HELIOS and the radioactive beam program at Argonne

Proc. 25th Winter Workshop on 

Nuclear Dynamics (2009) 000–000 

25th Winter Workshop 

on Nuclear Dynamics 

Big Sky, Montana, USA 

February 1–8, 2009 

HELIOS and the radioactive beam program at Argonne 

B. B. Back for the HELIOS collaboration - 

N. Antler 1 , B.B. Back 1 , S. Baker 1 , J.A. Clark 1 , C.M. Deibel 1,2 , 

B.J. DiGiovine 1 , S.J. Freeman 4 , N.J. Goodman 3 , Z. Grelewicz 1 , 

S. Heimsath 1 , B.P. Kay 1 , H.Y. Lee 1 , C.J. Lister 1 , S.T. Marley 3,1 , 

P. Müller 1 , R. Pardo 1 , K.E. Rehm 1 , J. Rohrer 1 , J.P. Schiffer 1 , 

J. Snyder 3 , M. Syrion 1 , J.C. Lighthall 3,1 , A. Vann 1 , 

J.R. Winkelbauer 3,1 , A. Woodard 1 , A.H. Wuosmaa 3 

1 Argonne National Laboratory, Argonne, IL 60439, USA 

2 Joint Institute for Nuclear Astrophysics, Michigan State University, East 

Lansing, MI 48824, USA 

3 Western Michigan University, Kalamazoo, MI 40908, USA 

4 University of Manchester, Manchester M13 9PL, UK 

Abstract. The radioactive beam program at the Argonne Tandem Linac Accelerator 

System (ATLAS) has been based chiefly on beams produced in flight 

using light-ion reactions such a (p,n), (d,n), (d,p), (d, 3 He), and ( 3 He,n). In the 

fall of 2009 a new source of radioactive ions will become available with the Californium 

Rare Isotope Breeder Upgrade (CARIBU), which will provide exotic 

neutron-rich beams of 252 Cf fission fragments. In order to take advantage of 

these beams, a new spectrometer, the Helical Orbit Spectrometer (HELIOS), 

has been developed for the study of light-ion transfer reactions in inverse kinematics. 

The properties of both installations will be discussed. 

Keywords: Spectrometer, Inverse kinematics, radioactive beams, Neutron-rich 

beams 

PACS: 25.60.Je, 25.60.Lg, 29.30.Aj, 29.30.Ep, 29.38.Gj 

1. Introduction 

The radioactive beam program at ATLAS dates back to the early nineties with 

a study of 18 F(p,α) 15 O and 18 F(p,γ) 19 Ne reactions using a 18 F beam (t 1/2 =110 

min). The 18 F material was produced at the University of Wisconsin cyclotron in

2 B.B.Back et al. 

Madison, Wisconsin via the 18 O(p,n) reaction, chemically extracted and transported 

to Argonne in a small plane and inserted into the ATLAS ion source for acceleration. 

This two-accelerator method has been used only for a few, sufficiently long-lived 

isotopes, such as 18 F [1], 56 Ni [2], and 44 Ti [3]. Subsequently an in-flight production 

method has been used to produce a large number of radioactive beams by interacting 

stable beams with p, d, and 3 He in a small, cooled gas target and transporting the 

reaction products from few nucleon transfer reactions to the experimental station 

for studies of reactions, mostly of astrophysical interest [4]. The latest scheme for 

obtaining radioactive ions for nuclear structure and astrophysics studies involves 

the capture of fission fragments from a strong spontaneous fissioning 252 Cf source 

in CARIBU, as described in the following section. In order to take full advantage of 

both these and the in-flight beams, a new spectrometer, HELIOS, has been built, 

which allows for optimal use of these beams in inverse kinematics studies of few 

nucleon transfer reactions. 

Transport 

cask 

Shielding 

Gas cell 

RFQ 

coolers 

Quadrupoles 

252 Cf source 

High resolution 

mass spectrometer 

High voltage 

platform 

Low energy beam 

Switching 

magnet 

To ECR charge breeder 

Fig. 1. Schematic view of the arrangement on the CARIBU high-voltage platform 

(V < 200 KV). The individual components are labeled in the figure. The 252 Cf 

source is shown in two positions and the shielding is shown in its open position 

allowing for the source transfer. 

2. CARIBU 

The CARIBU injector (see Fig. 1 and Ref. [5]), which will start operation in the fall 

of 2009, is based on a strong, up to 1 Ci, 252 Cf source, which emits about 10 9 fission 

fragments per second of very neutron-rich nuclei into the forward hemisphere, where 

they are stopped in a high-efficiency gas cell and extracted as 1 + ions. The ions 

are subsequently cooled in an RFQ cooler and mass selected in a m/∆m=20,000

HELIOS ... 3 

isobar mass separator. Singly-charged ions may be transported into a low-energy 

beam-line for study of nuclear properties such as precise measurements of masses in 

the Canadian Penning Trap (CPT), nuclear moments and charge distributions in a 

future co-linear laser setup, or decay properties using a tape transport system with 

γ,β detection of short-lived activities. Alternatively the singly-charged ion beam 

may be injected into a modified ECR source for charge breeding and extracted as 

highly charged ions for further acceleration in ATLAS. These high-quality exotic 

beams may therefore be utilized in all the ATLAS experimental stations. 

3. Why inverse kinematics? 

Much of the nuclear structure information available today has been obtained by 

light-ion reactions on stationary nuclear targets performed in so-called normal kinematics. 

Examples are studies of the single-particle strength in nucleon stripping 

(d,p), ( 3 He,d) or pick-up (p,d), (d, 3 He), ( 3 He,α) reactions, which have been used to 

explore the single particle/hole strength and spectroscopic factors in practically all 

nuclear species that can be reached with stable or long-lived targets. This detailed 

information is, however, by nature concentrated along the line of β-stability because 

of the target limitations. 

Fig. 2. Left panel: Illustration of inverse and normal kinematics for a (d,p) 

reaction with a heavy nucleus. Right panel: The laboratory energy of protons 

emitted from the 28 Si(d,p) reaction in either normal (gently sloping curves) or 

inverse (strongly sloping curves) kinematics is shown as a function of laboratory 

angle, θ lab. The corresponding range of center-of-mass angles, θ cm, is indicated. 

In recent years, transfer reaction studies have been extended to radioactive 

species by using inverse kinematics, i.e. with heavy radioactive beams impinging 

onto light solid targets containing deuterons or protons, or gas targets of Helium 

or Hydrogen isotopes. With the prospect of more precision radioactive beams becoming 

available in the near future it is anticipated that such transfer reactions


performed in inverse kinematics will play a central role in advancing our understanding 

of nuclei far from stability; first with relatively weak beams from HRIBF 

at ORNL, ISAC-2 at TRIUMF, Rex-Isolde at CERN, ReA-3 at NSCL, Spiral-2 at 

GANIL and CARIBU at ATLAS, but on a longer time scale abundant beams from 

FRIB will substantially expand the range of nuclear transfer reaction studies. 

The inverse kinematics method carries with it, however, substantial obstacles for 

obtaining sufficient energy resolution to be able to separate adjacent nuclear states 

when the ejectiles are measured as a function of emission angle. In many cases, the 

forward angles in the center-of-mass system correspond to very backward scattering 

angles in the laboratory, where kinematical compression of the nuclear states into 

a small energy interval can be quite severe as illustrated in Fig. 2. This effect 

limits the achievable Q-value resolution and the ability to identify particles using 

the standard ∆E-E technique. At first glance, these experimental limitations would 

appear to severely limit the utility of transfer reaction studies using radioactive 

beams. As explained in the following section, these problems can be overcome by 

employing a new concept in which the ejectiles are transported back to the beam 

axis in a homogeneous magnetic field aligned with the beam axis, a method that is 

realized in the recently commissioned HELIOS spectrometer at ATLAS. 

Fig. 3. Schematic illustration of the HELIOS concept. From the left, the beam 

enters the homogeneous, axial field region through the hollow Si detector array. 

Backward-going charged particles follow helical trajectories and are intercepted by 

the position-sensitive Si detector before returning to the beam axis. Coincidences 

with recoils detected in a forward detector may be used, if needed. 

4. The HELIOS concept 

The basic principle of this spectrometer [6] is that in a sufficiently strong, homogeneous, 

longitudinal magnetic field B, charged particles with mass m and charge 

q, emerging from the beam-target interaction point, will follow helical trajectories


and return to the beam axis after the cyclotron period 

t cyc = 2π m 

B qe , (1) 

where e is the unit charge. By placing a hollow, position-sensitive Si-detector array 

(see Fig. 3) to intercept the particles before they reach the beam axis, their energy, 

E, flight-time, t, and position, z, can be measured. 

These measured quantities determine completely the kinematic parameters of 

2-body reactions. The flight-time provides particle identification via the cyclotron 

period, Eq. 1, and the center-of-mass energy is given by 

E cm = E + m 2 V cm 2 − mV cmz 

, (2) 

t cyc 

where V cm and z are the velocity of the center-of-mass system, and the intercept 

position, respectively. V cm and z, along with the measured energy, E lab , determine 

the center-of-mass scattering angle, θ cm . The fact that the energy in the centerof-mass 

system equals the measured laboratory energy plus a constant offset, when 

measured at a fixed axial position, z, illustrates the second important property 

of HELIOS, namely that the kinematic compression is eliminated. This property 

allows for improved Q-value resolution in inverse kinematics measurements, which 

use heavy radioactive beams impinging on light H and He targets. Finally, the fact 

that particles return to the beam axis means that only a relatively small area needs 

to be covered with position sensitive Si detectors. A large acceptance can therefore 

be obtained at a moderate cost and complexity of the detection system. 

5. Building HELIOS 

The large, homogeneous magnetic field volume needed for the HELIOS spectrometer 

is provided by a 3 T superconducting solenoid obtained from a de-commissioned 

Magnetic Resonance Imaging magnet. The solenoid is installed in the former general 

purpose area at ATLAS such that the magnetic field axis coincides with the beam 

line. The 90 cm diameter warm bore of the magnet is enclosed by large end-flanges, 

which provides a vacuum enclosure that serves as the target chamber. The beam 

enters via a 1 cm diameter hole through the Si detector array support rod, which 

is clad on the outside with twenty-four 1 x 5 cm 2 position sensitive Si detectors. It 

is planned to upgrade the Si detector array to forty 2 x 5 cm 2 wafers in order to 

achieve almost 100% azimuthal acceptance over a length of >60 cm. The position of 

a particle impact is obtained by charge division in the resistive front electrode of the 

detectors. A fan-like structure supports up to nine targets, which can be selected 

by rotation of the mounting rail located at the bottom of the magnet bore. The 

position of the target fan and the Si detector array can independently be adjusted 

longitudinally. Combined with magnetic field settings up to a maximum of 3 T, this 

allows for optimal coverage of excitation energy and scattering angle of interest. A 

three-dimensional engineering drawing cut-through of the setup is shown in Fig. 4.


Fig. 4. Cut-through of the HELIOS spectrometer showing the end-flanges, the 

angle-adjustable supports for the Si-detector array, and the target-fan mounted 

off the rotatable rail at the bottom of the magnet bore. 

6. First results 

The spectrometer was commissioned in August 2008 using a stable 28 Si beam incident 

upon an 84 µg/cm 2 CD 2 target to measure single neutron states in 28 Si using 

the d( 28 Si,p) reaction. The magnetic field was 1.915 T. Fig. 5 shows a 2-dimensional 

spectrum of the proton energy, E p , versus the longitudinal position z p (the distance 

from the target) for several longitudinal target positions. The top diagonal trace 

corresponds to the ground state of 29 Si. Several excited states are visible at lower 

energies. Note that these traces are parallel, as expected on the basis of Eq. 2. 

The horizontal lines correspond to α contamination from a 228 Th source used for 

an energy calibration of the detectors at the beginning of the experiment. Fig. 6 

shows the excitation spectrum of 29 Si integrated over a wide range of angles. The 

energy resolution is clearly demonstrated by the fact that the E x =6.19 and 6.38 

MeV states are well separated. 

This commissioning experiment clearly validates the basic operating principle of 

the HELIOS spectrometer. The preservation of good energy resolution for inverse 

kinematic reactions will be of central importance for exploring charged particle


Fig. 5. The energy of protons, E p, is plotted vs. z p for the d( 28 Si,p) reaction. The 

diagonal traces represent states in 29 Si, whereas horizontal lines arise from a shortlived 

α-contamination in the chamber. Horizontal gaps of about 1 cm between 

the detectors are evident, and a proton background from fusion-evaporation on 

the carbon in the CD 2 target is seen. 

reactions with heavy radioactive beams, such as those becoming available from the 

CARIBU injector at ATLAS in the near future. 

7. Conclusions 

In this paper we have given a historical perspective on the radioactive beam program 

at ATLAS. This work has been ongoing for over a decade, starting with beams 

obtained from relatively long-lived activities produced at other accelerators, and 

continuing with a very productive program with in-flight produced radioactive, but 

relatively light, beams that have been used mostly for nuclear astrophysics studies. 

With the new CARIBU injector, which will be commissioned in the summer/fall of 

2009, the range of radioactive beams will be expanded to include very neutron-rich 

fragments from spontaneous fission of a strong, 1 Ci, 252 Cf source. In order to 

use these beams for transfer reaction studies, a new type of spectrometer has been 

built, which is ideally suited for the study of such reactions in inverse kinematics. It 

is expected that the CARIBU injector and the HELIOS spectrometer will allow for 

a wide range of detailed studies of the nuclear structure in the neutron-rich region 

of heavy nuclei from CARIBU as well as nuclear astrophysics studies with beams 

produced via the in-flight method.


Fig. 6. Excitation energy spectrum of protons from the d( 28 Si,p) reaction. A 

subtraction of the background from reactions on the carbon in the target has 

been performed. 

Acknowledgments 

This work was supported by the U.S. Department of Energy, Office of Nuclear 

Physics under contracts DE-AC02-06CH11357 and DE-FG02-04ER41320 and the 

National Science Foundation under grant PHY-08-22648 (JINA). 

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HELIOS and the radioactive beam program at Argonne

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