# Beamline design for particle production experiment, e907, at fnal\*

C. Johnstone^\#^, C. Brown, D. Carey, M. Kostin, R. Raja, FNAL,
Batavia,IL 60510, USA\
E.Hartouni, LLNL, Livermore, CA 94551, USA

Abstract

Experiment 907 at Fermilab will be conducted in the Meson Center beam
enclosure. The purpose of this experiment is to measure cross sections
for hadron production from nuclear interactions using pions, kaons and
proton beams in the momentum range from 5 to 120 GeV/c. Light to heavy
targets will be used to study the scaling laws of hadronic fragmentation
and light meson and baryon spectroscopy. Design aspects for the
experiment's beamline are presented. The lattice, in particular the
secondary beamline design, the primary target, and the collimation
system are covered.

## INTRODUCTION

MIPP (FNAL E-907) stands for Main Injector Particle Production
Experiment. A low-cost experiment, it uses existing hardware to measure
particle production off hydrogen and other nuclear targets including the
NuMI targets, with excellent initial and final state particle
identification. The resolution and statistics projected for the
experimental data represent a significant improvement over existing
measurements. Analysis of the experimental results will provide:

-   A test of the accuracy of and energy range over which a general
    scaling law of fragmentation applies given each particle type,

-   A measurement of particle production off NuMI targets with
    sufficient accuracy to predict the neutrino spectra at the near and
    far detectors in MINOS,

-   A starting point for the study of the dynamics of non-perturbative
    QCD and its associated resonances,

-   A better understanding of the propagation of particles in nuclei.

-   Improvement in the predictions of atmospheric neutrino fluxes,

-   Advance the design of accelerator-based neutrino factories,

-   Improvement of hadronic shower models in collider simulation
    programs (GEANT and MARS, for example).

A key feature of this experiment is the high-precision identification of
the produced particle species across the entire kinematic range that can
be accessed using the 120-GeV primary proton beam from the Fermilab Main
Injector. Precise particle identification is critical to accomplish the
goals stated above. The experiment achieves nearly complete resolution
for π, *K*, and *p* at a the secondary energy from 0.1-120 GeV/c by
measuring energy loss in a TPC, threshold Cerenkov radiation combined
with ring-imaged Ĉerenkov radiation, and time of flight.

Most the experimental beamtime will be spent in survey mode running. The
secondary beamline (in Meson Center) will allow acquisition of data at
one rigidity for K, and p simultaneously for one sign of the charge. The
incident secondary beam species will be tagged by threshold Ĉerenkov
counters upstream of the 1% interaction length experimental (or
secondary) target. In 126 hours of beamtime, one million events will be
acquired per incident particle species. The momentum range from 5-110
GeV/c will be scanned over a set of experimental targets that span the
periodic table from hydrogen to lead.

Further primary data will be acquired at 120 GeV/c by directing the Main
Injector primary beam directly onto the NuMI targets. In this way, the
pion production spectrum can be measured to 1% across the entire range
relevant for MINOS, which is 1-50 GeV/c. Since kaons are produced at
approximately 10% of the rate for pions, and both exhibit a fairly flat
longitudinal spectrum, particle identification across this momentum
range is critical for the measurement.

## The PRIMARY AND SECONDARY BEAMLINES

The length available to the experiment for primary beam control and the
secondary beamline totals only 128m, with 41m reserved for the threshold
and ring-imaging Ĉerenkovs. Due to enclosure restrictions and the need
for primary beamline magnets in this section (no superconducting magnets
are used), the primary portion must remain at least 20 m long. Less than
100 meters is left to accommodate the necessary secondary line
optics---optics appropriate for momentum selection and which also meet
the beam envelope and divergence criteria for the experiment. A picture
of the physical layout of both lines, plan and elevation view, is given
in Figure 1.

### Primary Beamline Layout

The primary beamline extends for well over a thousand meters through
various beam splits back to the Main Injector. Control over primary beam
characteristics is established through a focusing quadrupole doublet and
adjustable collimators located roughly 300 and 200 m upstream of the
primary target, respectively. Intensity on

![](media/image1.wmf){width="2.8125in" height="2.875in"}

![](media/image2.wmf){width="2.8625in" height="1.3694444444444445in"}

Figure 1. The plan and elevation view of the MIPP primary and secondary
beamlines.

the primary target will be attenuated by adjusting both beam optics and
the collimator aperture to achieve 10^11^ protons/spill, or less, with a
spill occurring about every sec into this line from the Main Injector.
Control over primary optics combined with collimator aperture is
necessary for proper secondary beamline operation---a spot size of ±2 mm
(95%) at the primary target must be maintained. (This requirement will
be discussed further in the following section.) The primary line will be
tuned and collimated accordingly to maintain a consistent spot size over
a large range in primary intensity. The primary intensity will depend on
the secondary beam energy and desired event rate.

### Secondary Beamline

Several considerations challenged the design of the secondary beamline,
a beamline which must accommodate both the demanded momentum resolution
and the precision in particle identification. The former requires the
momentum-dispersed beam size to be larger than the transverse beam size
and the latter requires low divergence at the Ĉerenkov detectors. At the
experimental target dispersion must be cancelled to contain the spot
size on the secondary target and associated detectors.

The challenge in meeting the required momentum resolution arose from the
fixed civil construction of the experimental and beamline enclosures.
The primary and secondary beamlines are straight horizontally with only
a slight increase in elevation at the experiment of 0.86 m relative to
the primary beamline. Significant dipole bend fields, or weaker fields
but longer distances, are required to produce significant separation of
beam momenta. In the absence of either, only small separation of
particles of different momentum can be achieved; in the case here only
0.3 m of vertical dispersion could be installed in the secondary line
and still respect the length and height constraints. In order to make an
accurate momentum cut, the transverse beam size must be smaller than the
desired momentum resolution otherwise particles with momenta outside the
cut overlap spatially. For this experiment a desired momentum resolution
was needed of about 1%. When combined with the low value of dispersion
(a 1% change in momentum represents only a 3 mm shift in the in particle
coordinates), this implied the transverse beam size at the momentum cut
or collimator must be less than or comparable to this shift. Hence, the
beam spot size at this point must be \~3 mm across, which is less than
or comparable to the primary target beam size. This criterion implies
point to point focusing, or slight demagnification of the primary beam
spot, further implying a strong sensitivity to the actual transverse
size of the beam on the primary target.

Due to the limitations in length of the secondary beamline, the
quadrupole focusing in the secondary line is strong and achromatic. With
a focal length of only about 10 m at 100 GeV/c, most of the quadrupoles
must operate at or near their peak fields to achieve the beam sizes
depicted in Figure 2. The resulting large chromaticity of the line
limits the momentum window to about 2-3% in practice.

![](media/image3.wmf){width="3.4229166666666666in"
height="2.270138888888889in"}

Figure 2. Beamline lattice functions: beam size is determined using an
emittance of 3-5π mm-mr.

Accurate particle identification further restricted the divergence of
the secondary beam to about 0.3-0.5 mr in the region of the Ĉerenkov.
When coupled with a beam size at the experimental target of about 1 cm
in radius, the beam emittance which produces an acceptable beam envelope
is about 3-5π mm-mr. To restrict the beam

envelope it was found necessary to scrape the emittance of the secondary
beam just after the primary target, and before the momentum collimator
to this value. The effect of secondary absorbers was to reduce
significantly the ratio of background to signal at the secondary target,
although at a price of reduced secondary beam intensity. The results of
extensive simulations of this beamline are given in the next section.

## BEAMLINE SIMULATION

The primary and secondary beamline were designed using MAD\[1\] given
the specific beam spot size at the primary target and a phase ellipse
constrained by the needs of the experiment at the Ĉerenkov and secondary
targets of 3-5π mm-mr. However, collimators, momentum selection and
actual performance of the secondary line were simulated in the MARS
code\[2\] using the primary beam distribution and secondary particle
production. The secondary particle distribution is ray-traced through
the entire beamline locating high loss points for radiation shielding
design and showing the effect of collimation and the size and divergence
of the beam as it progresses along the secondary line. This proved to be
a very effective approach to verification of the optics design of the
line. The histograms in Figure 4 show the performance of secondary
beamline for a large momentum bite, and for a more restrictive one which
involves additional phase space collimation.

![](media/image4.wmf){width="3.252083333333333in"
height="2.5590277777777777in"}

Figure 3. Beam profile on the 1-cm radius secondary target as simulated
by MARS\[2\]

It is interesting to note the effect of a tighter momentum bite and
scraping or secondary collimation of the beam to the design ellipse is
to reduce the signal to background significantly at the expense of
secondary intensity for a given primary intensity. Since we are not
currently at intensity limits for the primary beam, more severe cuts are
not a problem.

![](media/image5.wmf){width="3.575in" height="3.0in"}

![](media/image6.wmf){width="3.252083333333333in"
height="2.845138888888889in"}

Figure 4. Histograms showing the performance of the secondary beamline
and target for a ±5% dp/p at the momentum collimator: the signal and
background events for a 1 cm radius detection area at the secondary
target and no secondary collimation (8 x 10 ^-4^ signal protons and
10^-4^ background protons are detected for every proton on the primary
target). The second set of histograms show a more restrictive, ±1% dp/p
momentum cut, and additional collimation in the secondary line.

## References

\[1\] Hans Grote and F. Christoph Iselin, \`\`The MAD program
"Methodical Accelerator Design", CERN/SL/90-13.

\[2\] N.V.\~Mokhov, \"The MARS Code System User\'s Guide\",
Fermilab-FN-628 (1995);

N.V.\~Mokhov, O.E.\~Krivosheev, \"MARS Code Status\", Proc. Monte Carlo
2000

Conf., p. 943, Lisbon, October 23-26, 2000; Fermilab-Conf-00/181 (2000);

http://www-ap.fnal.gov/MARS/.