Vol. 49 (2018)

ACTA PHYSICA POLONICA B

No 3

THE BRIKEN PROJECT: EXTENSIVEMEASUREMENTS OF β-DELAYED NEUTRONEMITTERS FOR THE ASTROPHYSICAL r PROCESS∗
J.L. Taina , J. Agramunta , D.S. Ahnb , A. Algoraa,c , J.M. Allmondd
H. Babab , S. Baee , N.T. Brewerc , R. Caballero Folchf , F. Calvinog
P.J. Coleman-Smithh , G. Cortesg , T. Davinsoni , I. Dillmannf
C. Domingo-Pardoa , A. Estradej , N. Fukudab , S. Gok,b , C. Griffini
R. Grzywaczk , J. Hae , O. Halli , L. Harkness-Brennanl , T. Isobeb , D. Kahli
M. Karnym , G.G. Kissc,b , M. Kogimtzish , A. Korgulm , S. Kubonob
M. Labicheh , I. Lazarush , J. Leen , J. Liun , G. Lorussoo,b , K. Matsuib,pK. Miernikd,m , F. Montesq , B. Moonr , A.I. Moralesa , N. Nepalj
S. Nishimurab , R.D. Pagel , Z. Podolyaks , V.F.E. Pucknellh , B.C. Rascod
P.H. Regano,s , A. Riegog , B. Rubioa , K.P. Rykaczewskid , Y. Saitof
H. Sakuraib,p , Y. Shimizub , J. Simpsonh , P.A. Söderströmb , D.W. Stracenerd
T. Sumikamab , R. Surmant , H. Suzukib , M. Takechiu , H. Takedab
A. Tarifeno-Saldiviag , S.L. Thomasv , A. Tolosa-Delgadoa
V.H. Phongz , P. Woodsi
for the BRIKEN Collaborationa Instituto de Fisica Corpuscular (CSIC–Universitat de Valencia), 46980 Paterna, Spainb RIKEN Nishina Center, Wako, Saitama 351-0198, Japanc Inst. of Nuclear Research, Hungarian Academy of Sciences, Debrecen Pf. 51, 4001, Hungary
d Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USAe Dept. of Physics and Astronomy, Seoul National University, Seoul, 08826, Republic of Korea
f TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3, Canada
g Universitat Politecnica de Catalunya (UPC), Barcelona, Spainh STFC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UKi University of Edinburgh, Edinburgh, EH9 3FD, UK
j Central Michigan University, Mount Pleasant, MI 48859, USA
k Dept. of Physics and Astronomy, Univ. of Tennessee, Knoxville, TN 37996-1200, USAl University of Liverpool, Liverpool, L69 7ZE, UK
m Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warszawa, Polandn Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, Chinao National Physical Laboratory, Teddington, UK TW11 0LW, UKp Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japanq National Superconducting Cyclotron Lab., Michigan State Univ., East Lansing, MI 48824, USAr Department of Physics, Korea University, Seoul 136-701, Republic of Koreas Department of Physics, University of Surrey, Guildford GU2 7XH, UKt Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USAu Department of Physics, Niigata University, Niigata 950-2102, Japanv STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxfordshire, OX11 0QX, UKz Department of Nuclear Physics, Faculty of Physics, VNU University of Science, Hanoi, Vietnam(Received January 3, 2018)∗

Presented at the XXXV Mazurian Lakes Conference on Physics, Piaski, Poland,
September 3–9, 2017.

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An ambitious program to measure decay properties, primarily β-delayed
neutron emission probabilities and half-lives, for a significant number ofnuclei near or on the path of the rapid neutron capture process, has beenlaunched at the RIKEN Nishina Center. We give here an overview of thestatus of the project.
DOI:10.5506/APhysPolB.49.417

1. Introduction
Most of the nuclei formed during the initial stages of the rapid (r) neutron capture process will decay by β-delayed neutron (βn) emission. For
these nuclei, the separation energy of one or more neutrons in the daughter
nucleus Sxn is lower than the energy available for the decay Qβ , making theprocess energetically possible. The half-life T1/2 of the decay determines theinitial abundances of the nuclei along the reaction path. The decay of these
nuclei towards stability is altered by the neutron emission process, since eachneutron emitted reduces the final mass number A by one unit. In addition,
the injection of fresh neutrons to the system, at the moment that the neutron density is rapidly decreasing, can have a significant effect on the neutron
capture rate, increasing A by one unit per (n, γ) reaction [1, 2]. Thus, theprobability of x-neutron emission per decay Pxn shapes the final abundance
distributions. All of this underlines the importance of accurate decay datafor βn emitters in r process abundance calculations. However, obtaining
such data in the past has been hampered by the difficulty of accessing therelevant isotopes, far from the valley of β stability. In fact, reaction network
calculations of the r process rely heavily on theoretical estimates for T1/2
and Pxn as well as for other relevant quantities such as the capture crosssections σnγ and masses.
From the astrophysics point of view, in spite of the advances since thegeneral idea of the r process was laid out [3], there are still considerable uncertainties. It is not clear what are the exact conditions of the astrophysicalenvironment such as temperature, neutron density, entropy, rotation speed
and magnetic fields, that lead to a successful r process. Where such events
actually take place is also unknown. Two sites have been actively investigated: (1) core-collapse supernova events (CCSN) occurring at the endsof the lives of massive stars, and (2) the kilonova explosion ignited by themerging of two neutron stars or a black hole and a neutron star (NSM). The
relative contributions of the two to the evolution of the Universe and theobserved abundances of elements in stars is also unclear. The very recent
results of combined observations of gravitational wave, γ ray, X ray, UV,
optical, infrared and radio emission from a two-neutron star merger confirm
in a spectacular way that NSM are a major source of r process elements [4]The BRIKEN Project: Extensive Measurements of β-delayed Neutron . . .

419

and opens a new field for precision r process studies. In this situation, it
is fair to claim that a reduction in the uncertainty of the nuclear physics
input, in particular for values of Pxn and T1/2 , is a key to the progress ofinvestigations in this area.
With that idea in mind the Beta-delayed neutrons at RIKEN (BRIKEN)
Collaboration [5] was established with the aim of exploiting the currently
unsurpassed radioactive beam intensities by in-fight production available atthe Radioactive Isotope Beam Factory (RIBF) [6] and the powerful selection and identification capabilities of the in-flight separator BigRIPS, and
ZeroDegree spectrometer [7], by supplementing them with state-of-the-art
instrumentation that would allow us to study decay properties of very exotic
βn emitters.
2. Experimental setup
To determine neutron emission probabilities, we use a combination ofneutron counting and β counting, a widely used method. The BRIKEN
neutron counter is of the neutron moderation type, which uses a hydrogenous material, in our case high density polyethylene (HDPE), to bring down
the neutron energy from MeVs to meVs, and a detector that is very efficient for detecting thermal neutrons, in our case a 3 He filled proportionalcounter. The energy deposited by the exothermic reaction 3 He(n, p)3 H(Q = 0.76 MeV) induces a signal well-separated from the noise and witha relatively small background, providing a high degree of selectivity. With
this detection scheme, one can also reach high detection efficiencies by assembling a large number of tubes, a key feature for the study of weakly produced
isotopes. The fraction of initial neutrons effectively moderated and captured
depends on their initial energy and the geometry of the detector, resulting inan energy dependence of the neutron detection efficiency εn . This is an undesirable effect since the energy distribution is a priori unknown for most of
the nuclei of interest. Thus, the arrangement of tubes to achieve maximum
detection efficiency and minimum energy dependence is a complex problem
that is best tackled using Monte Carlo (MC) simulations.
The optimization of the BRIKEN neutron counter was carried out using a parametric approach [8] taking into account the tubes available to thecollaboration, coming from ORNL, UPC-Barcelona, GSI-Darmstadt, JINR
and RIKEN, with varying gas volumes and pressures. One of the designs
investigated includes γ-ray detectors, more specifically two CLOVER-type
HPGe detectors. This option reduces the maximum achievable efficiency
and flatness of the efficiency curve, but was the one chosen because of theenhanced research prospects provided by the significant gain in peak-tobackground ratio of β-neutron gated γ-ray spectroscopy [9]. The CLOVER
detectors are inserted perpendicular to the beam from opposite sides of the420

J.L. Tain et al.

εn [%]

PE moderator and achieve a γ-ray detection efficiency of about 3.5% for
1 MeV γ rays. The configuration finally installed includes 140 3 He tubes(see Fig. 1, left panel), becoming the largest detector of its kind ever built
for βn measurements, with a rather constant efficiency of around 68% up
to 1 MeV, dropping to 59% at 3 MeV (see Fig. 1, right panel), according
to MC simulations. Given the large energy window for βn emission in exotic neutron-rich nuclei, this variation can represent a source of systematic
uncertainty in Pxn values. However, we can use the number of neutrons detected as a function of distance to the source, which carries information on
the neutron energy as shown in the right panel of Fig. 1, to deduce de facto
the average detection efficiency for each decaying nucleus [10]. The simulated neutron detection efficiency was verified with a strong uncalibrated
252 Cf source using the neutron multiplicity counting method [11]. Due tothe close packing of 3 He tubes, the mean time for moderation and capture in 3 He is rather short, τ ∼ 30 µs. Thus, setting a time window forneutron detection of ∆tn = 200 µs ensures close to 100% detection efficiency. The signal from each tube, each HPGe crystal and other ancillarydetectors is digitized when it goes above a noise discrimination threshold(self-triggering), processed with trapezoidal filters and stored in list-mode
files as time-amplitude pairs, using the Gasific digital data acquisition system [12]. The resulting acquisition dead time is typically very small (lessthan 1%).

10

1: Total: Ring1: Ring2: Ring3: Ring4: Ring5: Ring6: Ring7

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

En [MeV]Fig. 1. (Color online) Left panel: Schematic lay-out of the 3 He tube distribution
inside the HDPE moderator around the square shaped AIDA hole. The color
indicates the tube type. Small medium-gray dots/green:
100 × 11.800 /5 atm —0000
RIKEN; small dark-gray dots/red:
1 × 23.6 /8 atm — UPC; small light-graydots/blue: 100 × 2400 /10 atm — ORNL; big gray dots/pink: 200 × 2400 /10 atm— ORNL. Tubes belonging to a single ring are connected with a line (Ring-1:
innermost, Ring-7: outermost). Right panel: Neutron detection efficiency as afunction of energy for each ring and the whole detector.

The BRIKEN Project: Extensive Measurements of β-delayed Neutron . . .

421

The β counter also acts as the ion implantation detector, namely the Advanced Implantation Detector Array (AIDA) [13]. A total of six Si DSSDs
spaced by 10 mm were installed in the AIDA nose, which is inserted longitudinally in the PE moderator. The stack of DSSDs is centered with respect
to the tubes and located between the two CLOVER detectors. Each DSSD
has 1 mm thickness and a size of 72 mm × 72 mm with 128 horizontal andvertical strips on each side. Each strip is read out independently and processed with both high-gain and low-gain electronic branches. This allows
one to measure precisely the low energy deposited by β signals shortly after
an implantation event which deposits a very high energy. A threshold is applied to each electronic channel to discriminate against the noise and reduce
data throughput. The threshold in the β branch ranges typically between
100 and 200 keV. The total data readout acquisition system stores the timeand signal amplitude of all strips that fired.
The identification of each implanted ion is provided by the BigRIPS
spectrometer. The ion velocity is measured by the time-of-flight between
thin plastic scintillators located conveniently along the particle trajectory.
The particle trajectory is reconstructed from the transverse position provided by two-dimensional parallel plate avalanche counters and combinedwith the magnetic field in the dipole magnets to obtain the particle’s magnetic rigidity. Combining velocity and magnetic rigidity, one obtains the
mass-over-charge ratio A/Q. The energy lost by the ion in dedicated ionization chambers provides its atomic charge Z. After identification of possible
charge states (ions which are not fully stripped of electrons), the two quantities together provide a unique identification of the ion in an event-by-event
basis.
The data from the three independent DACQ systems, BigRIPS, AIDA
and BRIKEN, have to be merged for the data analysis. The merging is doneon the basis of the respective time stamps which are synchronized thanks
to the use of a common clock signal distributed to all three systems. The
synchronization between them is constantly monitored during the measurement.
3. Experimental programme
Four measurement proposals have been approved until now. Altogether,
they span a very wide range of very neutron-rich isotopes from Co to Eu(see Fig. 2). They cover the region of the 1st r process peak, Z = 27–35
and A = 75–95 [14], the region of neutron-rich deformed nuclei, Z = 35–48
and A = 95–130 [15], the region of the 2nd r process peak, Z = 45–55 andA = 120–150 [16], and the region of deformed nuclei leading to the rareearth abundance peak (REP), Z = 55–63 and A = 150–167 [17]. The goal
is to measure in total 135 new P1n values, 85 new P2n values and 40 new
T1/2 values for nuclei relevant to r process calculations.

422

J.L. Tain et al.

90

60

80

Z

50

55
50

70

50

Pn [%]

Z

100

65

60
50

45

40

40

30
35

20

82

30

28

10

25
50

60

70

80

90

N

100

N

110

0

Fig. 2. (Color online) A plot of part of the nuclide chart showing calculated Pnvalues from Ref. [18] (color code on the right). Black triangles indicate isotopes
where experimental information exists. Black squares indicate stable nuclei. Greysquares indicate waiting-point nuclei for a hot r process trajectory. The region of
the chart covered by the four accepted BRIKEN proposals is roughly indicated byellipses.

The physics reach of the expected results extends beyond the astrophysical interest. Half-lives and Pxn values give information on the β-strength
distribution Sβ (Ex ) which carries information on the nuclear structure. T1/2
is mainly sensitive to the low excitation energy part of Sβ (Ex ), while Pxnvalues measure the fraction of Sβ (Ex ) above Sxn . The combined information
provides constraints on the theoretical models calculating Sβ (Ex ), thus improving their predictive power for nuclei as yet unmeasured. Alternatively,
these two quantities together can provide information about the nuclear shellstructure [19] or about the deformation of the nucleus [20, 21]. Additional
nuclear structure information can come from the precise location of excitedstates and their βn feeding intensity obtained with the CLOVER γ-ray detectors. The background reduction provided by neutron gating [22, 23] will
push the limit of such studies to more exotic nuclei.
The process of βn emission itself is not fully understood. For example,
the competition between γ and neutron emission from neutron unboundstates populated in β-decay has just started to be studied in a more systematic way [24–28]. The competition between β-delayed one-neutron andmultiple-neutron emission channels has not been properly addressed [29],the main reason being the scarcity of known βxn emitters. Currently, only
twenty three β2n, four β3n and one β4n emitters have been measured. As
mentioned above, we will substantially increase the database for β2n emitters providing the ground for an extensive systematic study. It is also un-

The BRIKEN Project: Extensive Measurements of β-delayed Neutron . . .

423

clear whether the multiple emission process occurs sequentially, or throughthe emission of a loosely bound neutron cluster, or as direct breakup into
the continuum [30]. Some insight into this problem may be obtained by
the measurement of angle-energy correlations between emitted neutrons, as
has been done in the case of β-delayed proton emission [31]. Because of
the neutron moderation process, the BRIKEN neutron counter can provide
such information only with limitations. However, given its large neutron
detection efficiency, it is worth to explore this possibility.
4. First measurements
The commissioning of the experimental setup took place in November
2016 parasitic to an experiment of M. Takechi and collaborators. The primary beam was 238 U at an energy of 345 MeV/u. The BigRIPS setting forthe secondary radioactive beam was centered on 76 Ni. In about 10 hoursof measurement, sufficient statistics were accumulated for 15 isotopes of Ni,
Cu, Zn and Ga which are β-delayed one-neutron emitters. This includes 475
78 Ni events, enough to determine its P value by direct neutron counting forn
the first time.
We observed a very large beam induced background in the neutron detector which reached a value of 250 cps (counts per second), to be compared with 0.4 cps from ambient background. This limits the sensitivityof our setup to β-delayed neutrons. A similar issue was found during the
BELEN-30 experiment at the GSI Fragment Separator in 2011 [32]. There
the problem was solved, for isotopes with low statistics and sufficient long
half-life, using in the analysis only the decay data coming between beam
spills. This is not possible at BigRIPS given the continuum time structureof the RIBF beams. However, we found that by vetoing neutrons coming
within ∆tn (200 µs) of the passage of ions through the plastic scintillator
at the end of the spectrometer, we effectively reduced the neutron detector
background. Reduction factors of about 4 for the one-neutron backgroundand about 15 for the two-neutron background were achieved. This procedure introduces a few percent beam-rate-dependent reduction on neutron
statistics in the analysis.
In order to obtain the P1n value we build for each identified implant
the histogram of β-implant time differences, tβ − timplant , both without any
condition and with the condition that only one neutron comes within ∆tnafter the β (see Fig. 3). These spectra are corrected for uncorrelated andcorrelated background and fitted with a function describing the evolution of
the decay activity of parent and descendant nuclei for all decay chains. The
fit function has the form of424

J.L. Tain et al.
33

countscounts

×10

82

Ga β

35

2.5 ×10822

Ga β1n

1.5
30

25

1
0.5
0

2

4

time [s]

6

0

2

4

time [s]6Fig. 3. (Color online) Time distribution of β signals with respect to implanted82
Ga ions for the commissioning run. Left panel: without conditions. Right panel:
neutron-gated. The uncorrelated background (in green) and the different decay
components in the fit are shown (blue: sum of all contributions, black: 82 Ga, red:82
Ge, pink: 81 Ge).

f (t) =Xi

ε̄βi λi Ni (t) +Xj
ε̄βj ε̄nj P1n
λj Nj (t) ,(1)jwhere index i runs over all β decays, index j runs over all βn decays andλ = ln(2)/T1/2 . The bar symbol above ε emphasizes that these efficiencies
are average quantities, weighted with the corresponding β-intensity distribution. The number of decaying nuclei at each moment Nk (t) is obtained
from appropriate solutions of the Bateman equations [33] which take theform ofk−1
kYXe−λi t,(2)
Nk (t) = N1
(bi,i+1 λi ) ×
Qk
j=16=i (λj − λi )i=1
i=1where N1 is the number of implanted parent ions and the branching ratioi or 1 − P ibi,i+1 at the branching point i in the decay chain is either P1n
1n
depending on the branch. All half-lives and descendant P1n values are kept
fixed, thus a simultaneous fit of both histograms provides directly the neutron emission probability of the parent. The systematic uncertainty due touncertainties on the T1/2 and Pn values used is derived from the distributionof results obtained by repeated fitting after varying the parameters within
uncertainties.
In the preliminary analysis, we assume that the neutron efficiency ε̄n isisotope-independent. The common assumption that the efficiency ε̄β is also
independent of the decay requires the use of very low β thresholds [12]. Thisis a challenging task in a complex detector such as AIDA. The effect wasverified by comparing the P1n values obtained for different ways of reconstructing a β event with different nominal thresholds and sorting conditions.
Relative variations of up to 7% were observed when the β efficiency increases

The BRIKEN Project: Extensive Measurements of β-delayed Neutron . . .

425from about 25% to about 40%. The preliminary results so far obtained show
a good agreement with previous literature values [34] for 76 Cu, 77 Cu, 79 Zn,
80 Zn and 82 Ga. Significant differences are observed for 78 Cu, 81 Zn and 82 Zn,
and more precise values are obtained for 79 Cu and 83 Ga. We obtain for the
first time Pn values for 76 Ni, 77 Ni, 78 Ni, and 80 Cu. Altogether, these results
demonstrate the excellent performance of the setup.
In May 2017, the first BRIKEN experiment took place. The measurements in the region around 78 Ni and for the formation of the 2nd r process peak at A = 130–140 were completed. A partial measurement for theheaviest elements relevant to the formation of the rare-earth peak was also
performed. All three experiments used a 238 U beam at 345 MeV/u on a Be
target, and the primary beam intensity reached 50 pnA.
As an example of the success of the experiment, we highlight some results from the 78 Ni region measurement. Figure 4 displays the combined
identification plot of implanted ions for the two BigRIPS settings used. The
figure indicates the current limits of measured T1/2 and Pn values. In about
95 hours of measurement 6.5 million ions of 86 Ge were implanted, a totalof 7500 implants were identified as 78 Ni ions, and more than 800 ions werecollected for the very exotic 92 As.
94

Z

Br

91

94

Se

34
33

88

32

85

31

95

Br

35

Se
92

As

As
90

Ge

Ge

82

87

Ga

Ga

80

85

Cu

82

Zn

30

Zn

77

29

Cu

76

28

79

Ni

Ni
Co

76

75

Co

27
2.6

2.65

2.7

2.75

2.8

2.85

A/Q
Fig. 4. (Color online) Preliminary particle identification plot (atomic number versus
mass-over-charge ratio) for the 78 Ni region run. The doubly-magic 78 Ni is encircled
in dotted black/red. The continuum line represents the limit of measured T1/2 . The
dot-dashed line is the limit of measured Pn .

A number of potential β2n emitters were measured for the first time.
In order to obtain the P2n value, another spectrum is added to the simultaneous fit. This histogram stores β-implant time differences when two,and only two, neutrons come within ∆tn after the beta. The histogram
is corrected for correlated and uncorrelated backgrounds. In addition, the426

J.L. Tain et al.

countscontribution of two-neutron emission to the one-neutron spectrum, appearing when one of the neutrons escapes detection, is also taken into account.
Theterm added to the fit function of Eq. (1) takes the form
Padditionalkk2
kof k ε̄β (ε̄n ) P2n λNk (t). As an example, Fig. 5 shows the background corrected spectra of β-implant time differences for one-neutron and two-neutron
detection corresponding to the decay of 91 As (3.2×103 implanted ions) showing the sensitivity of our setup to β2n emission. The analysis of the full setof data is in progress.
103
91

As:
: β1n
: β2n

2

10

10

1
10-1

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

time [s]Fig. 5. Time distribution of β signals with respect to implanted 91 As ions during
the 78 Ni region run gated on one-neutron detection (light gray) and two-neutron
detection (dark gray).

5. Conclusions and outlook
The BRIKEN setup has been commissioned showing an excellent performance and the first experiments have been performed successfully. The
next BRIKEN runs will take place in October–November 2017. During thisperiod, the experiment to study the region of neutron-rich deformed nuclei
with A = 95–130 will be performed. A second experiment will mainly focus
on neutron-gated γ-ray spectroscopy of 82 Cu decay to study neutron singleparticle levels in the 78 Ni region. The measurements to study the formationof the rare-earth peak will be completed in 2018.
Additional experiments are planned in the near future. Three new proposals have been presented. One aims to study the light mass neutronrich region Z = 9–16, A = 29–48 that includes a number of potential β3n
and β4n emitters. The second one concentrates on the medium-light nuclei
Z = 17–25, A = 47–67 around the shell gaps at N = 34 and N = 40. The
third challenging proposal aims at the heavy region Z = 72–83, A = 190–224
of importance for the formation of the 3rd r process peak, whose feasibility
at BigRIPS will be investigated.

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427

When completed, this experimental programme will have a major impact
on the quantity and quality of decay data for β-delayed neutron emitters,
addressing a significant number of key topics in nuclear astrophysics andnuclear structure.
This work has been supported by the Spanish MINECO under grants
FPA2011-24553, FPA2011-28770-C03-03, FPA2014-52823-C2-1/2, SEV2014-0398; by the National Measurement System Programmes Unit of BEIS
(UK); by STFC (UK) grant ST/L005743/1; by FP7/EURATOM contract
No. 605203; by JSPS KAKENHI grant No. 17H06090; by NSERC grants
SAPIN-2014-00028 and RGPAS 462257-2014 at TRIUMF; by the U.S. DOE
grant DE-AC05-00OR22725; by the U.S. NSF grant PHY-1714153; by the
National Science Centre, Poland (NCN) UMO-2015/18/E/ST2/002. Workpartially done within IAEA-CRP for Beta Delayed Neutron Data.
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