# Development of nuclear emulsions with View the 1µm source spatial resolution for the AEgIS experiment

M. Kimura, Stefano Aghion, O. Ahlén, Claude Amsler, Akitaka Ariga, Tadashi Ariga, A.S. Belov, Germano Bonomi, P. Bräunig, J Bremer, et al.

► To cite this version:

M. Kimura, Stefano Aghion, O. Ahlén, Claude Amsler, Akitaka Ariga, et al. Development of nuclear emulsions with View the 1µm source spatial resolution for the AEgIS experiment. 13th Vienna Conference on Instrumentation, Feb 2013, Vienne, Austria. pp.325-329, 10.1016/j.nima.2013.04.082. in2p3-00920259

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Development of nuclear emulsions with 1 µm spatial resolution for the
AEgIS experiment

M. Kimura<sup>a</sup>, S. Aghion<sup>b,c</sup>, O. Ahlén<sup>d</sup>, C. Amsler<sup>a,*</sup>, A. Ariga<sup>a</sup>, T. Ariga<sup>a</sup>, A.S. Belov<sup>e</sup>, G. Bonomi<sup>f,g</sup>, P. Bräunig<sup>h</sup>, J. Bremer<sup>d</sup>, R.S. Brusa<sup>i</sup>, G. Burghart<sup>d</sup>, L. Cabaret<sup>j</sup>, C. Canali<sup>k</sup>, R. Caravita<sup>l</sup>, F. Castelli<sup>l</sup>, G. Cerchiari<sup>l</sup>, S. Cialdi<sup>l</sup>, D. Comparat<sup>j</sup>, G. Consolati<sup>b,c</sup>, S. Di Domizio<sup>m</sup>, L. Di Noto<sup>i</sup>, M. Doser<sup>d</sup>, A. Dudarev<sup>d</sup>, A. Ereditato<sup>a</sup>, R. Ferragut<sup>b,c</sup>, A. Fontana<sup>g</sup>, P. Genova<sup>g</sup>, M. Giammarchi<sup>c</sup>, A. Gligorova<sup>n</sup>, S.N. Gninenko<sup>e</sup>, S. Haider<sup>d</sup>, S.D. Hogan<sup>o</sup>, T. Huse<sup>p</sup>, E. Jordan<sup>q</sup>, L.V. Jørgensen<sup>d</sup>, T. Kaltenbacher<sup>d</sup>, J. Kawada<sup>a</sup>, A. Kellerbauer<sup>q</sup>, A. Knecht<sup>d</sup>, D. Krasnický<sup>r</sup>, V. Lagomarsino<sup>r</sup>, S. Mariazzi<sup>l</sup>, V.A. Matveev<sup>e,s</sup>, F. Merkt<sup>t</sup>, F. Moia<sup>b,c</sup>, G. Nebbia<sup>u</sup>, P. Nédélec<sup>v</sup>, M.K. Oberthaler<sup>h</sup>, N. Pacifico<sup>n</sup>, V. Petráček<sup>w</sup>, C. Pistillo<sup>a</sup>, F. Prelz<sup>c</sup>, M. Prevedelli<sup>x</sup>, C. Regenfus<sup>k</sup>, C. Riccardi<sup>y,g</sup>, O. Røhne<sup>p</sup>, A. Rotondi<sup>y,g</sup>, H. Sandaker<sup>n</sup>, P. Scampoli<sup>a,z</sup>, J. Storey<sup>a</sup>, M.A. Subieta Vasquez<sup>f,g</sup>, M. Špaček<sup>w</sup>, G. Testera<sup>m</sup>, D. Trezzi<sup>c</sup>, R. Vaccarone<sup>m</sup>, S. Zavatarelli<sup>m</sup>

<sup>a</sup> Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics, University of Bern, 3012 Bern, Switzerland

<sup>b</sup> Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

<sup>c</sup> Istituto Nazionale di Fisica Nucleare, Sez. di Milano, Via Celoria 16, 20133 Milano, Italy

<sup>d</sup> European Organisation for Nuclear Research, Physics Department, 1211 Geneva 23, Switzerland

<sup>e</sup> Institute for Nuclear Research of the Russian Academy of Sciences, Moscow 117312, Russia

<sup>f</sup> University of Brescia, Department of Mechanical and Industrial Engineering, Via Branze 38, 25133 Brescia, Italy

<sup>g</sup> Istituto Nazionale di Fisica Nucleare, Sez. di Pavia, Via Agostino Bassi 6, 27100 Pavia, Italy

<sup>h</sup> University of Heidelberg, Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany

<sup>i</sup> Dipartimento di Fisica, Università di Trento and INFN, Gruppo Collegato di Trento, Via Sommarive 14, 38050 Povo, Trento, Italy

<sup>j</sup> Laboratoire Aimé Cotton, CNRS, Université Paris Sud, ENS Cachan, Bâtiment 505, Campus d'Orsay, 91405 Orsay Cedex, France

<sup>k</sup> University of Zurich, Physics Institute, Winterthurerstrasse 190, 8057 Zurich, Switzerland

<sup>l</sup> University of Milano, Department of Physics, Via Celoria 16, 20133 Milano, Italy

<sup>m</sup> Istituto Nazionale di Fisica Nucleare, Sez. di Genova, Via Dodecaneso 33, 16146 Genova, Italy

<sup>n</sup> University of Bergen, Institute of Physics and Technology, Alleegaten 55, 5007 Bergen, Norway

<sup>o</sup> University College London, Department of Physics and Astronomy, Gower Street, London WC1E 6BT, UK

<sup>p</sup> University of Oslo, Department of Physics, Sem Sælands vei 24, 0371 Oslo, Norway

<sup>q</sup> Max Planck Institute for Nuclear Physics, Saupfercheckweg 1, 69117 Heidelberg, Germany

<sup>r</sup> University of Genoa, Department of Physics, Via Dodecaneso 33, 16146 Genova, Italy

<sup>s</sup> Joint Institute for Nuclear Research, 141980 Dubna, Russia

<sup>t</sup> ETH Zurich, Laboratory for Physical Chemistry, 8093 Zurich, Switzerland

<sup>u</sup> Istituto Nazionale di Fisica Nucleare, Sez. di Padova, Via Marzolo 8, 35131 Padova, Italy

<sup>v</sup> Claude Bernard University Lyon 1, Institut de Physique Nucléaire de Lyon, 4 Rue Enrico Fermi, 69622 Villeurbanne, France

<sup>w</sup> Czech Technical University in Prague, FNSPE, Brehová 7, 11519 Praha 1, Czech Republic

<sup>x</sup> University of Bologna, Department of Physics, Via Irnerio 46, 40126 Bologna, Italy

<sup>y</sup> University of Pavia, Department of Nuclear and Theoretical Physics, Via Bassi 6, 27100 Pavia, Italy

<sup>z</sup> University of Napoli Federico II, Department of Physics, Via Cinthia, 80126 Napoli, Italy

The main goal of the AEgIS experiment at CERN is to test the weak equivalence principle for antimatter. We will measure the Earth's gravitational acceleration *g* with antihydrogen atoms being launched in a horizontal vacuum tube and traversing a moiré deflectometer. We intend to use a position sensitive device made of nuclear emulsions (combined with a time-of-flight detector such as silicon μ–strips) to

<footnote>* Corresponding author. Tel.: +41 227672914.</footnote>

<footnote>E-mail address: claude.amsler@cern.ch (C. Amsler).</footnote>
measure precisely their annihilation points at the end of the tube. The goal is to determine $g$ with a 1% relative accuracy. In 2012 we tested emulsion films in vacuum and at room temperature with low energy antiprotons from the CERN antiproton decelerator. First results on the expected performance for AEGIS are presented.

# 1. The AEGIS experiment

The main goal of the AEGIS experiment (CERN/AD6) is to test the weak equivalence principle (WEP) using antihydrogen ($\text{H}$). This principle of the universality of free fall has been tested with

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Top: Schematics of the AEGIS detectors. The vertex detector is made of nuclear emulsions. The time-of-flight detector (TOF) is needed to measure the velocities of the $\text{H}$ atoms. The thin window limits the position resolution (due to multiple scattering), but is needed to separate the ultra-high vacuum (UHV) part from the outer vacuum region (OVC) containing the emulsion films. Bottom: $\Delta g/g$ vs. number of particles for a vertex resolution of 1 µm (red) and 10 µm (blue). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)</figcaption></figure>

tremendous precision for matter, but not with antimatter particles, due to major technical difficulties related to stray electric and magnetic fields. In contrast, the electrically neutral $\text{H}$ atom is an ideal probe to test the WEP and the antiproton decelerator at CERN is a worldwide unique antihydrogen factory. In AEGIS the gravitational deflection of $\text{H}$ atoms launched horizontally and traversing a moiré deflectometer will be measured with a precision of 1% on $|\Delta g|/g$, using a position sensitive annihilation detector [1]. The required position resolution should be a few µm to achieve the 1% goal.

As we discuss in this paper, the antihydrogen annihilation point can be determined in a novel application of emulsion films [2] using the techniques applied to the OPERA experiment [3]. This is the first time that nuclear emulsions will be used in vacuum. The vertical precision on the measured annihilation point will be about 1 µm, an order of magnitude better than proposed originally with silicon µ-strip detectors [1]. Fig. 1 shows the principle of the experiment and the estimated number of annihilations needed to reach a given precision on $g$, as a function of position resolution.

# 2. Nuclear emulsions

Nuclear emulsions [4] are photographic films with extremely high spatial resolution, better than 1 µm. A track produced by a charged particle is detected as a sequence of silver grains (Fig. 2), where about 36Ag grains per 100 µm are created by a minimum ionizing particle. The intrinsic spatial resolution is about 50 nm. In recent experiments such as OPERA [3], large area nuclear emulsions were used thanks to the impressive developments in automated scanning systems.

For AEGIS, we developed nuclear emulsions which can be used in ordinary vacuum (OVC, $10^{-5}-10^{-7}$ mbar). This opens new applications in antimatter physics research. We performed exposures with stopping antiprotons in June and December 2012. A sketch of the experimental setup is shown in Fig. 3. The emulsion detector consisted of five sandwiches made of emulsion films deposited on both sides of (200 µm thick) plastic substrates ($68 \times 68 \times 0.3$ mm³).

A thin foil will be needed in the gravity measurement as a window to separate the $\text{H}$ beam line at UHV pressure from the

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Left: AgBr crystals in emulsion layers observed with a scanning microscope. Right: A minimum ionizing track from a 10 GeV/c pion.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3. Left: Experimental setup. Right: Picture of the vacuum flange holding the emulsion stack attached by a crossed bar frame.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Top: Typical antiproton annihilation vertices in the bare emulsion. Bottom: Tracks observed behind a thin silver foil in which the antiprotons annihilate.</figcaption></figure>

OVC section containing the emulsion detector. Thus for the tests half of the emulsion surface was covered by a 20 µm (SUS) stainless steel foil, while direct annihilation on the emulsion surface could be investigated from the other half.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Impact parameter resolution with a 20 µm stainless steel window.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. Emulsion films kept during 3.5 days in vacuum show cracks (A) which can be avoided by treating the films with glycerine before exposure (B).</figcaption></figure>

The 3D tracking and annihilation vertex reconstructions were performed at the University of Bern using the automatic scanning facility developed for OPERA [5]. Annihilation stars were observed
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Left: Crystal sensitivity vs. glycerine concentration. Right: Fog density (number of noise grains per $10^3 \mu m^3$) vs. glycerine concentration for films kept in vacuum for 3.5 days (squares), compared films kept at atmospheric pressure (dots).</figcaption></figure>

<table><caption>**Table 1**

Comparison between the reference films (OPERA) and the new emulsions from Nagoya University.</caption><thead><tr><th>Gel type</th><th>Development time (min)</th><th>Grain density (grains/100 µm)</th><th>Fog density (grains/µm³)</th></tr></thead><tbody><tr><td>OPERA</td><td>25</td><td>30.3 ± 1.6</td><td>10.1 ± 0.7</td></tr><tr><td>Nagoya 1</td><td>20</td><td>47.7 ± 2.0</td><td>1.9 ± 0.2</td></tr><tr><td>Nagoya 2</td><td>25</td><td>55.1 ± 2.6</td><td>3.0 ± 0.3</td></tr></tbody></table>

in the bare region not covered by the steel foil. A typical antiproton annihilation vertex in the emulsion layer is shown in Fig. 4 (top).

In December 2012 we also carried out measurements with a series of thin foils of varying compositions (Al, Si, Ti, Cu, Ag, Au, Pb) to determine the relative contributions from protons, nuclear fragments and pions as a function of atomic number. Fig. 4 (bottom) shows for instance the scanning of a $0.3 \times 0.3 \text{ mm}^2$ surface of a 5 µm thick silver foil. Tracks emerging from the annihilation vertex are clearly observed. The emulsions were exposed for up to several hours, leading to vertex densities of typically 0.5 vertices per mm². Note that the maximum track density that can easily be dealt with by the scanning facility is around $10^3$ tracks per mm², so that emulsions could be placed in the final apparatus for several days before being replaced.

Tracks from nuclear fragments, protons, and pions were reconstructed and the distance of closest approach between pairs of tracks was calculated. Fig. 5 shows the distribution of the distance of closest approach projected into the vertical direction (impact parameter), which is a measure of the resolution with which the annihilation point will be determined in the gravity measurement. The figure shows that with e.g. a 20 µm steel window a resolution of ≈1 µm on the vertical position of the annihilation vertex can be achieved.

### 3. Development of emulsions for AEGIS

We have tested the properties of nuclear emulsions in vacuum [2] which to our knowledge had not been studied before. Water loss in the gelatine which surrounds the AgBr crystals can produce cracks in the emulsion layer, thus compromising the mechanical stability (required at the µm level). We therefore developed a

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** A 10 GeV/c pion track in the reference film (OPERA, left) and in a highly sensitive one (right).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Principle of moiré deflectometer.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Photograph of miniature moiré deflectometer.</figcaption></figure>

treatment with glycerine which can efficiently prevent the elasti-
city loss in the emulsion (see Fig. 6). However, glycerine treatment
changes the composition of the emulsion layer and we therefore
had to determine the detection efficiency per AgBr crystal. This
was performed with minimum ionizing pions in a 6 GeV/c CERN
beam. The result (Fig. 7) does not indicate any changes in the
efficiency, which is typically 13% for glycerine concentrations
below 20%. However, the thermally induced background – the
so-called fog density – increases for glycerine treated emulsions.

Annihilation products from annihilating antiprotons (or $\overline{H}$
atoms) are emitted isotropically, in contrast to the $\tau$-decay
products measured in the OPERA experiment, which are forward
boosted. The efficiency of our automatic scanning system therefore
needs to be improved for tracks traversing the emulsion layers at
large incident angles. We are also investigating new emulsion gels
with higher sensitivity to increase the detection efficiency for
minimum ionizing particles. They were developed at Nagoya
University (Japan) and coated onto glass substrates in Bern. Glass
is well suited for highest position resolutions thanks to its superior
environmental stability (temperature and humidity), as compared
to plastic. A comparison between the OPERA films and the new
Nagoya gels is shown in Table 1, while Fig. 8 compares minimum
ionizing pion tracks between the two types of gel. The Nagoya
2 gel is roughly twice as sensitive as the OPERA one and shows a
much lower fog density.

**4. Proof of principle using a miniature moiré deflectometer**

In AEGIS [1] the Rydberg excited $\overline{H}$ atoms will be accelerated by
an electric field before traversing the deflectometer [6] consisting

<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Simulated intensity distribution of reconstructed vertices with position resolutions of 1 µm (red) and 10 µm (blue). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)</figcaption></figure>

of two identical gratings separated by a distance *L* of typically
40 cm (Fig. 9). The annihilation intensity will be measured along
the vertical direction *x* with emulsions located at the same
distance *L* from the second grating. The displacement *Δx* of the
moiré intensity pattern due to gravity will be measured. The $\overline{H}$
beam is not monochromatic and therefore *Δx* depends on the time
of flight of the $\overline{H}$ atoms through the relation *Δx* = $\overline{g}T^2$, where *T* is
the time of flight between the two gratings. The start of TOF is
given by the switch off time of the electric field for Stark
acceleration and the stop by the $\overline{H}$ annihilation time measured
by the silicon μ–strips (see Fig. 1) that associate a time of flight to
each annihilation vertex.

A proof of principle was performed in December 2012 with
emulsion films irradiated with antiprotons passing through a
small moiré deflectometer. A photograph of the deflectometer is
shown in Fig. 10. The device contained several pairs of gratings
with different spacings, as well as gratings in direct contact with
the films. The simulation in Fig. 11 shows as an example the
expected interference pattern at the emulsion layer, generated by
a pair of gratings (12 µm slit, 40 µm pitch, separated by L=25 mm).
The antiproton data is being analyzed and preliminary results are
quite encouraging.

**References**

[1] G. Drobychev, et al., <http://doc.cern.ch/archive/electronic/cern/preprints/spsc/public/spsc-2007-017.pdf>; A. Kellerbauer, et al., Nuclear Instruments and Methods in Physics Research Section B 266 (2008) 351.

[2] C. Amsler, et al., Journal of Instrumentation 8 (2013) P02015.

[3] T. Nakamura, et al., Nuclear Instruments and Methods in Physics Research Section A 556 (2006) 80.

[4] G. de Lellis, A. Ereditato, K. Niwa, in: Nuclear Emulsions, C. W. Fabjan, H. Schopper (Eds.), Springer Materials, Landolt-Börnstein Database (http://www.springermaterials.com), Springer-Verlag, Heidelberg, 2011.

[5] K. Borer, et al., Nuclear Physics A 834 (2010) 751.

[6] M.K. Oberthaler, et al., Physics Review A 54 (1996) 3165.