On the other hand, the group at Karlsruhe mentioned that their
samples have much smaller surface (1-8 cm) and larger thickness.
- Samples for fission measurements
The samples used in fission experiments can
not be thicker than few times 100 mm for a proper release of the fission fragments. Their
diameter can be typically of 1-3 cm.
- Samples for (n,xn) measurements
Usually there are two kind of samples: small
surface and thick samples are used when the g-radiation has to be detected,
while large surface and thin samples are needed when looking
at the conversion electrons.
The contact persons for the samples are:
E. Priorities for the year 2000 experiments
- Fission cross section measurements of 235U,
237Np, 238U and 239Pu with PPAC detectors.
- Capture cross sections of 197Au, 27Al, 19F
and natAg with C6D6 detectors and/or Moxon-Rae detectors.
- Capture cross sections of fissiles: 235U
and 238 U (simultaneuosly with FF-detectors).
- Test of g-detectors: 1p BaF2 and HPGe detectors.
F. Simulation of the C6D6 detectors for the year 2000
F. Gunsing presented a first layout of the
C6D6 detectors for the year 2000 experiments. The analysis of
the capture x-section measurements with C6D6 detectors needs
an accurate determination of its response functions, and this
should be attempted by Monte Carlo simulation. As commented by
J.L. Tain, this seems to be a delicate problem and related to
the materials surrounding the detector. A complete geometrical
description will be distributed as soon as possible.
On the other hand, some general comments on
the C6D6 setup were made by F. Gunsing:
- The pipe should be made of carbon and have
8 cm diameter
- The samples should also have 8 cm diameter
and variable thickness up to 1g/cm2.
- Some neutron absorber should be placed after
the collimator in order to reduce the background (a Li compound
is preferable to B, since no g-radiation follows after the capture).
- An on-line monitoring of the neutron beam
should be made. It has been observed in similar experiments
(Oak Ridge/Geel/Los Alamos) that there are deviations between
the charged particle beam current and the calibrated neutron
production. However, it was pointed out that an in-beam neutron
flux would certainly produce also background.
- A system of several filters could provide
some additional information of the background. However, the efficacy
of such system was questioned in presence of high energy neutrons.
G. Other business
N. Pavlopoulos commented on the possibility
of creating a PC-farm running Linux for the computing needs of
the collaboration. The farm would be maintained by the CERN computing
division. Also mentioned was the need for CPU requests to CERN
for the PS213.
H. Concluding remarks
- The structure of the MC simulation group
proposed by CIEMAT was accepted. CIEMAT will assume the co-ordination,
collection, distribution and archiving of all the information
related to MC simulation.
- The shielding at the 1st tube
diameter reduction point (70 m) has to be defined before middle
- The shielding at the 2nd tube
diameter reduction point (140 m) has to be defined before middle
- The people constructing a detector has to
be involved in the Monte Carlo simulation group. A list of the
detector contact persons is included in this minutes.
I. Actions to be taken
- The institutes which intend to contribute
for the simulations should provide a contact person for fast
communication as well as a list of available resources (computers
and codes) within the next 10 days following the meeting.
- N. Pavlopoulos will provide within a week
a detailed proposal for the samples to be used in the year 2000
experiments. Comments from the collaboration should follow urgently.
- The CERN group should provide as soon as
possible all the information related to the facility: technical
drawings with dimensions, materials and foreseen proton beam
- The groups responsible for every detector
type involved in the year 2000 experiments should provide detailed
detector geometries, characteristics and setups.
- All this information is expected to arrive
at CIEMAT within 2-3 weeks (please in electronic format).
- The parametrization of the main beam and
the background description, in the form of a neutron DST, has
to be urgently provided. CERN and CIEMAT are responsible for
this task. First results are expected before middle of November.
- The simulation of the different detectors
should start in parallel and first progress reports are expected
- The next meeting (to be held in December)
will be mainly devoted to the presentation of the first results.
- Two reports on the ongoing simulation of
the lead target activation and the effects of the beam window
on the time/energy correlation are expected from CERN.
Talk by E. Gonzalez
Talk by E. Radermacher
Talk by F. Gunsing
Talk by E. Gonzalez
(year 2000 experiments)
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