Tracking with the MINOS Time Projection Chamber

Accepted Manuscript Tracking with the MINOS Time Projection Chamber C. Santamaria, A. Obertelli, S. Ota, M. Sasano, E. Takada, L. Audirac, H. Baba, D. Calvet, F. Château, A. Corsi, A. Delbart, P. Doornenbal, A. Giganon, A. Gillibert, Y. Kondo, Y. Kubota, C. Lahonde-Hamdoun, V. Lapoux, D. Leboeuf, C.S. Lee, H.N. Liu, M. Matsushita, T. Motobayashi, M. Niikura, M. Nishimura, H. Otsu, A. Peyaud, E.C. Pollacco, G. Prono, H. Tokieda, T. Uesaka, J. Zenihiro

PII: DOI: Reference:

S0168-9002(18)30888-X https://doi.org/10.1016/j.nima.2018.07.053 NIMA 60999

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 23 March 2018 Revised date : 23 June 2018 Accepted date : 18 July 2018 Please cite this article as: C. Santamaria, A. Obertelli, S. Ota, M. Sasano, E. Takada, L. Audirac, H. Baba, D. Calvet, F. Château, A. Corsi, A. Delbart, P. Doornenbal, A. Giganon, A. Gillibert, Y. Kondo, Y. Kubota, C. Lahonde-Hamdoun, V. Lapoux, D. Leboeuf, C.S. Lee, H.N. Liu, M. Matsushita, T. Motobayashi, M. Niikura, M. Nishimura, H. Otsu, A. Peyaud, E.C. Pollacco, G. Prono, H. Tokieda, T. Uesaka, J. Zenihiro, Tracking with the MINOS Time Projection Chamber, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.07.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Manuscript Click here to view linked References

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Tracking with the MINOS Time Projection Chamber C. Santamariaa,b , A. Obertellia,b,c , S. Otad , M. Sasanob , E. Takadae , L. Audiraca , H. Babab , D. Calveta , F. Chˆateaua , A. Corsia , A. Delbarta , P. Doornenbalb , A. Giganona , A. Gilliberta , Y. Kondof , Y. Kubotad , C. Lahonde-Hamdouna , V. Lapouxa , D. Leboeufa , C.S. Leeb , H.N. Liua , M. Matsushitad , T. Motobayashib , M. Niikurag , M. Nishimurab , H. Otsub , A. Peyauda , E.C. Pollaccoa , G. Pronoa , H. Tokiedad , T. Uesakab , J. Zenihirob a

17

IRFU, CEA, Universit´e Paris-Saclay, F-91191 Gif-sur-Yvette, France RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan c Institut f¨ ur Kernphysik, Technische Universit¨ at Darmstadt, 64289 Darmstadt, Germany d Center for Nuclear Study, University of Tokyo, RIKEN campus, Wako, Saitama 351-0198, Japan e NIRS-HIMAC, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan f Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan g Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan

18

Abstract

9 10 11 12 13 14 15 16

b

We report on the performance of the MINOS Time Projection Chamber developed as a vertex tracker to study exotic nuclei produced from hydrogeninduced knockout via in-beam γ-ray and invariant-mass spectroscopy. Inbeam measurements with 4 He and 20 Ne beams at 200 and 350 MeV/nucleon, respectively, were performed at the HIMAC facility. The tracking algorithm for protons after quasi-free scattering is described. Realistic simulations and physics experiments are compared and show a good agreement. The vertex position resolution reaches 5 mm FWHM, mostly from re-scattering with the target and the Aluminum reaction chamber. The overall efficiency of vertex reconstruction is also benchmarked with the first experimental campaign data

Preprint submitted to Nuclear Physics and Methods A

June 23, 2018

performed at the RIBF, confirming an overall efficiency better than 90 % for physics experiments. 19

Keywords: Time Projection Chamber, tracking software, Hough

20

transform, Spectroscopy of exotic nuclei

21

1. Introduction

22

Away from the valley of stability, the nuclear structure has been shown

23

to evolve with respect to stable nuclei. However, many interesting short-

24

lived nuclei are hard to reach experimentally, even at state-of-the-art ra-

25

dioactive beam facilities. In-beam spectroscopy at intermediate energies has

26

been shown as a powerful method to perform the spectroscopy of the most

27

exotic species. The thickness of the target used for these experiments has

28

always been a compromise: the thicker it is, the more luminosity is provided

29

but also worsening the energy resolution due to uncertainty on the reaction

30

vertex position, i.e., the velocity of the projectile at the reaction vertex.

31

32

To address this limitation, the MINOS device [1, 2] was developed to re-

33

construct the interaction vertex within the target for knockout reactions at

34

the Radioactive Isotope Beam Factory (RIBF) of RIKEN, either coupled to

35

a gamma array for in-beam γ-ray spectroscopy or to particle detectors for

36

invariant-mass spectroscopy (of unbound states). MINOS was designed to

37

fulfill two criteria: (i) The detector offers a large geometrical acceptance for

38

a total efficiency of more than 80%. (ii) The vertex resolution should allow

39

a velocity reconstruction of a few ‰ along the beam axis. This corresponds

40

to a few millimeters resolution. Several experiments were performed with 2

41

MINOS at the RIBF since April 2014. First results from in-beam gamma

42

experiments in combination with the DALI2 NaI array [3] of neutron rich

43

nuclei have already been published [4–10]. Also, missing mass and invariant

44

mass spectroscopy experiments have been performed with MINOS at SAMU-

45

RAI [11, 12].

Figure 1: Principle of the MINOS active target detector system: a liquid H2 cryogenic target (blue cylinder) of 10-20 cm length and 40 mm diameter surrounded by a Time Projection Chamber (pink cylinder). The blueish grey parts are the cryogenic device with the cryostat on the left and the beam surrounding the target for thermal isolation. A (p, 2p) reaction induced by an incident A Z beam is schemed in the target, producing two protons tracked in the TPC. During the HIMAC tests, only the TPC was used. At the RIBF, the full device was implanted at the target focal point of the beam line, either in the F8 area of the BigRIPS line or in the SAMURAI area. 46

47

The full MINOS device is shown in Fig. 1. It is composed of a thick liquid

48

cylindrical hydrogen target [13, 14] (possible thicknesses ranging from 50 to

49

200 mm) and surrounded by an annular Time Projection Chamber (TPC)

50

equipped with a Micromegas [15, 16] pad detection plane to track recoil

51

protons from (p, 2p)-type reactions and to reconstruct the vertex position. 3

52

Thanks to this tracking concept, the beam energy at the interaction point

53

in the target is known and the system allows a resolution as good as with

54

thin solid standard targets. The MINOS system was developed from 2011 to

55

2013. In experimental conditions as illustrated in Fig. 1, beam particles go

56

through the liquid hydrogen target and a knockout reaction may take place

57

inside the target. The heavy fragment is scattered from the initial beam

58

direction at a small angle and is eventually identified downstream. The re-

59

coiling protons are ejected at a larger angle and in most cases at least one

60

of the two intersects the TPC surrounding the target. Charged particles,

61

while punching through the TPC, ionize the gas. The ionized electrons then

62

drift towards the anode, i.e. the Micromegas detection plane, in an electric

63

potential of typical value 120-250 V/cm2 . The tracks detected in the TPC

64

are exclusively straight lines as no magnetic field is applied and the charge

65

deposition along the track is uniform for energetic protons.

66

67

The signals from the detector are converted and read by dedicated read-

68

out electronics [17]. In the present study, they are composed of Front-End

69

Cards (FEC) equipped with AFTER chips, as for the T2K experiment [18].

70

Recently, the MINOS electronics have been equipped with state-of-the-art

71

AGET chips [19] from the GET project [20, 21]. They are read out in both

72

cases by the dedicated so-called Feminos cards [22]. All the cards are then

73

synchronized to a common clock and trigger by a Trigger Clock Module card

74

(TCM) [23, 24].

75

76

We present here the performance of the MINOS TPC, both with an in-

4

77

beam test at the HIMAC facility in Japan and with realistic simulations. In

78

section 2, we briefly introduce the experimental setup. Then the tracking

79

algorithm created for MINOS is detailed in section 3. In section 4 we expose

80

the experimental results and compare them with Monte Carlo simulations. In

81

section 5, we apply simulations for MINOS to realistic physics cases measured

82

at the RIBF.

83

2. Experimental setup

84

The TPC was tested at the HIMAC facility [25] for heavy ion therapy in

85

Chiba, Japan, in one of the two experimental rooms dedicated to physics ex-

86

periments. Beams of 20 Ne at 350 MeV/nucleon and 4 He at 200 MeV/nucleon

87

were used, the latter one being used in a parasitic way after the CAT-TPC

88

[26]. For the experimental setup illustrated in Fig. 2, CH2 or C targets of 30

89

mm diameter and 0.5 mm thickness were used, i.e., 5.73×1021 and 5.65×1021

90

atoms/cm2 respectively. The targets were mounted on Plexiglass frames for

91

an overall 40 mm diameter and placed about 120 mm apart along the beam

92

path inside an aluminum beam pipe identical to the reaction chamber used

93

during physics experiments. Note that several tests were performed during

94

the experiment and therefore the distance between the targets was modified

95

from one measurement to the other. Variations of the distance between the

96

two targets were of ±4 mm around 120 mm. Measurements were focusing

97

on the resolution, i.e., width of the reconstructed vertex position. For con-

98

venience, the beam pipe was kept in air and had 2 mm thickness and 72

99

mm internal diameter, with an equivalent thickness of 1.47 × 1021 atoms/cm2

100

inside. As a remark, the effect of straggling in the aluminum tube could be 5

101

reduced by cutting some part of the pipe for Mylar windows, for experiments

102

that really need to minimize angular straggling. The MINOS TPC had an

103

internal diameter of 80 mm, an external diameter of 180.8 mm, and a length

104

of 300 mm and was mounted on the beam pipe as shown in the picture on

105

the left side of Fig. 2.

106

Figure 2: (Left) MINOS TPC at the HIMAC facility. (Right) MINOS experimental scheme seen from above of the in-beam test at HIMAC, with distances in millimeters.

107

A MicroMegas detection plane with a ”projective” pad geometry (cf. Fig. 15

108

of Ref. [1]) was used on the anode plane of the MINOS TPC. It is composed

109

of 18 rings of 2 mm radius containing each 256 radial segments. Pads on

110

the outer ring of the TPC have therefore a larger area than those on the

111

inner ring. The 4608 pads for the projective geometry were readout with the

112

digital electronics described in the section above. Data taking was performed

113

with the following settings for the electronics: 50 MHz sampling frequency,

114

240 fC gain range and 200 ns shaping time. The gas used in the TPC was a 6

115

116

117

triple mixture of Ar (82%), isoC4 H10 (3%) and CF4 (15%). From simulations with the Magboltz software [27], the longitudinal and transverse diffusions √ were estimated at ∼200 µm ` (` being the drift length in cm). The cathode

118

was set to 5 (5.4) kV voltage leading to a field of 152 (166) V/cm and the

119

Micromegas stage was set to 450 (430) V on the mesh which corresponds to

120

an amplification gain around 5800 (2900) for the

121

the data taking, impurities were monitored online with O2 impurities main-

122

tained below 60 ppm and H2 O impurities below 1000 ppm.

20

Ne (4 He) beam. During

123

124

Plastic scintillators were positioned in the beam path upstream the TPC

125

for beam triggering and timing and two layers composed each of two plastic

126

scintillators of 220 mm width, 630 mm length, and 20 mm thickness were

127

placed on both sides of the TPC as shown in Fig. 2. Their position was set

128

to maximize the detection of the protons coming from the (p, 2p) quasi-free

129

scattering kinematics. The trigger was composed of a coincidence with the

130

beam plastic scintillator upstream and a coincidence on the left and right

131

scintillators on the sides of the TPC. Recoiling charged particles produced

132

at large scattering angles were detected by the plastic scintillator side layers

133

and provided a trigger information for two-particle events in the TPC. As

134

no particle identification was performed in the TPC itself or downstream our

135

setup, (p, 2p) events were not unambiguously identified. The vertex position

136

resolution and the detector efficiency detailed in the following sections were

137

consequently extracted with unidentified particles which could worsen the

138

vertex determination and impact the overall resolution.

7

139

3. Tracking algorithm

140

The tracking algorithm for MINOS has two main constraints from the

141

physics experiments. (i) The detector is intended to be used for experiments

142

with low beam intensity. A software with the highest tracking efficiency

143

should therefore be favored. (ii) A proper compensation of target effects, for

144

example a good Doppler reconstruction for in-beam gamma spectroscopy ex-

145

periments, implies a resolution in the beam direction for the vertex position

146

of 5 mm FWHM or better.

147

148

Several methods have been applied for tracking such as a Kalman filter for

149

the PANDA prototype TPC [28] or a back-tracking of the photons with

150

simulation-based pulse-shape analysis of the signals for the AGATA spec-

151

trometer [29]. As the TPC tracks are linear, the Hough Transform (HT) [30]

152

is a good compromise between computing time and reconstruction accuracy

153

in our case. The HT has been a constant source of ideas for pattern recog-

154

nition, first for the detection of straight lines [31] but later also applied to

155

the detection of arbitrary patterns [32]. For the recognition of straight lines

156

in two dimensions, the HT changes each point in the coordinate space into a

157

straight line in the parameter space. Each line is specified in the real space

158

by two parameters (r, θ), r being the algebraic minimum distance between

159

the line and the origin and θ being the angle from this orthogonal vector and

160

a given reference axis. The parametrization becomes unique once the angle

161

θ is restricted to [0, π], r being either negative or positive in this standard

162

representation.

163

A first objective of the tracking algorithm is to find the multiplicity of tracks 8

while Npads ≥ nmin

Raw  event   (  x,  y,  q(t)  )   if qmax ≥ qmin Hough  transform  2D  

Hough transform k if Npads ≥ nmin & Cring

else

Keep (xnew, ynew) track i Discard pads for next iteration else

if Npads ≥ nmin

if 0 < Ntrack < 5 Fit of q(t) signals (xnew, ynew, znew, qmax) for every track

Hough  transform  3D  

Hough transform 3D if Npads ≥ nmin else & Rmin ≥ 15 Keep (xf, yf, zf) final track j

Discarded track if Nfinal track = {1,2}

3D Fit 4 parameters for each track if Nfinal track = 2 if Nfinal track = 1 Vertex finding with beam & track (xv, yv, zv) vertex position

Vertex finding with the 2 tracks (xv, yv, zv) vertex position

Figure 3: Scheme of the overall tracking algorithm with details on the applied conditions in Table 1.

9

164

per event in order to distinguish the one- and two-proton-like events. A first

165

selection of the number of particles per event is done, called event filtering,

166

by applying an adapted Hough filter to the Micromegas detection plane (xy).

167

Moreover, the tracking algorithm must filter off any possible noise contribu-

168

tions from the tracks (electronics noise, delta electrons) before being able to

169

reconstruct the vertex of interaction. The general algorithm scheme shown

170

in Fig. 3 is explained in details in the following paragraphs.

171

3.1. Event filtering

172

At a first stage, only the 2-dimensional information from the Micromegas

173

(charge projection on pad plane with no drift time information) are consid-

174

ered for each event. We therefore obtain a set of (x, y)j points for each event,

175

placed in the annular disk forming the detection plane. Physical tracks come

176

from the inner disk where the targets are located, imposing limits on the

177

Hough parameter space. We adapted the HT to the geometry of MINOS.

178

Each track segment in the TPC resulting from a reaction in the target is

179

determined by the crossing points on the inner and outer radii of the TPC.

180

The intersections are parameterized with the angles θint and θext respectively,

181

as shown in Fig. 4. For each (x, y)j pad on the Micromegas plane there is a

182

range of possible tracks coming from inside the beam pipe passing through

183

it with (θint , θext )i parameters. For every hit pad (x, y)j in the TPC, an iteration over all possible θint is made with a binning of ∆θint = 2◦ . For each possible value, there is one and only one line, crossing at the same time the hit point and the point located at the inner radius Rint at an angle θint , with coordinates (xint = Rint cos θint , yint = Rint sin θint ). The point (xext , yext ) at which this line crosses the outer radius 10

Figure 4: View of the HT parameterization used with the MINOS geometry. One possible track (in red) crossing the point (x, y)j (in purple) is shown. The range of possible angles θint and θext for this specific hit pad (x, y)j are shown in orange, they correspond to tracks coming from inside of the beam pipe.

is given by:

  y = (y − p x ) + ext int 1 int  x2 + y 2 = R 2 . ext ext ext

yint −y x , xint −x ext

(1)

184

Only one solution for (xext , yext ) corresponds to a track from a particle emit-

185

ted from the θint position of the inner tube of the TPC. This unique solution

186

determines θext .

187

We apply this iteratively and calculate all the possible (θint , θext )i combi-

188

nations for all the hit pads j in one event. Those values are stored in a

189

2-dimensional histogram. Physical tracks are identified by maxima in the

190

Hough space. Once a maximum is found in the Hough space, all the pads on

191

the line parameterized within the (θint , θext )max histogram binning are con-

192

sidered part of the same track and removed for the next iteration. The track

193

is considered only if it contains more than nmin = 10 pads as physical tracks

11

194

should all cross the detector given the transparent nature of the TPC. This

195

value for the minimum number of pads in a track was chosen from diffusion

196

considerations to remove the most number of stray pads from background.

197

An additional condition Σinner = 3 on the number of hit pads is applied in

198

the most inner rings of the TPC to rule out charged particles created outside

199

the target region. The procedure is applied several times until the number

200

of remaining pads with signal is smaller than nmin .

Figure 5: Two-dimensional HT applied to a two-particle event, with the treatment of the first track on the top and of the second track on the bottom. From left to right is shown the hit pads before the filter, the curves in the Hough representation of the event, and finally the hit pads after filtering out the points in the new-found track.

201

An example is given in Fig. 5, in which two tracks are recorded on the detec-

202

tion plane on the top left figure. In the top middle plot, all the possible cor-

203

responding (θint , θext )i couples are represented in the modified Hough space. 12

204

Two maxima can be distinguished, with angles around 350°and 160°which

205

correspond to the track circled in red and blue in the top left and bottom

206

left figures respectively. On the top right side is shown the pads considered

207

as part of this first cluster, which match the track seen at first sight in the

208

red circle. Those pads are removed for the next iteration presented in the

209

bottom part of the panel. The remaining track in the blue circle on the left

210

is parametrized in the Hough space and the second maximum leads to the

211

second cluster of pads in the bottom right figure.

212

3.2. Track filtering and time information In a second step all events with at least one track out of the previous HT are recovered and the software considers the previously discarded information on each pad. The charge deposited as a function of time or pulse shape gives the time and charge information. This signal is driven by several parameters in the electronics: the shaping time, the digitization frequency and the gain. An empirical formula for the signal q(t) as a function of the trigger time tpad relative to an external trigger (given by the beam plastic scintillator), the shaping time τ and the amplitude A can be written as:   3   t−t t − tpad t − tpad −3 τpad q(t) = A × e sin + qb τ τ

(2)

with the signal baseline qb , a constant fixed by the electronics. This analytical formula of the response function of the electronics has been obtained by fits to measured waveforms [33]. Therefore, by fitting with this function, one can extract the time tpad (in nanoseconds) at which the signal is produced in the TPC and the maximum of the function which is the charge qpad (in femtocoulombs) deposited on the 13

pad. tpad is the time it takes for the electrons to drift from the ionization at the track location to the detection plane relative to the trigger time tmin . The drift time is proportional to the position zpad at which the ionization took place inside the TPC along the beam direction: zpad = (tpad − tmin ) × vdrif t

(3)

213

Knowing the drift velocity vdrif t of the gas (see section 4.1 for its determina-

214

tion), a three dimensional picture of charge deposition can be deduced along

215

the track in the TPC.

216

217

Standard two-dimensional HT are then applied [31] in the (xy), (xz) and (yz)

218

planes once for each track of the selected events in order to filter the possible

219

delta electrons produced along the track (see Fig. 6). The track is taken into

220

account only if at least Rmin = 15 rings are hit when the charged particle is

221

not passing through the cathode. Otherwise for low scattering angle particles

222

that punch through the cathode, the final track is still required to contain

223

at least nmin = 10 pads to be registered. This value for the number of rings

224

allows for a few pads to be removed in the reconstruction and cleaning steps

225

from restrictive algorithms. It has been determined empirically to yield the

226

best reconstruction efficiency, and checked on a sample of data event-by-

227

event. As a result, one finds the final number of tracks for each event and

228

only the final one- and two-particle events are kept in the present analysis.

229

3.3. Track fitting and interaction vertex The fit of the tracks is performed with a minimization of the distance between every hit point and the reconstructed track weighted by charge in 14

Figure 6: HT showing the filtering of a track. From left to right the three 2-dimensional HT in the (xy), (xz) and (yz) planes are represented. From top to bottom the hit pads before the filter, the curves in the Hough space of the event, and finally the hit pads after filtering are shown.

the three dimensional space. In the present study, the determination of the vertex position in three dimensions is done for two-charged-particle events. As two straight lines in three dimensions generally do not intersect, the vertex position is determined as the midpoint of the line segment of minimum distance connecting the two tracks. To calculate the minimal distance between two tracks in three dimensions, we consider two vectors of coordinates → − − x and → x belonging to each line. The distance D between them is given a

b

− − by D2 = k→ xb − → xa k2 and can be analytically minimized. We can define the − vertex position of vector → x as the middle of this segment and we eventually v

15

get:

→ − − xa + → xb → − xv = 2 D=Dmin

(4)

230

The target being aligned along the beam for the physics experiments, the ver-

231

tex position zv along the beam direction is the most important parameter for

232

Doppler reconstructions of the gamma ray spectra or invariant mass measure-

233

ments. We therefore focus in the following on this parameter to extract the

234

resolution obtained during the HIMAC experiment. In physics experiments,

235

not only two-charged particle but also one-charged particle events are taken

236

into account for the reconstruction of the vertex using this particle track and

237

the beam trajectory in Eq. 4 instead. In this study, we will concentrate on

238

the two-charged particle events to extract the TPC performance.

239

4. Experimental results

240

In the following, the procedure to extract the drift velocity from the in-

241

formation collected by the TPC is described in Section 4.1. This allows us

242

to reconstruct the hit tracks in three dimensions and to apply the tracking

243

method described in Section 3 to calculate the resolution for the vertex po-

244

sition in Section 4.2. The pad multiplicity is studied in Section 4.3 as an

245

important parameter to check the gas transverse dispersion properties of the

246

TPC.

247

In the following experimental results are compared to simulations. They

248

are done in three consecutive steps: (i) a GEANT4 (v10.00) [34] simulation

249

including a reaction process (INCL v09.06.47) [35, 36] to generate events

250

and produce charged particles in the device, (ii) a Monte Carlo simulation to

251

reproduce the drift and the amplification of the ionized electrons towards the 16

Figure 7: (Left) Measured time in µs and extracted position or drift length z along the beam line inside the TPC for a

20

Ne run (350 MeV/nucleon). The time and distance 0

correspond to the Micromegas plane. Its calculated derivative is also shown in red. (Right) Deduced drift velocity as a function of the electric field applied in the MINOS TPC for several runs with different settings during the HIMAC measurements. Statistical error bars are smaller than the markers. The experimental points are compared to Magboltz simulations (blue line) with no impurities.

252

Micromegas detection plane, (iii) the reconstruction of the vertex position

253

via the above tracking software used also for experiment. Details on the

254

simulations are given in Ref. [1].

255

4.1. Drift velocity

256

The drift velocity vdrif t is a key parameter of the TPC and depends on

257

the electric field and gas composition including impurities such as H2 O and

258

O2 . It is determined by Eq. 3 from the distribution of trigger times tpad

259

inside the TPC for all pads and events, as shown on the left side of Fig. 7 in

260

blue. The histogram representing all possible drift times in the TPC should

261

reflect its length. As a convention, we choose the Micromegas plane of the 17

262

TPC to be the origin of the beam line axis, and therefore the statistics in

263

the plot must start at 0 mm and stop at zmax = 300.0(2) mm. With these

264

conditions, we can deduce the drift velocity by measuring the minimum and

265

maximum measured trigger times tmin and tmax inside the TPC such as in

266

Fig. 7: vdrif t =

267

mixture of Ar(82%) + CF4 (15%) + C4 H10 (3%), a drift velocity of 4.42(2)

268

cm/µs is found. We also show the calculated derivative of the drift time in

269

red in Fig. 7 as further reference for the reader. The rise and fall of the drift

270

times clearly correspond to peaks in the derivative.

271

During the performance measurements at HIMAC, several configurations of

272

electric field were taken, and the extracted drift velocities are plotted in Fig.

273

7. They are in very good agreement compared to Magboltz simulations [27],

274

consistent with no impurities in the TPC gas.

275

zmax . (tmax −tmin )

In the case of a 152 V/cm electric field and a gas

With these drift velocities, we can extract the vertex position along the

276

beam direction with the tracking algorithm.

277

4.2. Resolutions

278

With the tracking algorithm, we can extract the vertex position for (p, 2p)-

279

like events in the case of two-particle events with the fit of the tracks and the

280

reconstruction of the vertex point. The conditions applied for the tracking

281

are summarized in Tab. 1. The resolution of the vertex position is then ex-

282

tracted from the vertex position along the beam line for all filtered events as

283

shown in Fig. 8 on the left side for a run of

284

at 5 × 104 particles per second (pps) with two CH2 targets (relative distance:

285

120(4) mm). We obtain a resolution at full-width-half-maximum of 5.3(2)

286

mm and 6.6(3) mm for the first and the second CH2 targets, respectively. 18

20

Ne beam at 350 MeV/nucleon

Energy threshold of the pads

qmin ≈ 9000 e−

HT in 2D

∆θint/ext = 2◦

(binnings and intervals)

θint/ext ∈ [0◦ , 360◦ ]

Minimum number of points in track

nmin = 10

Condition Σinner

Npads ≥ 3

in the 4 first rings of the TPC Events with number of tracks

0 < Ntrack < 5

considered in 3D HT in 3D

∆ρxy/xz/yz = 2◦

(binnings and intervals)

∆ρxy/xz/yz = 3 mm ρxy ∈ [−45 mm, 45 mm] ρxz/yz ∈ [−300 mm, 300 mm] θxy/xz/yz ∈ [0◦ , 180◦ ]

Minimum number of rings touched

Rmin = 15

(if not crossing the cathode) Table 1: Conditions applied in the present study for the tracking algorithm.

19

Figure 8: (Left) (Filled histogram) Vertex position zv along the beam axis for a run with a 20

Ne beam at 350 MeV/nucleon. (Non-filled histogram) Results of a GEANT4 simulation

performed with the same conditions than the experiment and with the same tracking software, normalized to the number of counts in the experiment. (Right) Same with a 4 He beam at 200 MeV/nucleon.

287

The resolution of the vertex position is extracted from the vertex position

288

along the beam line for all filtered events as shown in Fig. 8 on the right side

289

for a run of 4 He beam at 200 MeV/nucleon at 1.5 × 104 pps with the same

290

setup. We obtain a resolution at full-width-half-maximum of 5.5(1) mm and

291

7.6(1) mm for the first and the second CH2 targets, respectively.

292

Simulations are performed in order to realistically compare our results. There

293

is an overall fairly good agreement between experimental data and simula-

294

tions. The experiment exhibits a slightly worse resolution for the second

295

target with both incident beams. This could come from the more forward

296

position of the created proton tracks: they need to travel farther in the TPC,

297

i.e., the ionized electrons experience a longer drift and therefore diffusion,

298

which affects the average multiplicity of the track. However, the simula-

299

tions exhibit a poorer resolution for the first target than for the second one.

20

300

Moreover, the integral of the two peaks were compared. The data shows an

301

equal amount of reactions occurring in the first and second target, while the

302

simulations exhibit a loss of counts in the second peak.

303

This difference between simulations and data was investigated to understand

304

the simulation bias. Plotting the proton angle distributions for the two differ-

305

ent beams, as seen in Fig. 9, the simulation discrepancy shows a difference in

306

the kinematics as compared to the experiment, especially for the 20 Ne beam.

307

The second target being placed at the middle of the TPC, the geometric effi-

308

ciency is smaller than for the first target as the tracks produced in the inner

309

tube outside of the TPC cover a wider solid angle in this case. However, the

310

simulations have taken into account the relative positions of the targets and

311

TPC, as demonstrated by the smaller number of counts in the simulations for

312

the second target. Comparing these considerations with the data exhibiting

313

a similar number of reactions in both targets, the supplementary counts in

314

the second targets could come from multiple scattering in the targets, which

315

are not accounted for in the simulations.

Figure 9: Angles (in degrees) of the charged particles emitted from the regions of the two CH2 targets, on the left for the first target with zv =[5,50] mm and on the right for the second target with zv =[125,170] mm. The data points (in black) are extracted from a run with a

20

Ne beam at 350 MeV/nucleon and compared to simulations (in blue).

21

316

4.3. Pad multiplicity

Figure 10: The top left panel shows the pad multiplicity as a function of the radius on the Micromegas plane. Pad multiplicities for the first, ninth and eighteenth (last) ring as a function of the position along the beam axis are shown in the other panels. The data points (in black) are extracted from a run with a 4 He beam at 200 MeV/nucleon and compared to simulations (in blue).

317

For the projective pad geometry, we have defined the pad multiplicity as

318

the number of pads hit for each ring of the TPC annular detector. As the

319

pads on the outer rings have a larger area, the ring multiplicity should de-

320

crease as one goes from the center of the TPC to the outer radius. The ring

321

multiplicity is extracted for each two-particle event which passed the track-

322

ing algorithm, and averaged over the number of events taken into account. 22

323

Its absolute value depends on the energy threshold taken for each pad, in

324

our case around 9000 electrons on the detection plane. The result for a 4 He

325

beam at 200 MeV/nucleon and a

326

a detector gain around 5800) is illustrated in the upper left part of Fig.10

327

and Fig.11 respectively as a function of the ring radius. The effect of the pad

328

size is clearly visible in this graph with a downward slope when going to the

329

exterior of the TPC, i.e., to larger radii.

20

Ne beam run at 350 MeV/nucleon (with

330

Figure 11: Same plot as Fig.10 but for a run with a

331

332

333

20

Ne beam at 350 MeV/nucleon.

There is a specific dispersion for a given gas mixture inside the TPC, here of √ about ∼200 µm `, which is a function of the distance the ionized electrons have to travel in the gas. As a result, the tracks coming closer to the cathode 23

334

are more dispersed when they get to the detection plane. We calculate the

335

ring multiplicities for the inner, middle and outer rings of the TPC as a

336

function of the position along the beam axis of the track points. The results,

337

as seen in the other three figures of Fig.10, reveal a slight slope in dispersion

338

as we go farther from the Micromegas plane, i.e., for longer drifts. They are

339

compared to simulations which show a very good agreement.

340

5. Physics cases simulations

341

Simulations were performed to obtain vertex position resolutions and de-

342

tection efficiencies for physics cases measured at the Radioactive Isotope

343

Beam Factory of RIKEN. In the following, we present cases that are part of

344

the RIKEN Proposal for Scientific Program (PSP) related to the search for

345

first 2+ states in neutron-rich nuclei [37]:

346

at an incident energy of 250 MeV/nucleon. Those two reactions were per-

347

formed in May 2014 and May 2017, respectively, and their data analysis

348

is ongoing. The simulations have been made in the same framework (see

349

Section 4) as the one benchmarked on HIMAC data in the present work.

350

The total efficiency of a (p, 2p) reaction to be analyzed can be decomposed

351

in three parts: the geometrical efficiency, the detection efficiency, and the

352

software efficiency. (i) The geometrical efficiency is given by the position and

353

size of the TPC as well as target. In other words, this is the angular coverage

354

by the TPC for which protons enter the detection area. (ii) The detection

355

efficiency is the probability for the protons crossing the TPC to actually

356

create a signal detected above the noise. We consider for this TPC a 100%

357

detection efficiency as for all tracks in (p, 2p) reactions we have completely 24

79

Cu(p, 2p)78 Ni and

53

K(p, 2p)52 Ar

358

continuous tracks without loss of signal. (iii) The software efficiency is then

359

determined as the efficiency of the tracking algorithm to reconstruct (p, 2p)

360

events, taking also into account the efficiency to detect only one of the two

361

tracks.

362

In the case of the spectroscopy of

363

at 250 MeV/nucleon on a 150 mm thick LH2 target is considered and 400

364

000 events are generated by GEANT4 simulations. We consider that (p, 2p)

365

reactions can be identified as it is usually the case in experiments by use of

366

a spectrometer downstream the target, either SAMURAI or the ZeroDegree

367

spectrometer at the RIBF. In the TPC, the gas ionization produced by proton

368

tracks is simulated as well as the electron drift towards the detection plane.

369

The algorithm described in Section 3 is used to extract the two proton events

370

and measure the total efficiency of the detector and the vertex resolution in

371

the beam direction. (p, 2p) events for which only one proton is detected in the

372

TPC are also treated and the vertex is determined from the intersection of the

373

found track and the beam trajectory from beam trackers. Out of all (p, 2p)

374

simulated reactions, 92(2)% were reconstructed with the successful tracking

375

of one (15(2)%) or two (77(2)%) protons in the TPC, demonstrating the

376

large efficiency provided by the MINOS design. The rare unidentified (p, 2p)

377

events correspond to a proton scattered at small angle and a second proton

378

scattered at low energy which does not reach the TPC, in very forward angles

379

of the (p, 2p) kinematics. Results for the efficiency are summarized in Tab. 2.

380

The reconstruction of the vertex are compared to the real position as shown

381

in Fig. 12. A vertex position resolution along the beam axis of 4.3(1) mm

382

at full-width-half-maximum is obtained.

52

Ar via (p, 2p) knockout, a beam of

25

53

K

53

K(p, 2p)52 Ar

79

Cu(p, 2p)78 Ni

Beam energy

250 MeV/nucleon

250 MeV/nucleon

Target length

150 mm

100 mm

(p, 2p) events simulated

2568 (100%)

2349 (100%)

(p, 2p) events in the TPC

2473 (96(2)%)

2295 (98(2)%)

(p, 2p) events analyzed

2361 (92(2)%)

2195 (93(2)%)

2 protons analyzed

1982 (77(2)%)

1787 (76(2)%)

1 proton analyzed

379 (15(2)%)

408 (17(2)%)

FWHM vertex position resolution

4.3(1) mm

4.5(1) mm

Table 2: Simulation results for the

53

K(p, 2p)52 Ar and

79

Cu(p, 2p)78 Ni physics cases.

Detected events correspond to (p, 2p) events with charge depositions in the TPC. Analyzed events correspond to events fully treated by the tracking algorithm and leading to a vertex position. Here the vertex position resolution is obtained from two proton tracks analyzed in the TPC, but it can also be determined with only one proton in the TPC by taking the beam position with a resolution defined by the position of the beam trackers and their resolution. The effect of the beam tracking resolution is determined in the simulations section of Ref. [1]. 383

79

Cu(p, 2p)78 Ni, a beam of

79

384

In the case of

385

ing on a 100-mm thick LH2 target is considered. A geometrical efficiency for

386

the detection of one or two protons of 98(2)% is found, with an efficiency

387

of 76(2)% to detect the two protons from (p, 2p) reactions. This detection

388

efficiency is comparable to the previous case with a slightly larger geomet-

389

rical efficiency understandable from the smaller target length. The results

390

are summarized in Table 2. The vertex position resolution is estimated to be 26

Cu at 250 MeV/nucleon imping-

Figure 12: Difference in millimeter between the vertex reconstruction and the real position of the (p, 2p) reaction along the beam direction for the simulation of (left) 53 K (p, 2p) 52 Ar and (right) 79 Cu (p, 2p) 78 Ni at a beam incident energy of 250 MeV/nucleon for both cases and with a 150 mm and 100 mm thick LH2 target, respectively. In both cases, a fit of the distributions is shown in red. 391

4.5(1) mm full-width-half-maximum along the beam direction, as illustrated

392

in Fig. 12.

393

394

The simulated total MINOS efficiency can be confirmed directly from data.

395

We used data from the

396

with a 150-mm long hydrogen target and DALI2 [3] at the RIBF during

397

one experimental campaign [38] in 2017. The 2+ 1 state of

398

to lie at 1178(18) keV [39] from a previous measurement. The statistics in

399

+ the 2+ 1 → 01 photopeak of the gamma spectrum measured by DALI2 for

400

events with and without the coincidence of a vertex reconstructed in MINOS

401

TPC were compared. Both spectra were Doppler corrected without using

51

K (p, 2p)

50

Ar reaction measured using MINOS

27

50

Ar is known

402

the vertex information. Therefore, their line shape is the same. Considering

403

the Doppler-corrected photopeak events ensures that the γ rays indeed come

404

from reactions inside the target. Fig. 13 illustrates the ratio of counts in

405

these two gamma spectra with an energy bin of 40 keV after background

406

subtraction. The mean value is 90(5)%. The quoted uncertainty mainly arose

407

from background subtraction and limited statistics. A detection efficiency

408

for tracking the two protons was found to be 80(6)% and only one proton

409

11(3)%. We also extracted the MINOS efficiency from the

410

channel in another experimental campaign in 2015 [4] using a 100-mm long

411

hydrogen target. A total MINOS efficiency of 92(1)% was found, with 75(1)%

412

efficiency to track the two protons and 17(2)% to reconstruct only one proton,

413

in agreement with the simulations. Note that for this last measurement we

414

only considered the statistical uncertainties in the errors.

415

6. Conclusion and perspectives

69

Co (p, 2p)

68

Fe

416

We reported on the in-beam validation of the MINOS cylindrical time

417

projection chamber by use of intermediate-energy heavy-ion beams at the

418

HIMAC facility. A set of two CH2 targets were inserted inside the TPC. Ne

419

beams at 200 and 350 MeV/nucleon and a He beam at 200 MeV/nucleon were

420

used for the measurement. The TPC performances were tested at different

421

drift fields and the drift velocities obtained from the measurements show a

422

very good agreement with Magboltz simulations in the domain of interest.

423

This benchmarks our method to monitor the changes in drift velocity due to

424

oxygen and water impurities.

425

Dedicated algorithms and software have been developed for track reconstruc28

Figure 13: The total MINOS efficiency as a function of the γ-ray Doppler corrected energy with respect to the photopeak position of (p, 2p)

50

50

Ar. It was extracted using the data from

51

K

Ar. The square points show the ratio of counts for an energy bin of 40 keV, and

the red solid line shows the mean value of these square points.

426

tion based on the Hough transform. A vertex resolution of 5-7 mm FWHM

427

has been found for (p, 2p)-like reactions that includes interactions on both

428

carbon and hydrogen of the target. Monte-Carlo simulations have been per-

429

formed and show a good agreement with experiment. Furthermore, the pad

430

multiplicity is correctly reproduced, which shows our correct simulation of

431

the electron drift and diffusion in the TPC gas.

432

To benchmark further the tracking algorithm in terms of efficiency, we have

433

performed simulations of physics cases in realistic RIBF conditions. The TPC

434

demonstrates a 100% detection efficiency of protons in the MINOS energy

435

domain. The total efficiency of MINOS to reconstruct a reaction vertex after

436

a (p, 2p) reaction is shown to be ∼92%, ∼16% and ∼76% from 1p and 2p

437

events respectively, mostly from the not totally complete angular coverage.

29

438

A vertex position reconstruction better than 5 mm FWHM is found. The

439

MINOS efficiency has also been benchmarked with the experimental results

440

of

441

+ the 2+ 1 → 01 photopeak in the Doppler-corrected gamma spectra with and

442

without the vertex reconstruction in the MINOS TPC.

443

To increase the vertex resolution, one could think of software improvements

444

on the tracking algorithm using for example a 3D Hough transform [40] or

445

hardware modifications of the setup to minimize the angular straggling of

446

the protons.

447

MINOS is a very efficient detector for gamma-ray spectroscopy of exotic nu-

448

clei thanks to thick-target induced reactions reconstructed with either the

449

same or better vertex resolution as standard thin targets. Concerning the

450

gamma-ray spectroscopy at the RIBF, the resolution limitation comes from

451

the DALI2 scintillator array. The combination of higher resolution gamma-

452

ray arrays with MINOS would open new possibilities. In particular, the

453

MINOS concept is expected to bring a large gain in sensitivity to in-beam

454

gamma experiment with new generation Ge tracking arrays [29, 41].

51

K (p, 2p)

50

Ar and

69

Co (p, 2p)

68

Fe reactions by using the statistics in

455

456

This work has been funded by the European Research Council through the ERC

457

Starting Grant MINOS-258567. C. Santamaria has been supported by the IPA

458

program at the RIKEN Nishina Center. A. Obertelli thanks the Japanese Society

459

for the Promotion of Science (JSPS) for its support.

460

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461

[2] http://minos.cea.fr.

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tween CEA-IRFU, CENBG, GANIL (France) and NSCL (US) labora-

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34