Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 209–211, 2002

209

The Present and the Future of JAERI Recoil Mass Separator
S. Mitsuoka,∗ H. Ikezoe, K. Nishio, K. Satou, and L. Dan
Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Received: February 1, 2002; In Final Form: April 5, 2002
A recoil mass separator has been in operation at the JAERI tandem-booster facility since 1995, for in-flight separation of low-yield reaction products recoiling out in the beam direction. The current status and future plan together
with a brief description of the design performance are presented.

1. Introduction
The JAERI Recoil Mass Separator (JAERI-RMS) was constructed at the tandem-booster facility at the end of March in
1995. Immediately after the performance test,1, 2 the RMS was
utilized for various experiments where extremely low-yield reaction products of sub-nb cross sections were separated in flightfrom the high flux of the primary beam and other reaction products. At an early stage of the experiments, the RMS was usedto separate new neutron deficient isotopes in order to study thedecay properties of short-lived α-emitting isotopes near the proton drip line in the light actinide region.3, 4 For the next threeyears, the excitation functions of evaporation residue cross sections were measured in order to study the reaction mechanismof sub-barrier fusion of well-deformed nuclei.5, 6 In such experiments, it is important to collect rare fusion products with hightransport efficiency in wide charge distribution while keeping
low background. For this purpose, the RMS will be improvedin this year by installing a new quadrupole magnet. And recently, for another type of research in nuclear astrophysics, a test
experiment has been started to separate low-energy radioactive
nuclear beam (RNB) by using the RMS.7 In future, more intense RNB will be supplied to the RMS beam line from a newly
planned upgrade tandem-booster facility.8
In the next section, the ion optical design and performanceof the RMS is described. Short descriptions of the experiments
which have been or are being carried out by using the RMS ispresented in sect. 3. The future plan is briefly introduced insect. 4.
2. Ion Optical Design and Performance
The schematic layout of the RMS is shown in Figure 1 andthe ion optical properties are listed in Table 1. The RMS has asymmetric configuration of Q1 Q2 -ED1 -MD-ED2 -Q3 Q4 -O. Two25◦ electric dipoles (ED1 and ED2 ) and one −50◦ magnetic
dipole (MD) disperse reaction products by their mass/charge(m/q) ratio and cancel out the energy/charge dispersion. The
design value of m/q dispersion is 0 ∼ 1.5 cm/% (variable). Twoquadrupole doublets (Q1 Q2 and Q3 Q4 ) allow to focus the recoils
irrespective of their angular spread and also to change the m/q
dispersion at the focal planes. The octupole magnet (O) is forthe correction of a non-linearity of the m/q dispersion. Theseelectric and magnetic components are arranged on a rotating
platform (−5◦ to 40◦ ), 9.4 m in length.
The RMS was designed especially to reduce a backgroundoriginating from the scattered beams from the anode surface of
the ED1 .1 For this purpose, the ED1 anode is vertically split into
two parts separated by 1 cm, so that the primary beam can passthrough this gap without hitting the anode surface. The slit anode also allow to monitor a beam current during the experiment
by a Faraday cup located behind the ED1 anode.
The performance of the RMS was tested,1 by using α particles∗Corresponding author. E-mail: mitsuoka@nucleus3.tokai.jaeri.go.
jp. FAX: +81-29-282-5927.

from an 241 Am source and beams of 127 I and 28 Si from the tandem accelerator. A nat Ag target was bombarded with 127 I beamof 120 MeV and the recoiling Ag particles at θ = 20◦ were led
to the RMS. The particles passing through the RMS were detected by a position sensitive silicon detector (45 mm × 8 mm)
at the focal plane. It was confirmed that the mass 107 and109 of Ag isotopes with the same charge state 25+ were separated with no background. Fusion evaporation residues werealso separated at θ = 0◦ (the beam direction) in the reactions of28
Si + 59 Co,89 Y,nat Ag,nat In at 130 and 150 MeV. The mass resolution was obtained to be A/∆A  300, which is nearly equal tothe calculated value. The capabilities of the background suppression were 3 ×1010 for 59 Co, 4.5 ×1010 for 89 Y, 3 ×1011 fornatAg, and 2 ×1012 for nat In targets. This means that the split anode adopted in the RMS is really effective on the reduction ofbeam background.
The solid angle and transport efficiency of the RMS weremeasured,2 by using elastic recoils of 197 Au in 28 Si + 197 Au andalso 5.486 MeV α from a source. The obtained values of solid
angle were 11 and 21 msr for the focusing conditions with massdispersion of 1.1 and 0 cm/%, respectively. The energy acceptance was measured by changing the energy setting of the RMS
in small steps after selecting a charge state, and the mass acceptance was measured by changing the charge state with a constantenergy. Good agreements between the data and the ion-optical
calculation were obtained.
The absolute values of transport efficiency were measured2 in
30
Si + 141 Pr reaction by detecting γ rays of long-lived evaporation
residues of 167 Lu and 167 Yb with the charge state q = 18+ , whichwere implanted into a catcher foil placed just behind the target
or at the focal plane of the RMS. The charge-state distributions
were also measured and compared with available phenomenological formulas. The absolute transport efficiency multipliedby the charge fraction was consistent with the calculated result.
3. Experiments
At an early stage of experiments using the RMS, new neutron
deficient isotopes 209 Th (Ref. 3) and 212 Pa (Ref. 4) were produced via hot fusion reactions of 182 W(32 S, 5n) at 171 MeV and182
W(35 Cl, 5n) at 183 MeV, respectively. The fusion evaporation
residues separated by the RMS were implanted into a doublesided position sensitive silicon detector (73 mm × 55 mm). Theenergies and two-dimensional positions of the implanted evaporation residues and those of the α particles in their subsequentα-decay chains were registered together with the times associated with the detection. The observed short-lived activities wereidentified on the basis of time- and position-correlated α-decaychains. The α-decay energies and half-lives were determined to209
Th, and 8.270(30) MeV
be 8.080(50) MeV and 3.8+6.9−1.5 ms for+6.1
212
and 5.1−1.9 ms for Pa. The production cross sections were1 nb for 209 Th and 0.5 nb for 212 Pa, when the RMS transport
efficiency of 19% was assumed. In the same manner, we haveobserved (i) a new spontaneous fission decay of 264 Sg producedin 238 U(30 Si, 4n) at 158 MeV,9 (ii) a new isomeric state in 217 Pac 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

210 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Mitsuoka

MDED1

ED2

50°

25°

25°

40°

Q3 Q4 O

Q1Q2
Beam
TargetFaraday cup

Detector

5°

Rotating platform

01mFigure 1. Schematic drawing of the JAERI-RMS.

TABLE 1:The dimensions and ion-optical properties of theJAERI-RMS.

Energy acceptance±12%±4%Mass acceptance (m/q)
Mass resolution
300
Mass dispersion
1.5 cm/%Solid angle
21 msrMx magnification
1.66
My magnification
1.17−5◦ ∼ 40◦
Platform rotation angle
Overall length
9.4 m
DipolesMD
Radius of curvature
1m
Deflection angle
50◦
Radius of EFB∗
3.9 m
16◦
EFB∗ shim angle
Gap12 cm
Maximum rigidity
10 kGmQuadrupoles and Octupole
Q1
Q2
Bore diameter / cm
1010
Effective length / cm
30
20
Maximum field gradient1.3 1.3/ kG·cm−1 for Q, kG·cm−3 for O∗
Effective field boundary(variable)(variable)(variable)(variable)

ED1 and ED24m25◦
0◦
12 cm
20 MV
Q3Q4151830
400.8 0.8O
15
157.1

produced in Pt( Si, 5n) at 163 MeV, and (iii) α decay ofTh populating the low-lying two excited states in 213 Ra in
28
Si + 198 Pt reaction.11194

28

10217

For the last three years, we have investigated the sub-barrier
fusion between heavy projectiles and well-deformed targets
in the reactions of 60 Ni + 154 Sm and 32 S + 182 W (Ref. 5), and76
Ge + 150 Nd and 28 Si + 198 Pt (Ref. 6), where the same compound
nucleus 214 Th and 226 U are formed, respectively. This is aimed
to study an appealing speculation of gentle fusion12 or hugging
fusion,13 which were theoretically proposed as a novel way tosynthesize the superheavy elements. It was found that the fusion
probability depends strongly on the orientation of nuclear deformation. The measured excitation functions showed the large fusion hindrance in the low energy region where only the collisionat the tip of the deformed target is possible, while no fusion hindrance was observed at higher energies where the side collisions
become possible. This supports the original idea of hugging
fusion that the compact configuration at side collision is morefavorable for the complete fusion than the elongated configuration at tip collision. Moreover, we have studied (i) the effect ofnuclear deformation in 76 Ge + 150 Nd reaction by directly comparing with the case of spherical projectile and target combina-tion in 82 Se + nat Ce reaction,14 (ii) the effect of neutron numberin 60,64 Ni + 154 Sm reactions to see how the n-rich projectile actson the fusion probability,15 and (iii) the effect of nuclear shellstructure in 82 Se + 134,138 Ba reactions.16 The details of these experimental results are shown by Ikezoe et al. elsewhere in thisproceedings.
Recently, as another type of experiments by using the RMS,
we started direct-exclusive measurements in astrophysical reactions like (α, n) or (α, p) with radioactive nuclei of 8 Li, 10 Be,
12
B, 14,15 C, or 16,17 N (Ref. 7). Especially, 8 Li(α, n) is the key
reaction for nucleosynthesis reaction network of r-process inSupernovae.7 Radioactive nuclear beams (RNBs) are produced
via nuclear transfer reaction, for example, (d, p) reaction by irradiating deuteron gas target with heavy-ion beams from Tandem accelerator, and then separated from the primary beamby the RMS. Low-energy RNBs are implanted into a multiplesampling and tracking proportional chamber (MSTPC),17 which
is filled with helium gas working as both the detector gas andthe target for the secondary reaction. The neutron TOF is measured by a neutron detector array surrounding the MSTPC. The
analysis of the first experiment in α(16 N,19 F)n is in progress.
4. The Future Plan
We have two plans (i) to improve the transport efficiency of
the RMS for wide charge distribution and (ii) to supply more intense RNBs as well as intense stable nuclear beams to the RMS
beam line.
As for the plan (i), a new quadrupole singlet will be installed,in this year, between the magnetic dipole (MD) and the second
electric dipole (ED2 ). In general use of the RMS, fusion products of interest with another charge states can not pass the gap ofED2 , due to the mass/charge (m/q) dispersion produced by MD.
The new Q magnet bends horizontally the m/q dispersed particles to the central axis. It is estimated that evaporation residueof Sg (Z = 106) produced in Kr + Yb reaction, for example, withfive charge states can be transported to the focal plane of theRMS.
As for the plan (ii), JAERI and KEK (High Energy Accelerator Research Organization) has jointly proposed a new RNB
project to produce RNBs (0.1 ∼ 8 MeV/u) with high quality andsufficient intensity for studying nuclear physics, nuclear astrophysics, material science, and so on.8 This joint project has been
started since this year FY2001 to construct an ISOL-based RNB
facility at JAERI by utilizing the existing accelerators of both theJAERI tandem-booster and the KEK-Tanashi RNB linac complex. The RNB with energy up to 1 MeV/u will be available
from FY2004. After the next two years, a pre-booster linac will

The Present and the Future of JAERI Recoil Mass Separator
be installed to increase the RNB energy up to 8 MeV/u by theexisting superconducting booster linac. Intense stable nuclear
beams will be also available from the linac complex independent of the tandem for studying, for example, SHE synthesis byusing the RMS.
Acknowledgement. The authors would like to acknowledge
the accelerator crew of JAERI tandem facility.
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