# XEMIS: A new Compton camera with liquid xenon

J. Masbou, J.-P. Cussonneau, J. Donnard, L. Gallego Manzano, O. Lemaire P.

Leray, A.-F. Mohamed Hadi, E. Morteau, L. Scotto Lavina, J.-S. Stutzmann,

D. Thers, et al.

► To cite this version:

J. Masbou, J.-P. Cussonneau, J. Donnard, L. Gallego Manzano, O. Lemaire P. Leray, et al., XEMIS: A new Compton camera with liquid xenon. 2014 IEEE 18th International Conference on Dielectric Liquids (ICDL), Jun 2014, Bled, Slovenia. pp.1-4, 10.1109/icdl.2014.6893166. hal-04969845

HAL Id: hal-04969845

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# XEMIS : A new Compton camera with liquid xenon

J. Masbou, J.-P. Cussonneau, J. Donnard, L. Gallego Manzano, O. Lemaire P. Leray, A.-F. Mohamed Hadi, E.

Morteau, L. Scotto Lavina, J.-S. Stutzmann, D. Thers

SUBATECH, UMR 6457, Ecole des Mines de Nantes, CNRS / IN2P3 / Université de Nantes,

4 rue Alfred Kastler BP 20722, 44307 Nantes cedex 3, France

e-mail address : julien.masbou@subatech.in2p3.fr

S. Bassetto, P. Briend, M. Gorski,

Air Liquide Advanced Technologies,

2, rue Clémencière, BP 15, 38360 Sassenage, France

N. Beaupere, T. Carlier, M. Cherel, F. Kraeber-Bodéré

CRCNA, UMR Inserm892 - CNRS 6299, Institut de Recherche en Santé de l'Université de Nantes,

8 quai Moncousu - BP 7072, 1 44007 Nantes cedex 1

**Abstract** — In the context of medical imaging system, we develop an innovative technique, called 3 gamma. It consists in a direct 3D reconstruction of each decays of $^{44}$Sc radionuclide with a resolution below the centimeter. This breakthrough in instrumentation technique is only possible by the use of a new detection medium (liquid xenon) and a new detection structure compared with conventional imaging technique. Thanks to an ultra-low noise front-end electronics (below 100 electrons ENC) operating at liquid xenon temperature and a fast UV sensitive PMT, high spatial resolution and high energy resolution are achievable in 3D. This is particularly important for Compton imaging since all interactions in the medium have to be identified to derive the incoming gamma ray direction.

A prototype (XEMIS1) is now in test at Subatech and shows promising results. We achieve an energy resolution of 8.9% (FWHM) at 1.2 MeV with an electric field of 1kV/cm. All the cryogenic system is fully operational with a high purification rate and shows a very good stability. I will review all these aspects and introduce the next step of the project, XEMIS2, a larger prototype dedicated to the 3 gamma imaging of small animals.

**Keywords**— Medical Imaging; Compton Imaging; Liquid xenon; 3 gamma.

## I. INTRODUCTION

Subatech is developing the 3 γ imaging, a new concept of medical imaging which aims to perform direct 3D reconstruction of special radionuclides [1]. It is based on the use of a 3 γ emitter radionuclide, in particular the $^{44}$Sc. This radionuclide emits a $\beta^+$ ($E_{mean} = 632$ keV, $E_{max} = 1473$ keV) decaying in 2 photons of 511 keV emitted back-to-back (branching ratio 99.9%) with a gamma ray ($E = 1157$ keV) in quasi-coincidence. The two back-to-back 511 keV photons from the positron annihilation gives a line of response (LOR) and the arrival path of the third gamma is reconstructed according to the Compton kinematics thanks to the following equation:

$$ \cos(\theta) = 1 - m_e c^2 \frac{E_1}{E_0 (E_0 - E_1)} \quad (1) $$

where θ is the scattering angle, $m_e c^2$ the mass of the electron (511 keV), $E_0$ is the energy of the gamma (1157 keV) and $E_1$ is the energy deposited by the Compton interaction. Since in dense medium the direction recoil of the scattered electron is generally not measurable, revolution symmetry appears. This leads to the reconstruction of a cone of presence containing the incident γ ray and not a straight line. This reconstructed cone is determined by 3 parameters: the apex (3D position of the first hit), the axis Δ (given by the line connecting the two hits) and the aperture angle θ. The 3D position of the radionuclide can be thus reconstructed by locating the intersection point between the LOR and the cone. Figure 1 shows the principle of the 3 γ imaging technique.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Principle of 3 γ imaging technique with a Compton camera.</figcaption></figure>
This device would benefit from a large acceptance and no dead area to have a high efficiency for both LOR and Compton reconstruction. The goal is thus to build a monolithic volume filled with a dense medium to have a high stopping power for γ rays. The use of liquid xenon (LXe) is then particularly relevant [6]. Firstly, LXe is a relative high Z (Z = 54) and dense medium (~3 g/cm³) which is very useful to have a high stopping power for γ rays (~84% of 1.157 MeV gamma rays could be detected with 12 cm length volume). Secondly, LXe has a high scintillation (~16 000 UV photons/MeV) and ionization yield (~54 400 electrons/MeV) at a 2 kV/cm electric field and finally, the use LXe gives the advantages to have a monolithic and homogeneous active volume.

## II. THE INSTRUMENT

XEMIS (XEnon Medical Imaging System) is an experiment developed to demonstrate the feasability to reconstruct the Compton sequence of a MeV - γ ray interacting with the instrument. The system is composed of 5 different major parts: (A) the cold-head, (B) the purification loop, (C) the storage and recovering, (D) the rescue system and (E) the cryostat containing the instrument.

A. A Pulse Tube Refrigerator (PTR) which is specially designed and optimized for xenon liquefaction [7] is selected to be the cold-head of XEMIS. It is used to provide stable cooling power up to 200 W at 165 K. An electrical heater up to 180 W is assembled between the copper cold head and PTR in order to adjust the net-cooling power by compensating the extra cooling power from the PTR. All this liquefaction part is connected to the cryostat and is maintained at a distance of 2 meters for the purpose of reducing mechanical noise due to the vibration of the PTR.

B. A recirculation system shown on Figure 2 driven by an oil-free membrane pump is used to purify xenon continuously in order to reduce the electronegative contamination (O₂, CO₂, NO₂, etc.) from outgassing. These impurities have to be removed as much as possible because the ionizing electrons created by the interaction of γ rays can be captured by electronegative molecules instead of drifting to anode. Two rare-gas purifiers (SAES PS4-MT3-R/N getter) connected in parallel are used to purify the gaseous xenon. Since the getters can only purify gaseous xenon, the liquid xenon must be evaporated to gaseous state before entering purification loop. Therefore, a coaxial heat exchanger made of stainless steel is placed in order to recuperate the vapor heat during the circulation. This heat exchanger is designed to have 95 % of efficiency up to 50 NL/min (4.92 g/s) [5].

C. The storage and recovering system of the Xenon is made of classical commercial gas bottle B50.

D. A 4 m³ stainless steel tank is connected to the cryostat in case of recuperation in emergency.

E. The instrument in the cryostat is composed of a Time Projection Chamber (TPC) immersed in a vessel filled with LXe. When a γ ray interacts inside the TPC, the energy deposit creates direct scintillation (VUV photons of 178 nm) and ionization signals. By applying an external low electric field (E<sub>drift</sub>) tunable up to 2 kV/cm provided by 24 copper field rings, ionizing electrons will drift toward the anode with constant velocity and can then be collected. The active volume of the TPC is 28 x 28 mm² (X-Y plane) with 12 cm length (Z direction). The anode is segmented into 64 x 64 square pixels with a size of 3.5 x 3.5 mm². This anode is connected to 2 ultra-low noise front-end readout electronics of 32 channels each based on the IDeF-X HD-LXe chip [4] which has around 100 electrons NEC at LXe temperature. A 670 lpi (line per inch) micromesh made of a 5 µm thick copper foil is placed 200 µm above the anode and is used as a Frisch grid. In this space, an electric field with a strength higher than 50 times the electric field present in the drift space must be applied to ensure a 100 % electron transparency. A Hamamatsu R7600-06MOD-SSY99 photomultiplier tube (PMT), placed at the other side of the TPC, is used to detect VUV scintillation signals.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Xemis 1 gas circulation containing the condensation and purification loop.</figcaption></figure>

## III. CALIBRATION

In order to check the validity of all parameters we developed a calibration setup at Subatech. It is based on a ²²Na source (β + γ emitter E<sub>mean</sub> = 545 keV E<sub>max</sub> = 1257 keV and 1274 keV γ ray in 99.9% of disintegration) placed outside the
<figure><img src="image_4.png" /><figcaption>Fig. 3. Xemis 1 equipped with the 511 keV calibration line triggered with a BaF₂ crystal and in coincidence with the xenon PMT.</figcaption></figure>

TPC at 12.5 cm distance from the anode as shown in Figure 3. This source can thus be used for 511 keV calibration by triggering in coincidence with a BaF₂ crystal placed behind the source and the xenon PMT. Waveforms of the 64 pixels electric signals and scintillation signals from PMT are recorded with a V1740 Caen DAQ system and a V1720 Caen DAQ system, respectively.

## IV. RESULTS

In order to ensure a good Compton reconstruction, the TPC must have good features in terms of spatial resolution, energy resolution and electronegative impurity.

### A. Spatial resolution

With the TPC, the X and Y positions are given by the relative signal present on each of the 64 pixels of the anode. This 64 pixels segmentation allows a spatial resolution below 1 mm in each X and Y direction [2]. The Z position is determined by measuring the time difference between the PMT signal and the collection time of electrons created by the Compton interaction. This time difference multiplied by the drift velocity gives the Z value. The drift velocity of the charge carriers is extracted from data by calculating the time difference between the beginning and the end of the TPC as presented in Figure 4. To determine the Z resolution, a gaussian error function is adjusted on the first edge defined by the very beginning of the sensitive medium. The resolution in the Z-direction is given by the dispersion of the gaussian error function and is measured below 100 µm [3].

### B. Xenon purity

As described previously, electronegative impurities in xenon capture drifting electrons. This absorption law is described by an exponential model $S(z) = S(0)e^{-z/\lambda}$, where $S(z)$ is the signal amplitude at the z altitude and $\lambda$ is the attenuation length. All the amplitudes measured are then corrected accordingly to their altitude. This effect is clearly visible on Figure 5 where the amplitude of the collected signal produced by a 511 keV is exponentially dependent on the depth of

<figure><img src="image_5.png" /><figcaption>Fig. 4. Depth interaction profile for photoelectric 511 keV events. The beginning and the end of the TPC are determined by edges on left and right. A gaussian error function is adjusted on the first edge defined by the very beginning of the sensitive medium to determine the Z resolution</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. 511 keV photoelectric peak measurement of the xenon as a function of the depth of interaction fitted by an exponential law. The slope parameter of the exponential law is used to measure the purity and gives the drifting length of the electrons.</figcaption></figure>

interaction. Absorption length above 1 m has been measured.

### C. Energy resolution

The energy deposited at the interaction point is determined by measuring the amplitude of the produced signal on the anode. The Figure 6 shows energy resolution at 511 keV for photoelectric events $^{22}$Na and an electric field of 1 kV.cm$^{-1}$. Energy resolution on the ionization signal of 5 % at 511 keV (and 8.9 % FWHM at 1 MeV) can be obtained with a drift field of 1 kV/cm.

## V. XEMIS 2

A first work has been initiated with the open GATE collaboration ([9], [10]) to implement the Xenon as a medium in GATE in order to be able to quantify and share the benefit of this new imaging technique. XEMIS2, presented in Fig. 7, is a new geometry able to achieve a full 3 $\gamma$ imaging with liquid Xenon. The aim of this new instrumental development is to detect both the $\beta^+$ disintegration and the third photon with only one device. This geometry is a hollow cylinder of 24 cm length designed to contain two different readout planes on the
<figure><img src="image_7.png" /><figcaption>Fig. 6. Energy Resolution measured at 511 keV with <sup>22</sup>Na and an electric field of 1 kV.cm<sup>-1</sup>.</figcaption></figure>

top and the bottom part of the cylinder. Each plane is equipped
with a thin micromesh Frisch Grid and contains an anode
segmented in pixel of 3.175 x 3.175 mm². The anode will be
connected to 200 Front-End electronics readout cards of 64
channels each. There will be a total amount of 25000
electronic channels. This new geometry will hold around
180 kg of LXe. A new concept of REcovery and STOrage
system for Xenon (ReStoX) is developed by Subatech and
AirLiquide, which is designed to be able to store xenon at a
temperature between -108°C (liquid phase) and 20°C.
XEMIS2 will be equipped with a small ReStoX system to
ensure a rapid filling and recovering of the Xenon. The inner
radius of the camera is 7 cm and the outer radius is 19 cm
giving a depth volume of 12 cm for an orthogonal incident γ
ray. The UV-light produced by the interaction of γ rays is
collected by an assembly of 192 Hamamatsu PMT. By a
particular trigger system on the scintillation light, it is possible
to read only pixels that are susceptible to contain information
in order to reduce the flow of data and to be able to follow the
rate.

ACKNOWLEDGMENT

This work is supported by the E.U. and the region Pays de
la Loire in France.

<figure><img src="image_8.png" /><figcaption>Fig. 7. XEMIS 2 device devoted to the small animal 3 γ imaging</figcaption></figure>

REFERENCES

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[2] A.F. Mohamad Hadi et al., Simulation of Liquid Xenon Time Projection Chambers and 3γ Camera with GATE, IEEE NSS/MIC Conference, 2667-2670, 2012.

[3] T. Oger et al. A liquid xenon TPC for a medical imaging Compton telescope, NIMA, A695:125-128, 2012.

[4] O. Lemaire, Development of a Readout Electronic for the Measurement of Ionization in Liquid Xenon Compton Telescope Containing Micro-patterns, IEEE NSS/MIC Conference, 858-861, 2012.

[5] W. T. Chen, Improvement of purification system using a combination of PTR and cohexial heat exchanger, ICEC24 ICMC2012, 2012.

[6] E. Aprile and T. Doke, Liquid Xenon Detectors for Particle Physics and Astrophysics, Rev.Mod.Phys., 82:2053-2097, 2010.

[7] T. Haruyama et al., High-power pulse tube cryocooler for liquid xenon particle detectors, Cryocoolers 13 (2004) 689-694.

[8] J. Thomas, D. A. Imel and S. Biller, Statistics of charge collection in liquid argon and liquid xenon, Phys.rev.A., 38(11):5793-5800, 1988.

[9] OpenGATE collaboration: http://www.opengatecollaboration.org/

[10] G. Santin, et al., GATE: A GEANT4-based simulation platform for PET and SPECT integrating movement and time management *IEEE Trans.Nucl.Sci.*, NS-50:1516-1521, 2003.