Proposal number (User Support Office use only)
Date: October 12, 2018

Proposal for Nuclear Physics Experiment at RI Beam Factory(RIBF NP-PAC-18, 2017)

Title of Experiment:

High Resolution In-Beam Gamma-Ray Spectrometer for the RIBF
[ ] NP experiment [ ] Detector R&D[X] Construction[ ] Update proposal (Experimental Program: NPaaaa-aaaa )[ ] Re-evaluation (Experimental Program: NP aaaa-aaaa)[ ] GARIS[ ] RIPS[ ] CRIB[ ] KISS[X] BigRIPS [X] ZeroDegree [ ] SHARAQ [ ] OEDO
[ ] RI Ring
[ ] SAMURAI[ ] DALI2[ ] GRAPE

Category:
Experimental
Devices:
Detectors:

Spokesperson (Only one person, Multiple spokespersons not accepted):

Name
Institution
Title of position
AddressTel.
Email

Pieter Doornenbal
RIBF, RIKEN
Researcher
2-1 Hirosawa, Wako, Saitama 351-0198, Japan+81 (0)48 462 1111
Fax
+81 (0)48 462 4464
pieter@ribf.riken.jp

Co-Spokesperson (if any):

Name
Institution
Title of position
AddressTel.
Email

Kathrin Wimmer
University of Tokyo
Lecturer
7-3-1 Hongo, Bunkyo-ku, TOKYO 113-0033 Japan+81 (0)3-5841-4216
Fax+81 (0)3-5841-7642
wimmer@phys.s.u-tokyo.ac.jp

Name
Institution
Title of position
Address
Tel.
Email

Fax

In-House Contact Person (if any):

Name
Institution
Title of position
Address
Tel.
Email

Fax

Beam Time Request Summary:

Please indicate requested beam times of TU ser−T uning and TU ser−DataRun only. TBigRIP S andTotal times will be given by RIKEN.
TBigRIP S:(User Support Office use only )Days

Tuning time with BigRIPS for secondary
beam settings

TU ser−T uning
Total
Beam Time

Tuning time for users’ own equipmentDays

and/or detectors using primary/secondarybeams

TU ser−DataRun
Days

Beam-time for data runs

(User Support Office use only)

Total

Days

Beam Summary
Primary Beam:Particle

EnergyE/A (MeV)

Intensity(pnA)

Secondary Beams:

Beam-On-Target Time for
DATA RUN

RI Beams
Isotope

EnergyE/A(MeV)Rate of
Isotope(/s)

Total Beam
Rate @F3 (/s)

Days

Fill each row for each BigRIPS setting, with objective isotope, its energy, rate and total beam
rate which cannot exceed 107 /s in BigRIPS due to radiation regulation.

Keywords:

[ ]
[X]
[ ]
[ ]
[ ]
[X]
[X]
[X]

[X]
[X]
[ ]
[ ]
[ ]
[ ]
[ ]
[X]

New isotope search
Lifetime measurement
Mass measurement
Superheavy elementβ-γ spectroscopy
In-beam γ-ray spectroscopy[ ] 2+
1 study
Nucleosynthesis[ ] r process[ ] rp process
Nuclear structurearound [ ] 32 Mg[ ] 42 Si[ ] 78 Ni
100
132
[ ]
Sn
[ ]
Sn
[ ] Others (Nuclear reaction
Shell evolution
Nuclear moment
Spin-isospin excitation
Nuclear force
Nuclear equation of state
Exotic atom
Isospin symmetry)

Readiness:

Estimated date ready to run the experiment
Date which should be excluded if any

2020—

Summary of Experiment:

It is proposed to construct a high-resolution, Ge based γ-ray spectrometer composed of the
MINIBALL array as well as AGATA and GRETINA type cluster detectors named DAGATA,RCNP Quad, and LBNL P3. The envisioned in-flight resolution and efficiency are 0.15–1.5 %(σ) and 9.4%, respectively, for 1 MeV γ-rays emitted at 100 MeV/u. This setup will pave theway to detailed nuclear structure and reaction studies currently not feasible at the RIBF.

List of Collaborators (including spokespersons):

Name
N. Aoi
H. Baba
F. Browne
C. Campbell
M. Carpenter
A. Corsi
M.L. Cortés
H. Crawford
M. Cromaz
P. Doornenbal
P. Fallon
A. Gillibert
H. Hess
E. Ideguchi
T. Isobe
V. Lapoux
H. LiuA. Macchiavelli
M. Niikura
O. Möller
S. Nishimura
A. Obertelli
V. Panin
N. Pietralla
P. Reiter
L. Riley
H. Sakurai
M. Seidlitz
D. Suzuki
S. Thiel
V. Werner
N. Warr
K. Wimmer
Y. Yamamoto

Institution
RCNP
RIKEN
RIKEN
LBNL
ANL
CEA Saclay
RIKEN
LBNL
LBNL
RIKEN
LBNL
CEA Saclay
University
Cologne
RCNP

of

RIKEN
CEA Saclay
TU Darmstadt
LBNL
The University ofTokyo
TU Darmstadt
RIKEN
TU Darmstadt
CEA Saclay
TU Darmstadt
Universityof
CologneUrsinus College
The University ofTokyo/RIKEN
Universityof
Cologne
RIKEN
Universityof
CologneTU Darmstadt
Universityof
Cologne
The University of
Tokyo
RCNP

Title or position
Professor
Team Leader
Postdoc
Researcher
Researcher
Researcher
Postdoc
Researcher
Researcher
Researcher
Researcher
Researcher
Researcher

Emailaoi@rcnp.osaka-u.ac.jp
baba@ribf.riken.jp
frank@ribf.riken.jp
cmcampbell@lbl.gov
carpenter@phy.anl.gov
acorsi@cea.fr
liliana@ribf.riken.jp
hlcrawford@lbl.gov
mcromaz@lbl.gov
pieter@ribf.riken.jp
pfallon@lbl.gov
alain.gillibert@cea.fr
hess@ikp.uni-koeln.de

Assosciate Professor
Researcher
Researcher
Postdoc
Researcher
Assistant Professor
Researcher
Researcher
Professor
Postdoc
Professor
Professor

ideguchi@rcnp.osaka-u.ac.jp

Professor
Chief Scientist

lriley@ursinus.edu
sakurai@ribf.riken.jp

Postdoc

seidlitz@ikp.uni-koeln.de

Researcher
Researcher

daisuke.suzuki@ribf.riken.jp
thiel@ikp.uni-koeln.de

Researcher
Researcher

vw@ikp.tu-darmstadt.de
warr@ikp.uni-koeln.de

Lecturer

wimmer@phys.s.u-tokyo.ac.jp

Researcher

yamamo@rcnp.osaka-u.ac.jp

isobe@riken.jp
valerie.lapoux@cea.fr
hliu@ikp.tu-darmstadt.deaomacchiavelli@lbl.gov
niikura@nucl.phys.s.u-tokyo.ac.jp
moeller@ikp.tu-darmstadt.denishimu@ribf.riken.jp
aobertelli@ikp.tu-darmstadt.devalerii.panin@riken.jp
pietralla@ikp.tu-darmstadt.depreiter@ikp.uni-koeln.de

5

Detailed Description of the proposed experimentHigh Resolution In-Beam Gamma-Ray
Spectrometer for the RIBF

Physics motivation
Since its first beam in 2006 the Radioactive Isotope Beam Factory (RIBF) of the RIKEN Nishina
Center (RNC) provides highest intensity primary beams at 345 MeV/u. The great potential of
the facility is best demonstrated by the search and discovery of more than 100 new isotopes [1–3]using a 238 U primary beam at the fragment separator BigRIPS [4]. In addition to 238 U, primarybeams at the RIBF include 18 O, 48 Ca, 70 Zn, 124 Xe, and 78 Kr. In-beam γ-ray spectroscopyusing secondary beams mostly employ the ZeroDegree spectrometer [4] for reaction productidentification and the DALI2 NaI array [5] for γ-ray detection. Owing to the high secondary
beam intensities as well as detection efficiency, experiments performed at the RIBF with themost exotic nuclei are feasible nowhere else in the world. Notable examples of the RIBF’s
capabilities include the first γ-ray spectroscopy of 54 Ca [6], 78 Ni [7], and 70 Kr [8]. Overall, inbeam γ-ray experiments currently account for ≈40 % of the physics publications from NP-PAC
approved experiments at the RIBF with fast radioactive beams. A complete list of obtained
results can be found in Ref. [9].
So far, spectroscopic experiments, carried out with the NaI(Tl) based spectrometer DALI2 [5],have been largely limited to even-even nuclei in the vicinity of shell closures. Using a germanium based γ-detector will significantly enhance the capabilities of the RIBF facility and open
new possibilities for experiments. With this construction proposal, we combine the MINIBALL
array [10], which contains 8 six-fold segmented triple clusters, an AGATA type triple [11] fromthe TU Darmstadt (referred to as DAGATA), a GRETINA type quad [12] from RCNP (referred
to as RCNP Quad), and a GRETINA type triple from LBNL (referred to as LBNL P3) with
the BigRIPS fragment separator and the ZeroDegree spectrometer [4]. This combination of theworlds highest secondary beam intensities with a Ge-based γ-ray spectrometer of 34 crystals
in total will provide unique opportunities for nuclear structure as well as nuclear astrophysics
studies.

1

Physics case

The RIBF provides currently worldwide uniquely high secondary beam intensities at intermediate
energies ranging from 100 to 300 MeV/u. The proposed addition of the high resolution Ge-based
γ-ray spectrometer will allow to access the most exotic nuclei and thus unprecedented science.
Unique to the RIBF is the availability of a high intensity 238 U beam, giving access to the mostneutron-rich nuclei around 78 Ni as well as heavier nuclei beyond 132 Sn which are of importancefor the astrophysical r-process [13]. γ-ray spectroscopy experiments in this region of the nuclear
chart can to date only be performed at the RIBF.
Shell evolution towards the drip-lines, new magic numbers
Neutron-rich nuclei below 78 Ni display many interesting features, halo systems have now

6

1 PHYSICS CASE
been suggested at N ∼ 20 [14], while the classic shell gaps N = 20, 28 disappear for neutronrich nuclei resulting in a large region of deformation [15, 16]. Tracking the evolution of
the neutron f7/2 and p3/2 orbitals from N = 20 to 28 requires detailed spectroscopy of theeven-odd nuclei. So far this has only been done for the Si isotopes [17] up to N = 26.
Neutron knockout reactions with Ne or Mg isotopes would particularly benefit from theimproved resolving power compared to DALI2.
In the Ca isotopic chain two new sub-shell closures have been established experimentally [6,18, 19]. Theoretical calculations [20] show that three nucleon forces play an important rolein the microscopic description of these nuclei. Spectroscopy of Ca isotopes at RIBF has
recently been extended to N = 36 for 2+
1 states and single-particle properties studied in theneutron and proton removal reactions from 54 Ca within the SEASTAR project [21]. Beyond
Ca, the Ti isotopes show a staggering of collectivity which is not yet understood in either
beyond mean field [22] as well as shell model [23] calculations. Lifetime measurements
beyond the 2+
1 states, the odd Ti isotopes, as well as spectroscopic factors obtained fromknockout reaction will provide more stringent tests of the robustness of the closed-shell coresand the role of three-body forces for non-magic isotopes. Generally, due to the onset ofcollectivity and complexity of the level structures, odd-even nuclei beyond Z = 20 have beenchallenging with DALI2, as seen already for 55 Sc [24]. An abundance of new spectroscopic
details may be revealed by utilizing high-resolution in-beam γ-ray spectroscopy.
Collectivity and breakdown of the harmonic oscillator shell closure at N = 40 has been
established through measurements of collective properties, excitation energies, and level
lifetimes (see for example Refs. [25–27] for most recent experimental data). A new islandof inversion has been predicted with a center at 64 Cr [28]. Experimental data on spin
and parity assignments as well as level occupations is sparse in the region of neutronrich N = 40 nuclei. High-resolution spectroscopy of the even-odd nuclei 67,69 Fe, 63,65 Cr,
and 57,59 Ti from one-neutron knockout reactions will fill this gap and reveal whether its
inversion extends down towards 60 Ca.
Excited states in 78 Ni have recently been studied at the RIBF facility with a high intensity
238 U primary beam [7]. Quantification of the magnitude of the shell gap along N = 50
and Z = 28 as predicted in Ref. [29] requires precise measurements of odd nuclei throughknockout reactions. Lifetime measurements of neutron-rich Zn, Ge, and Se nuclei with
N > 50 to delineate the onset of collectivity beyond 78 Ni will be possible as well.
Structural changes around the doubly-magic 132 Sn are subject of great and persisting experimental as well as theoretical interest. Latest state-of-the-art Monte-Carlo Shell-Modelcalculations with an unprecedented large configuration space allowed for a detailed description of the structural evolution along the tin isotopic chain [30]. Predictions from this newinteraction can be tested by knockout reactions from even tin isotopes as well by B(E2)↑
measurements in 130,132,134 Sn.
North-east of 132 Sn, several interesting phenomena occur. E.g. the 6+ seniority isomers
along the Sn chain have been explored in the EURICA campaign [31]. The excitationenergies of the 2+
1 states in even-even isotopes are asymmetric with respect to N = 82
exhibiting lower values for N < 82. Filling the 2f7/2 neutron shell up to N = 90 sometheories predict a similar behavior as for the Ca isotopic chain, resulting even in a new shellclosure. However, in-beam data are hard to obtain for these neutron-rich isotopes. (p, 2p)
reactions in combination with high-resolution γ-ray spectroscopy open for the first time
also access to odd isotopes. Moreover, the (p, 2p) reactions will allow for the extraction ofoccupation numbers, shading new light on the underlying single-particle structures.

N = Z: the rp-process, mirror symmetry, and the quest for 100 Sn
Along the N = Z line of the nuclear chart, a number of interesting physics cases can

7
be studied. Proton-rich nuclei along N = Z constitute the path of the rapid protoncapture process (rp-process), that powers type I X-ray bursts. While direct measurementsof (p,γ) reaction rates on these radioactive isotopes are not feasible in all cases, indirect
measurements of excited states in the final nuclei as well as spectroscopic factors from(d,n) reactions can significantly reduce the uncertainties associated with the calculation ofelement abundances in the rp-process [32].
N ∼ Z nuclei also provide an interesting testing ground to investigate the breaking of
charge symmetry and charge independence. Measurements of Coulomb energy and mirror
energy differences have been limited so far to A = 70 and low spin values [8, 33]. Datafor higher (medium) spin values and odd-even nuclei are required to clarify the role ofisospin-non-conserving forces and neutron skin effects [34]. The high intensity 78 Kr and
124 Xe primary beams at RIBF combined with the high in-beam energy resolution of the
Ge array allow for detailed spectroscopy of nuclei between A = 50 and 100. In addition to
nuclear structure information, new data on waiting point nuclei of the rp-process predictedin this region can be obtained.
The region around 100 Sn has attracted a lot of interest. B(E2)↑ values in light tin isotopes
are larger than expected [35, 36], and the ordering of the neutron νg7/2 and νd5/2 orbitals
above N = 50 seems to be inverted [37]. Spectroscopy of 101,103 Sn and 101,103 In throughproton or neutron knockout and well as (p,p′ ) reactions will contribute to the understandingof unexpected high collectivity in the light even-even Sn nuclei.
Shape evolution and coexistence around A ∼ 100 and in the rare earth region
Between N = 50 and N = 82, nuclei show an interesting interplay of single-particle andcollective excitation modes. In the Sr and Zr isotopes, a sudden onset of collectivity at
N = 60 is accompanied with shape coexistence at low excitation energies [38]. In the Kr isotopes, however, the associated drop in the E(2+1 ) excitation energy is not observed [39, 40].
Additional information could come from lifetime measurements and off-yrast spectroscopy
through secondary fragmentation reactions and spectroscopy of odd-even nuclei. High intensity radioactive beams with Z & 50 are unique to the RIBF facility. Measurements ofcollective properties, excited state lifetimes, and proton inelastic scattering will shed newlight on the evolution of collectivity and shape coexistence in this region. Measurements ofsingle-particle properties will give hints on the underlying microscopic structure and new
constraints for astrophysical r-process calculations.
Octupole collectivity in A ∼ 140 and 90 nuclei
In regions where neutrons and protons occupy orbitals at the Fermi surface with ∆j =ℓ = 3 strong octupole correlations can break the reflection symmetry of nuclei, resulting
in octupole deformed shapes. Octupole collectivity is expected below the Ra-Th region
for neutron-rich nuclei in the region around 90 Se and 144 Ba [41]. In the first case, first
spectroscopy of 90 Se has only been performed recently [42], and measurements beyond thespectroscopy of states are necessary to asses the possibility of octupole deformation, forexample by lifetime measurements.

2

High-resolution in-beam γ-ray spectroscopy at the RIBF

It is proposed to couple a high-resolution Ge array with the BigRIPS fragment separator and the
ZeroDegree spectrometer at the RIBF. Experimental details are briefly described in the following section, highlighting the unrivaled possibilities for in-beam γ-ray spectroscopy experiments
offered at the RIBF.

8

2 HIGH-RESOLUTION IN-BEAM γ-RAY SPECTROSCOPY AT THE RIBF

Figure 1: Overview of the RIBF facility. Not shown are the initial acceleration stages. The
BigRIPS fragment separator is used to purify and identify the radioactive ion beams (production
target – F7). The array will be placed the focal point F8. Reaction products are uniquely
identified using the ZeroDegree spectrometer (F8 – F11).

2.1

BigRIPS and ZeroDegree spectrometer

A layout of the RIBF facility is presented in Fig. 1. Primary beams impinge on the production
target (F0); their fragmentation or fission products are subsequently separated in the two-stage
separator BigRIPS. Event-by-event identification is facilitated by Bρ, ∆E, and time-of-flightmeasurements between the focal points F3 and F7. Positions and angles of beam particles
are measured using parallel plate avalanche counters (PPAC), the time-of-flight is measured
with plastic scintillators at F3 and F7. At F7, an ionization chamber allows for unique Z
identification through an energy loss measurement. Maximum rates are several tens of kHz.
The total momentum acceptance amounts to ±3 % with a momentum resolution of 3420 (p/δp).
A relative A/Q resolution of 0.034 % has been achieved [43], allowing for the separation of
fully stripped from hydrogen-like charge states. Production cross sections of radioactive ionshave been measured for a wide range of nuclei [2, 3, 44] providing validation of the theoreticalfragmentation and fission cross sections employed in LISE++ simulations. Secondary beam
intensities for experiments can therefore be predicted reliably allowing for an easy planning ofexperiments at the RIBF.
In-beam γ-ray spectroscopy experiments are performed at F8 with thick solid secondary reaction
targets (Be, C, Au, Pb etc.) in the order of 1 g/cm2 depending on the case of interest. The
size of the beam spot on the target amounts typically to σ = 5 mm. Tracking of the beam
onto the target is facilitated by a set of two PPACs in front of the target (position resolution
σ = 0.7 mm), the scattering angle can be reconstructed using a third PPAC behind the target(before STQ17 in Fig. 1) with an accuracy of typically 5 mrad [36]. For the high-resolution array,we envision to use either solid targets, the liquid hydrogen target MINOS [45], or plunger for
lifetime measurements. For experiments with solid targets, the present setup at F8 with three
PPACs and a beam pipe of 150 mm diameter can be maintained.
The ZeroDegree spectrometer will be used for identification of reaction products after the secondary target located at F8 (see Fig. 1). Particle identification will be obtained in an analogous
way to the BigRIPS separator by using Bρ, ∆E, and time-of-flight measurements between F8
and F11. Typical beam energies after the target will amount to 100 − 200 MeV/u. Therefore,
for high Z nuclei not all reaction products will be in the fully stripped charge state. With the910

DALI2 (a)8

∆ Eγ /Eγ (%)∆ Eγ /Eγ (%)

2.2 High resolution Ge spectrometer

10

8

6

6

4

4

2

2

0
0

20 40 60 80 100 120 140 160 180

laboratory detection angle θ (deg)00

Ge array (b)intrinsic resolution
velocity uncertainty
angle uncertainty tracking
total tracking
angle uncertainty Miniballtotal Miniball20 40 60 80 100 120 140 160 180

laboratory detection angle θ (deg)Figure 2: Energy resolution of high-resolution array (b) in comparison with DALI2 (a). The total
in-beam energy resolution (black) depends on the intrinsic energy resolution (blue), the position
resolution for the Doppler correction (green) as well as the velocity uncertainty due to the energy
loss in the target (red). Uncertainties are calculated for a 200 MeV/u beam impinging on a 4 mm
Be secondary target (hβi = 0.55, ∆β = 0.016) emitting a 1 MeV γ ray. Great improvements in
the intrinsic energy as well a position resolution of the high-resolution array compared to DALI2
allow for spectroscopy with about 1–3 % final energy resolution for in-beam experiments. This
assumes Doppler correction on the segment level for MINIBALL, and a position resolution of
5 mm (FWHM) for DAGATA, RCNP Quad, and LBNL P3 detectors. Detectors are placed at a
distance of 200 mm.
high resolution for the mass to charge ratio A/Q it is possible to discriminate different charge
states up to Z ≈ 55, A ≈ 150. Additionally, a total kinetic energy detector can be placed at
the end of the beam-line after the last ionization chamber at F11. With a momentum resolutionof p/∆p = 1240 the large acceptance mode offers sufficient accuracy for parallel momentum
distributions to deduce angular momentum L assignments.

2.2

High resolution Ge spectrometerIn-beam γ-ray spectroscopy at the RIBF has so far been mostly restricted to the spectroscopy ofeven-even nuclei. Spectroscopy of odd nuclei is challenging due to the limited energy resolution,
except for certain cases close to magic numbers [24, 46]. The in-beam energy resolution depends
critically on the intrinsic energy as well as position resolution, as shown in Fig. 2 (a).
Here, we propose to combine the existing MINIBALL array, DAGATA, the RCNP Quad, andthe LBNL P3 to a common array of 34 crystals. The latter three are 36-fold segmented, position
sensitive γ-ray tracking detectors, while MINIBALL consists of eight six-fold segmented triple
clusters. It is noted that the collaboration has experience in running and maintaining such
position sensitive detectors.
Obtainable energy resolutions following Doppler correction depend largely on distance and angleof the placed detectors. Assuming detector distances of 200 mm, the resolution is improved by afactor three for MINIBALL detectors and about a factor five to ten for γ-ray tracking detectors
compared to DALI2, but these numbers also depends on the energy loss inside the reaction
target. Fig. 2 shows the parameters of a typical in-beam γ-ray experiment at the RIBF. For
larger distances between detectors and target, better resolutions can be obtained.
The nominal efficiency of MINIBALL amounts to 7.8 % at 1.3 MeV [10] using add-back. For ex-

10

2 HIGH-RESOLUTION IN-BEAM γ-RAY SPECTROSCOPY AT THE RIBF

y
10 cm
x
10 cm

z
10 cm

Figure 3: (Left) Support structure for the array. The system will be placed on rails (red arrow)and can be height adjusted to the BigRIPS/ZeroDegree beam height of 1700 mm. Detectors canbe placed at desired angles. (Right) Possible configuration of the array as seen from upstream
in GEANT4 simulations. MINIBALL crystals are shown in gray, the RCNP Quad and LBNL
P3 in yellow and purple, and DAGATA in red, green, and blue.
periments at RIBF, the distance to the target is larger than for the typical experiments performed
at REX-ISOLDE. Thus a reduced efficiency is expected. For experiments with fast beams performed at GSI, the array was placed at angles of 51 and 85◦ at about 220 mm distance, resulting
in a resolution of 3 % and an efficiency of 3 % for a 1.3 MeV γ-ray emitted at 100 MeV/u [47].
Due to their position resolution of 5 mm (FWHM), the tracking detectors can be positioned
much closer to the reaction target. Assuming a distance of 130 mm, which sets the resolution toabout 1–3 %, the total efficiency of the entire array amounts around 9 %. Due to the beam pipe
diameter of 150 mm, such close distances are not available for all angles.

2.3

Support structure

The MINIBALL support structure, shown in Fig. 3 fits into the F8 area, which offers 2 m ofspace along the beam axis between the F8 chamber and STQ17. The beam height at F8 is 1.7 m.
All 11 detectors can be attached to the three rings. Angles and distances can be varied according
to the specific requirements of the experiment.

2.4

Simulations of resolution and efficiency

Initial simulations based on the GEANT4 framework [48] have been carried out. Parameters andobtained results are summarized in Tab. 2. The simulations comprised MINIBALL, DAGATA,
Detector

Crystals

Angle

Distance / mm

Resolution / %Efficiency /%MINIBALLMINIBALL
RCNP Quad, LBNL P3DAGATA

18
6
7
3

30
65
65
65

200
200
130
130

1.03
1.52
0.15
0.16

4.1
1.1
3.1
1.1

Total

34

9.4

Table 2: Simulated energy resolution (σ) and efficiency for a 1 MeV γ-ray emitted at 100 MeV/u.
The RCNP Quad, the LBNL P3, and DAGATA can be placed much closer to the target due totheir enhance position resolution. Presented values include intra cluster add-back and Doppler
correction based on segment level for MINIBALL and (link-algorithm based) interaction point
clustering and tracking to find the first interaction point for the tracking detectors.

5000

with target, σ = 2.54 %4000

no target, σ = 1.03 %1000

with target, σ = 1.52 %800

no target, σ = 1.56 %

600

2000

400

1000

200

8000
7000

200

400

600

800

0

1000

Eγ (keV)°

RCNP Quad and LBNL P3 at 65 (c)with target, σ = 0.33 %6000

no target, σ = 0.15 %

5000
4000
3000

°

2 Miniball Clusters at 65 (b)

1200

3000

0

counts / 1 keV

counts / 4 keV°

6 Miniball Clusters at 30 (a)6000

counts / 1 keV

counts / 4 keV

11

200

400

600

800

1000

Eγ (keV)°

3000

DAGATA triple at 65 (d)2500with target, σ = 0.32 %2000

no target, σ = 0.16 %

1500
1000

2000
500

1000
0

200

400

600

800

1000

Eγ (keV)

0

200

400

600

800

1000

Eγ (keV)Figure 4: Doppler Corrected energy spectra for a 1 MeV γ-ray emitted at 100 MeV/u without
target and with a 3 mm Be target. The latter uses 79 Cu secondary beams. See text for details.
the RCNP Quad, and the LBNL P3. Simulations were carried out for a 1 MeV γ-ray emitted at
100 MeV/u. As seen in Fig 2, MINIBALL crystals are best placed at forward angles, while angles
around 60◦ are ideal for the tracking detectors. For the angles given in Tab. 2, this resulted
in a resolution between 0.15–1.5 % (σ) and an efficiency of 9.4 %. A position reconstruction
resolution of 5 mm (FWHM), which has been verified in various experiments, was taken for the
tracking detectors. Obtained Doppler corrected energy spectra are displayed in Fig. 4, while theconfiguration is shown in Fig. 3.
Usage of thick solid target may impede the obtainable energy resolution significantly, as illustrated in Fig. 4 for a 79 Cu beam impinging on a 3 mm-thick Be target. Such cases will
greatly benefit from liquid hydrogen targets in combination with the time projection chambersurrounding MINOS. In 2020, a new compact Si tracker named Strasse should be available atthe RIBF [49]. It could be considered for experiments requiring the luminosity provided by athick H2 target with a significant gain in energy resolution.
It is noted that the employed configuration may vary from the one presented here, dependingon the experimental program. However, MINIBALL detectors are shorter than the tracking
detectors. Due to the limited space at F8 (1255 mm between the F8 standard focal point and
STQ17), MINIBALL detectors fit at forward angles.

3

Required equipment

The RIBF has ample experience with hosting Ge detector arrays, for example hosting EURICA
from 2012–2016 [50, 51]. Though, some infrastructure from the EURICA campaign exists andcan be used for the array, vital instrumentation is missing. Particularly, high voltage supply,
monitoring, UPS, and a reliable liquid nitrogen system are required as infrastructure.
A harmonized data acquisition system is indispensable for a smooth readout of the individual
systems. This electronic readout will be based on the readout implemented for GRETINA,
requiring a total of 568 readout channels due to the six-fold segmentation of a MINIBALL

12REFERENCES

Budget required for Array
Item
Travel
Pumping system
UPS
Liq. N2 Dewar
Shipping
Digitizer
VME crates
Computer Cluster

57,400 ( x 1k U )
Cost
2,000
1,600
2,000
3,500
2,000
10,0001,400
10,000Item
Maintenance Ge
HV Power
Liq. N2 system
Cooling pipes
Rail system
Trigger modules
IOC

Cost
2,000
5,000
3,000
400
4,000
2,500
4,000

Table 3: Estimated costs necessary to implement the described high resolution array at the
RIBF.
crystal and 36-fold segmentation of DAGATA, RCNP Quad, and LBNL P3 (40 per tracking
type crystal, 7 per MINIBALL crystal including core signals). The total readout costs (digitizer,
trigger module, VME crate, computing power, etc.) are estimated to be 120,000U per channel.
Some of the electronics are already available. However, readout electronics for DAGATA andMINIBALL array are necessary, but may be partially borrowed from the CAGRA project [52]. It
is estimated that electronic equipment for 200 readout channels will be needed to be purchased.
Other incurring costs include the shipment of the MINIBALL detectors and support structurefrom the University of Cologne, and setting up a rail system at F8. Furthermore, travel support
is necessary to setup the detectors at the RIBF. A summary of expected costs is provided in
Tab. 3.

4

Project time-line

In parallel to this construction proposal, the collaboration is submitting a Letter of Intent tohost MINIBALL at the RIBF in 2020. The next MINIBALL Steering Committee meeting will
be held by the end of 2018.
It is foreseen to hold a dedicated workshop in spring 2019, either at RIBF or in Europe, to discuss
the physics proposals and details of the setup. Applications for beam time with this proposed
array in future NP-PACs will be open to all RIBF users. The MINIBALL array will be available
for experiments at the RIBF in the calendar year 2020, while other detectors of the array may
be further available.

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