KVI Newsletter

RIJKSUNIVERSITEIT GRONINGENKVI

number 10

August 2003

Computer drawing of the BINA detection system.

Preface

In this 10th KVI newsletter you will find someof the research highlights obtained by the var-ious groups during the last half year. Also,information is given to the users of the KVIfacilities about the next meeting of the KVIProgramme Advisory Committee.KVI had a prolonged shutdown in the begin-ning of 2003 to allow for building and furnish-ing of the hall for the TRIµP magnetic sep-arator. The period was also used for main-tenance and improvement of various compo-nents of AGOR. As a result of this operationthe vacuum in the cyclotron and the injectionline improved significantly. Also, the beamquality and extraction efficiency of the 85 and90 MeV/u deuteron beams strongly improved.This was very useful for the (d,2He) measure-ments which took most of the beam time inApril and May. Furthermore, as a consequenceof the improvements the reliability of the op-eration of AGOR in the first half of 2003 (onlythree months of running) increased relative to2002, with 90% availability of the scheduledbeam on target.Progress has been achieved in all big projects.The first quadrupoles of the TRIµP magneticseparator as well as the lasers have been deliv-ered. The first phase of upgrade of the Plas-tic Ball has been accomplished and the Plas-tic Ball has been set up at the BBS ready for

the next experimental campaign in the Fall of2003. BINA (the extended version of SALAD)is also being installed. Furthermore, the Boardof Governors of the University and ExecutiveBoard of FOM approved our proposal to builda Storage Area Network (SAN) that will behoused at the Computer Centre and will in-crease from 2 Tb this year to 16 Tb in 4 yearsfrom now.We had three Ph.D. graduations in this period:Patrick Hendriks, Karsten Ermisch and DavidHeesbeen. David did his research in the HER-MES collaboration at DESY under the super-vision of Gerard van der Steenhoven. Further-more, three graduate students as well as sev-eral technical staff members were appointed inthis last period. We were also fortunate tohave Prof. Alex Korchin visiting us for fourmonths and Profs. Michael Urin and CzabaKorpa starting a six-month long visit at thebeginning of June. I would like to take the op-portunity to welcome them all at KVI, and tothank our guests and the KVI staff for all theircontributions to the success of the research ef-forts and the various activities undertaken in2003. In particular, I would like to thank Su-san Schadmand for her help in setting up theAGOR User Group and her contributions to itas chair and member.

Groningen, July 2003 Muhsin N. Harakeh

2

News and announcements

• The Programme Advisory Committee(PAC) visited the KVI in December2002. The PAC considered fifteen pro-posals, which were all accepted. A re-port of the meeting and some apprecia-tive comments of the PAC on the statusof the laboratory and its activities can befound in this Newsletter.

• The AGOR User Group did not conveneofficially in the past year. There wereand will be a few changes in its compo-sition, however: Peter Dendooven tookover the Secretary position from Ad vanden Berg, Susan Schadmand had to leavethe committee after the regular term oftwo years and Nives Blasi will have to doso in the course of this year. The term ofthe chairmanship of Dieter Frekers willfinish at the end of this year, but he willstay on as a member for another year.

• Reinhard Morgenstern has accepted a(half-time) position as a member of theBoard of the Mathematics and ScienceFaculty of the University of Groningen.As a consequence, he decided to giveup his tasks as Scientific Deputy Direc-tor of KVI and Director of the FAN-TOM research school. Klaus Jungmannhas taken over the deputy directorshipof KVI and Herbert Lohner is now theFANTOM Director.

• KVI staff member Rob Timmermans hasbecome the new Theory Professor atKVI. The extra-ordinary professorship inTheory of John Tjon of the University ofUtrecht has ended in December 2002 dueto his retirement.

• As of 1 February 2003, Johan Messchen-dorp is a new staff member at KVI. Hewill work for the programmes ‘Interact-ing Hadrons’ and ‘Nuclear structure andits implication for astrophyics’ and forthe coming years he will especially de-vote himself to the field of spin physics.

• KVI Director Muhsin Harakeh has beenelected Chairman of NuPECC for the pe-

riod 1 January 2003 until 31 December2005.

• On 8–13 September 2002 the fourteenthinternational IISC (‘Inelastic Ion SurfaceCollision’) conference, organised by theKVI Atomic-Physics Group, was held onthe Dutch island Ameland.

• On 19–21 Februari 2003 a workshop on‘Inelastic scattering with radioactive nu-clei’ was held at KVI. This workshopwas organised in the framework of FIN-UPHY (the European collaboration ofnuclear-physics institutes) and the newGSI plans, in which KVI is consideringto participate.

• On 27–28 February 2003 a workshop wasorganised in Groningen by the Euro-pean HINDAS collaboration (‘High andIntermediate energy Nuclear Data forAccelerator-driven Systems’), in whichKVI is participating.

• On 10–11 March 2003 a workshop on‘Advanced electromagnetic calorimetryand its applications’ was held in Julich.This workshop, organised by COSYJulich and KVI, was held in the frame-work of FINUPHY.

• KVI decided to participate in the Dutch‘HIgh-School Project on AstrophysicsResearch with Cosmics’ (HISPARC).This project aims at interesting high-school pupils in physics by setting upa network of detectors on high-schoolbuildings for the detection of high-energetic cosmic radiation. The newKVI staff member Johan Messchendorpwill coordinate the project in Groningen.

• The autumn meeting of the FANTOMresearch school, with the topic ‘Visuali-sation’, was held from 18 until 22 Novem-ber 2002 in Tecklenburg (near Munster).From 2 until 6 June 2002 the eighteenthgeneral FANTOM week, on ‘Trappingand manipulating atomic and subatomicparticles’, took place in Leuven. This

3

week was co-organised with the Euro-pean networks NIPNET, HITRAP andIONCATCHER, in all three of whichKVI is involved. During this week, aninternational evaluation committee wasaround for a site visit in the frameworkof the renewal-recognition procedure forthe period 2004–2009 of FANTOM in theNetherlands. The report of the com-mittee was positive, which will definitelyhelp FANTOM to become recognised forthe next six years.

• On 8 and 9 May 2003 the third FAN-TOM Research Symposium was held atKVI. Staff members from the variousFANTOM partner institutes presentedtheir research plans for the near future.The aim of this type of symposium is tobring together the FANTOM researchersand to facilitate research collaborationswithin FANTOM. The contributions tothe symposium can be found on the web:www.fantom.kvi.nl/symposium03.html.

Forthcoming events

• The KVI Programme Advisory Commit-tee (PAC) will convene on 21–22 Novem-ber 2003. A call for proposals can befound in this Newsletter.

• The 19th general FANTOM study week,on ‘Interplay between theory and experi-ment’, will be held from 3 until 7 Novem-

ber 2003 in the Olympic Stadium in Am-sterdam, the Netherlands. On 24–28May 2004, the 20th general FANTOMstudy week will be held in Emmen, theNetherlands. The topic of this week willbe ‘Low-energy QCD’.

4

AGOR status report

Facility operationIn 2002 and the first half of 2003 the oper-ation of the AGOR facility has again beenvery satisfactory. In 2002 some 3500 hoursof beamtime were used for experiments andmachine studies. The down-time was againreduced significantly, resulting in an overallavailability of 85% as compared to the plannedschedule. In the first half of 2003 the cyclotronwas operated for 14 weeks only because of theprolonged shutdowns needed for the installa-tion of the TRIµP separator, the Plastic Balland the SALAD polarimeter (BINA). Duringthis running period machine availability wasagain better than in 2002; it exceeded 90%.All planned experiments have been carried outwith only minor rescheduling.

DevelopmentBeams with energies close to the focussinglimit have posed problems in terms of internalbeam losses and low extraction efficiency sincethe start of the operation of the facility in 1996.Measurements of various beam properties haveshown these problems to be due to large-scalevertical motion of the beam, the amplitude ofwhich strongly increases when approaching thefocussing limit. This motion is superimposedon the betatron motion caused by misalign-ment and mismatch in the central region of thecyclotron. A number of different mechanismscan contribute to this phenomenon, which haveas a common denominator defects in the mid-plane symmetry of the cyclotron. From mea-surements for various beams it was concludedthat at least part of this symmetry defectis due to a misalignment of the main coilswith respect to the iron poles (vertical offsetand/or tilt). As correction of a vertical offsetof the coils is a relatively simple operation, this

radius [mm]

400 600 800be

am p

ositi

on [m

m]

-6

-4

-2

0

2

Figure 1: Vertical beam position for 85 MeV/udeuterons before (dotted line) and after (solid line)changing the position of the main coils.

hypothesis was tested by lowering the coils by0.3 mm with respect to the original position,which had been determined using field mea-surements. This corresponds to about half thecorrection needed when vertical offset would besolely responsible for the observed beam move-ments. In figure 1 the vertical beam position asa function of the radius in the cyclotron mea-sured before and after lowering the coils is dis-played, showing a significant reduction of thevertical excursions.

A further result has been a drastic im-provement of the extraction efficiency for theQ/A = 0.5; 90 MeV/u beam, which made itpossible to deliver this much-wanted beam tothe users for experiments. Work on a furtherincrease of the energy towards the design value95 MeV/u is foreseen in the near future.

Sytze Brandenburg

5

TRIµP progress report

The TRIµP project at KVI aims at setting upa facility to produce, separate and slow downradioactive isotopes and to store them in trapsin order to perform precision experiments con-cerning fundamental symmetries and interac-tions in physics. In the past year the projecthas been progressing according to a schedulewhich foresees readiness for first experimentsin the year 2005.

A magnetic double-separator system hadbeen designed which will allow to utilise a va-riety of nuclear reactions, including fragmen-tation, direct or inverse-kinematics fusion andevaporation reactions, for creating radioactiveisotopes with beams from the AGOR super-conducting cyclotron. The arrangement offour dipole and eight quadrupole magnets sep-arates the nuclides of interest from other re-action products and the primary beam. Themagnets have been ordered from two compa-nies, Danfysik in Denmark and SIGMAPHI inFrance. The first four quadrupoles have ar-rived at KVI and are presently being tested.The modifications of the infrastructure to ac-commodate TRIµP at KVI have been startedand during an AGOR beam shutdown in Jan-uary/February 2003 a new room — the T-cell— has been created in the large north exper-imental hall by installing a shielding wall anda new access door to the experimental hall.The start of the magnet-system installation isplanned for November 2003 and is expected tolead to readiness for first beam tests in spring2004. Design work has been started on the twopossible target stations. This includes a low-temperature hydrogen-gas target in connec-tion with which a collaboration with a groupfrom TUNL, North Carolina, USA, has beenstarted. We expect the hardware to be builtthere. The radioactive isotopes are created attypically MeV/u energies. They need to beslowed down in collisions with matter to the eVrange. In order to design the optimal stoppingdevice a series of test experiments was startedin which charge-exchange and neutralisationprocesses are studied. In particular such rel-evant cross sections for ion-gas interaction are

measured, which are presently not or not wellknown. The experiments take place in a dif-ferentially pumped gas-cell setup attached tothe low-energy accelerator of the KVI Atomic-Physics Group. These activities are part of theEuropean GAS CATCHER RTD network.

In the TRIµP facility singly charged ionsfrom the stopper device will be collected,further cooled in a gas-filled radiofrequencyquadrupole cooler, at the output of which aPaul trap will serve as an ion collector andbeam buncher. A prototype has been built anddemonstrated already successfully the satisfac-tory operation of several novel radiofrequencyelements including a new RF coupling scheme.Cooling of ions from a low-energy ion gun hasbeen observed. The design of the actual deviceis under way.

The TRIµP programme foresees a largenumber of experiments with many differentisotopes. For every element different atomtrapping schemes are needed and in particularfor every isotope different optical wavelengthsare needed. In some cases several lasers will beneeded simultaneously. We have successfullyestablished an optical laboratory. Single opti-cal mode semiconductor lasers have been con-structed including the necessary driving andstabilisation electronics. In particular, due tothe VIDI grant (of the Dutch organisation forscientific research NWO) of Lorenz Willmanna TI:Sapphire laser and a dye laser could beinstalled very recently which will be employedin spectroscopy of Radium atoms as a prepara-tion of a search for permanent atomic electricdipole moments.

It must be mentioned that also the AGOR-Cyclotron Group has started to prepare fordelivering beams to the TRIµP facility. Thisincludes, for example, upgrades towards highbeam currents and very heavy metal beams.The TRIµP Group appreciates in particularthe contributions of all the KVI staff membersinvolved, which will ensure the availability ofthe new facility on schedule for new experi-ments on fundamental interactions.

Klaus Jungmann

6

A 4π hadron detector for few-body studies

One of the experimental programmes at KVIfocuses on obtaining high-precision data infew-nucleon scattering processes below thepion-production threshold. The goal is tostudy the details of the nucleon-nucleon andthree-nucleon interactions through a compar-ison with predictions from state-of-the-art ef-fective nucleon-nucleon potentials and modelsbased on a chiral-symmetry expansion. Forthis purpose, differential cross sections andanalysing powers are measured in inelastic andelastic proton-proton, proton-deuteron, anddeuteron-proton scattering.

Very recently, a systematic study of thethree-nucleon interaction has been made bymeasuring cross sections and vector-analysingpowers in �p+d scattering for several bom-barding energies up to 190 MeV at the KVIfacility exploiting the Big-Bite Spectrometer(BBS) [1]. One of the outcomes of this ex-periment is that effective models based ona well-understood two-nucleon interaction arenot sufficient to describe the three-nucleon pdsystem. Furthermore, the observed deficien-cies between experiment and theory stronglydepend on the incident proton energy. Includ-ing the most modern type of three-nucleon po-tentials in the calculations resolves only a partof the discrepancies.

The number of observables which one canobtain by studying the elastic �p + d reaction islimited to differential cross sections and vectoranalysing powers. For a more detailed study ofthe 3N interaction, additional observables needto be obtained. For this, pioneering measure-ments in the �d+p elastic and break-up channelshave been performed at KVI using the Small-Angle Large-Acceptance Detector (SALAD).These studies give access to tensor-analysingpowers, and, in case of break-up, allow to scana larger kinematic region. Furthermore, exper-iments in the elastic p+d reaction which focuson measuring polarisation transfer coefficientshave been carried out recently with the BBS.

Figure 1 shows a sample of data of the d+pbreak-up reaction at Ed=130 MeV obtainedwith SALAD and analysed by our Polish col-laborators (A. Micherdzinska et al.). Thecross section is obtained for a specific config-

Figure 1: d + p break-up cross section for an-gular configuration θ1=25◦, θ2=15◦, φ12=90◦, andEd=130 MeV. The variable S is deduced from thekinematical correlation between the energies of thefinal-state protons.

uration of the final-state protons covered bythe SALAD detector. The lines represent theresults of a theoretical calculation based onan NN potential (dashed line), and with theinclusion of a three-nucleon potential (solidline). Note that the data are of high statisticalquality and that the agreement between dataand calculations is very good for this partic-ular configuration. The phase-space coverageof SALAD for the break-up channel is how-ever limited. The detector only covers pro-tons with polar angles of less than 30◦ andlimits the energy of the protons to more than20 MeV. Furthermore, SALAD does not in-clude a polarimeter and is therefore not ca-pable of measuring polarisation transfer coef-ficients. To overcome these experimental de-ficiencies, a new 4π hadron detector will beemployed in the near future at KVI. This de-tector carriers the name ‘Big Instrument forNuclear-polarisation Analysis’ (BINA).

A schematic drawing of the detector is de-picted in figure 2. The detector consists of aforward part (‘forward wall’) and a backwardpart (‘backward ball’). The forward wall mea-

7

sures the position and energy of hadrons atscattering angles in the range θ=10◦–35◦. A∆E-E hodoscope accounts for particle discrim-ination and a set of wire chambers for the de-termination of the scattering angles. In ad-dition, a carbon analyser can be installed suchthat the forward wall functions as polarimeter,thereby allowing to measure spin-transfer co-efficients. The backward ball operates as scat-tering chamber and detector simultaneouslyand consists of 150 plastic phoswich elements.Such design allows to measure and discrimi-nate protons and deuterons down to an energyof less than 1 MeV. Together with the forward

wall, the detector covers nearly 4π in spacewhich is advantageous for the determinationof the pd break-up reaction. The installationof the BINA detector will take place in thesummer of 2003. The first detector tests areplanned for November of 2003.

Johan MesschendorpNasser Kalantar

[1] K. Ermisch et al., Phys. Rev. Lett.86 (2001) 5862 and PhD dissertation,Rijksuniversiteit Groningen (2003) un-published.

Figure 2: Computer drawing of the BINA detection system.

8

Virtual bremsstrahlung in proton-proton scattering at 190 MeV

The high quality of the present state-of-the-art nucleon-nucleon interaction models for de-scribing elastic-scattering data have set highgoals for future experiments. These must ob-tain high-precision and high-statistics data fora study of higher-order effects. To meet thosegoals the ‘Interacting Hadrons’ research pro-gramme was initiated at the KVI with the ad-vent of the AGOR superconducting cyclotron.In the framework of this programme a numberof studies was undertaken, e.g. to study three-nucleon force effects and the nucleon-nucleoninteraction.

To address the nucleon-nucleon dynamicsa study of bremsstrahlung in proton-protonscattering employing SALAD and Plastic Ballwas undertaken. In a previous experimentemploying SALAD and TAPS [1] a feasibil-ity study of virtual bremsstrahlung was per-formed. That pilot experiment produced adata set of about 600 p + p → p + p + e+ + e−

virtual-bremsstrahlung events. It was realisedthat a full coverage of the backward angles aswell as good lepton/photon discrimination isneeded to increase the efficiency of detectinglepton pairs.

The Plastic Ball detector [2] consists of tri-angularly shaped ∆E − E phoswich detectormodules each covering approximately 17 msr.A thin layer of CaF2 in front of a thick plasticscintillator guarantees good charged-particleidentification properties. The detector mod-ules are arranged in a hollow sphere.

The Plastic Ball setup for the experimentdiscussed here used about 550 individual de-tector modules covering polar angles in thelab system between 50◦ and 160◦ with full az-imuthal coverage. The experiment used a pro-ton beam of 190 MeV with a typical beam cur-rent of 1.8 nA. The target was a liquid hydro-gen cell of about 1.7 cm3 with areal density of72 mg/cm2. The trigger selected events withat least two clusters of minimum energy in aring of 5 or 6 Plastic Ball modules.

The subsequent data analysis has resultedin a selection of 25000 ppe+e− events. How-ever, due to the problem caused by externalpair-production in the target-cell material (fig-ure 1) this data set was still contaminated by

θlab (deg.)φ la

b (d

eg.)

0

50

100

150

200

250

300

350

40 60 80 100 120 140 160

Figure 1: GEANT3 simulation: external pair-production density in different parts of target-cellmaterial presented as a function of lab angle. Thetrapezoidal shape on the picture shows where thetarget cell is located and the circle marks the areajust opposite to the target.

false events. To avoid these background dataa further selection was made on the invariantmass of lepton pairs Me+e− > 20 MeV whichaccording to simulations have negligible contri-butions from external pair-production. By im-posing this cut, only about 3500 events werefinally analysed. Nevertheless, the remainingdata was sufficient to allow the extraction of in-teresting observables, though under restrictedkinematical conditions.

The study of virtual bremsstrahlung is ad-dressing properties of the nucleon transitioncurrent by measurement of virtual photons, i.e.dilepton pairs. Virtual photons introduce ad-ditional degrees of freedom, due to the longitu-dinal polarisation, thus allowing longitudinal-transverse (LT) decomposition of the transi-tion amplitude resulting in six response func-tions (RF) which include longitudinal, trans-verse and four interference RF’s Wi, with i =T,L,TT,TT′,LT,LT′.

The RF’s in the LT-decomposition can beextracted by using the particular dependenceof the transition amplitude on the so-called di-hedral angle φl and the energy-sharing angleθl. The dihedral angle is defined as an an-

9

gle between the reaction plane (given by theincoming-proton and the virtual-photon mo-mentum vectors) and the plane spanned by themomentum vectors of the dilepton pair. Theenergy-sharing angle θl is the angle betweenthe relative dilepton momentum and the pho-ton momentum. The interference terms enterthe transition amplitude with factors of differ-ent sine and cosine harmonics of φl, while WT

and WL are multiplied only by factors contain-ing θl.

Using the particular harmonic it is possibleto extract different RF’s, but a good coverageof φl is necessary in order to suppress interfer-ence with other RF’s. In our data full coverageof φl was obtained (figure 2), allowing us toextract the interference RF’s with minimisedmutual interference.

The comparison of the data with theory isdone by means of a Monte Carlo phase-spaceintegration. An event generator was used toproduce a data set for which all observableswere calculated per event using the calculationdeveloped by [3] on basis of the Low EnergyTheorem. This data set was treated in thesame way as the experimentally obtained dataset, i.e., the detector acceptances were imposedby applying appropriate kinematical cuts. Inthis way we were able to directly compare re-sults of measurements to calculations at theexpense of introducing statistical uncertaintiesinto the calculated data set.

φl (deg.)

coun

ts

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300 350

Figure 2: Experimentally obtained distribution ofthe dihedral angle φl.

-2

-1

0

1

2

-2

-1

0

1

2

20 40 60 80-2

-1

0

1

2

20 40 60 80 100

-3

-2

-1

0

1

2

WTT W’

TT

WLT W’

LT

Mγ [MeV]

arb.

uni

ts

preli

minary

Figure 3: Preliminary: extracted interference re-sponse functions WTT, W ′

TT, WLT, W ′LT compared

to LET calculation (full line) plotted as a functionof the virtual-photon invariant mass Mγ. Responsefunctions are plotted in an arbitrary scale definedby sign(Wi) log(|Wi|).

Preliminary results of the determination of in-terference RF’s are shown in figure 3. Notethat due to the Monte Carlo procedure de-scribed above the averages of RF’s integratedover the whole data set are compared. Thereseems to be a ‘sign problem’ in the extrac-tion of the WLT which is still being pur-sued. Additionally, the absolute normalisationis presently not yet solved to our satisfaction.It still remains to be estimated to which confi-dence the real-photon channel can be used forthis purpose.

In the following period the Plastic Ball isgoing to be mounted and used together withthe BBS. The first experiment with the newlycoupled setup will be the measurement of vir-tual photon yields in the pd −→ 3He e+e− cap-ture reaction.

Mladen Kis

[1] J.G. Messchendorp et al., Phys. Rev.C61 (2000) 064007.

[2] A. Baden et al., Nucl. Instr. and Meth.203 (1982) 189.

[3] A.Yu. Korchin, O. Scholten and D. VanNeck, Nucl. Phys. A602 (1996) 423.

10

Proton-decaying states in 22Mg and nucleosynthesis of 22Na in novae

The thermonuclear runaway model of classi-cal novae provides a framework capable of ex-plaining many of their features. Astronomi-cal observations increasingly constrain currenthydrodynamic simulations. One important ob-servational test of the models would be the de-tection of γ-rays emitted by nuclei synthesisedin a nova outburst. Searches will be under-taken by space-borne γ-ray observatories suchas the recently launched INTEGRAL of theEuropean Space Agency. Since classical novaesynthesise 22Na, the 1.275 MeV γ-ray emittedby its β+ daughter 22Ne is a good candidate forsuch detection efforts. An observational cam-paign conducted with the COMPTEL instru-ment on NASA’s Compton Gamma Ray Ob-servatory failed to positively detect any 22Naγ-rays from galactic novae, placing a tight up-per limit on their 22Na production and ejec-tion.

There are two paths by which 22Na issynthesised in novae, 21Na(β+)21Ne(p, γ)22Naand 21Na(p, γ)22Mg(β+)22Na. At low temper-atures, the former path dominates, while athigh temperatures the 21Na(p, γ)22Mg rate cancompete with the 21Na β+ decay rate, and thelatter path becomes important. Knowledge ofthe 21Na(p, γ)22Mg rate is essential for makingquantitative predictions about the total 22Nayield in a nova explosion. At low tempera-tures, this reaction proceeds dominantly by di-rect capture, but at higher temperatures, reso-nant contributions from excited states in 22Mgbecome important. Recently, the first directmeasurement of the resonance strength of the5.714 MeV state in 22Mg that dominates the21Na(p, γ)22Mg reaction rate at nova tempera-tures was reported by a group working at TRI-UMF.

Here we describe a measurement of proton-decay branching ratios of states in 22Mg ly-ing just above the proton separation energy.We obtain an upper limit on the proton-decay branching ratio of the 5.714 MeV state.Combining this upper limit with an indepen-dent measurement of the lifetime of this state,we calculate an upper limit on its resonancestrength that is consistent with the TRIUMFmeasurement. We use the results of these

two measurements to determine the most likelyvalue of this resonance strength. Taking intoconsideration the experimental information onhigher-lying states in 22Mg that may also con-tribute to the 21Na(p, γ)22Mg rate in novae, wecalculate the astrophysical reaction rate andperform hydrodynamic calculations of novaoutbursts in order to predict their 22Na γ-rayfluxes.

The detailed balance theorem implies thatthe same information can be gained fromstudying the decay of a resonance as can belearned from measuring its formation. There-fore, we have performed a measurement of theproton-decay branching ratios of the states in22Mg relevant to the 21Na(p, γ)22Mg reactionrate in novae. The measurement was carriedout at the KVI using a recoil coincidence tech-nique through which we have detected bothproton- and γ-decaying recoils with 100% geo-metric efficiency in the Big-Bite Spectrometer.A 55 MeV/u 24Mg beam provided by the vari-able energy, superconducting cyclotron AGORbombarded a 1 mg cm−2 (CH2)n target to pop-ulate states in 22Mg via the (p, t) reaction ininverse kinematics. Both triton ejectiles and22Mg recoils entered the magnetic spectrom-eter, which was positioned at 0◦. The 22Mgrecoils deexcited by the emission of γ rays, re-taining their identities as 22Mg, or by the emis-sion of protons, resulting in the formation of21Na decay products. Proton decays were iden-tified through 21Na-triton coincidences, whileγ decays were observed as 22Mg-triton coinci-dences.

Heavy recoils and decay products weredetected in two phoswich detectors, whiletwo vertical drift chambers recorded the po-sitions and angles of the triton ejectiles be-fore they were stopped in a separate array ofsix phoswiches. Particles were identified us-ing energy loss, total energy, and time-of-flightinformation provided by the phoswich detec-tors. The experimental technique and appara-tus were previously employed in our measure-ment of 19Ne α-decay branching ratios.

Strong kinematic forward focusing in thebombardment of a proton target by 1.3 GeV24Mg projectiles made possible full angular and

11

momentum acceptance for recoils and decayproducts. There are two solutions of the re-action kinematics at laboratory angles around0◦, one in which tritons are emitted forwardin the centre-of-mass system, and one in whichthey are emitted backward. We detected thoseemitted backward in the centre-of-mass sys-tem, which have laboratory energies circa 19MeV/u. For triton ejectiles detected at labora-tory scattering angles of 4◦ or less in this mea-surement, the 22Mg recoils emerged at scat-tering angles up to 0.3◦. The impulse deliv-ered to the 21Na decay product in a proton de-cay results in a small angular spread about theoriginal 22Mg trajectory, but the high incident-beam energy and low decay energies of thestates studied limited the laboratory scatter-ing angles of the 21Na decay products to 0.5◦.Simultaneous detection of both ejectile and re-coil or decay product is possible because thelarge momentum acceptance of the spectrome-ter can accomodate the 12% magnetic-rigiditydifference between 21Na decay products andtriton ejectiles.

Excitation energies of the 22Mg recoils werereconstructed from the measured momenta ofthe triton ejectiles. The spectrum associatedwith γ-decay, obtained from 22Mg-triton coin-cidences, is shown in figure 1, and that withproton-decay, obtained from 21Na-triton coin-cidences, is displayed in figure 2. States in22Mg are labeled by their excitation energies.Several recent experiments have studied thestates near the proton threshold. Compar-ing the excitation energies of the seven statesobserved in this measurement with the litera-ture values, we find a root-mean-square devia-tion of 6 keV. An excitation-energy resolutionof 90 keV full-width-at-half-maximum was ob-tained. This resolution is insufficient to com-pletely separate the 5.96 and 6.05 MeV states,so a fit consisting of Gaussians and a constantbackground was used to determine the yield tothese two states in the ‘γ-decay’ and ‘proton-decay’ spectra.

The proton separation energy in 22Mg is 5.5MeV, so of the states observed here, only thoselying at 5.71-, 5.96-, and 6.05 MeV can decayby proton emission. While proton decay is thedominant deexcitation mechanism for the twostates around 6 MeV, figure 2 reveals no statis-

600

500

400

300

200

100

0

Cou

nts

/ 20

keV

6.05.55.04.522Mg Excitation Energy (MeV)

4.40

5.04

5.305.45

5.71

5.966.05

22Mg-triton coincidence(γ decay)

Figure 1: 22Mg excitation energy spectrum ob-tained from 22Mg-triton coincidences, representingγ decays of states in 22Mg.

1000

800

600

400

200

0

Cou

nts

/ 20

keV

6.05.55.04.522Mg Excitation Energy (MeV)

5.96

6.05

21Na-triton coincidence(proton decay)

Threshold

Figure 2: 22Mg excitation energy spectrum ob-tained from 21Na-triton coincidences, representingproton decays of states in 22Mg. The proton-decaythreshold lies at 5.5 MeV.

tically significant evidence for proton decays ofthe 5.714 MeV state. Hence, we set an upperlimit on the proton-decay yield from this stateusing Bayesian statistics with a uniform priorprobability density function. As the thresholdsfor the emission of neutrons and α particles lieat higher excitation energies, only proton andγ decay are possible for these states. There-fore, the proton-decay branching ratio is givenby Bp ≡ Γp/Γ = Γp/(Γp + Γγ).

Of the three proton-decaying states ob-served here, γ-decays of only the 5.714 MeVstate have been studied previously. Its meanlifetime has been measured to be 40±15 fs. Inthe case of the other two states, no experimen-tal information on the radiative or total de-cay widths is known. However, since the 5.962MeV state is of marginal importance for the

12

21Na(p, γ)22Mg reaction rate at nova temper-atures, we have assumed that it has the sameradiative width as its analog state in 22Ne forthe purpose of gauging its contribution to thereaction rate. As the 6.046 MeV state playsno role at nova temperatures, we have madeno effort to estimate its decay widths in theabsence of experimental data.

Using all of the available experimental in-formation, we have calculated the astrophysi-

cal rate of the 21Na(p, γ)22Mg reaction at tem-peratures from 0.01 to 1 GK. In collaborationwith J. Jose of the Institut d’Estudis Espacialsde Catalunya in Barcelona we have includedthis calculated reaction rate in hydrodynamicnova simulations in order to predict the yield of22Na from classical novae. The results of thesesimulations are currently being analysed.

Barry Davids

13

Catching some sun

In 1996, bright X-Ray emission was discov-ered from comet Hyakutake. This was a bigsurprise; comets are cold, dusty iceballs whileX-Ray emission is associated with high-energyprocesses. In the following years, more andmore comets were observed in X-Ray and inthe Far Ultraviolet (FUV) and it became clearthat this emission was a general feature ofcomets. The launch of two new, very sen-sitive X-Ray observatories (NASA’s CHAN-DRA and XMM-Newton by the EuropeanSpace Agency) enormously increased the ob-servational possibilities. Spectra acquired withthese satellites allowed to test the various theo-ries developed to explain the high energy radia-tion. It turned out that the cometary emissionwas produced by interaction between cometsand the solar wind, or to be more precise,charge exchange between cometary neutralsand highly charged ions from the solar wind[1,2].

The solar wind is a plasma streaming out ofcoronal holes, magnetic discontinuities in theouter regions of the Sun. It consists of pro-tons, 5% helium and a small fraction of heavyparticles (oxygen, carbon, neon, etc). Due tothe temperatures in the corona (over a milliondegrees), all ions in the wind are highly ionised.A rough distinction can be made between fastwind (800 km/s), originating from deep coro-nal holes near the Sun’s poles, and slow wind(below 400 km/s) coming from shallower holesaround the equator, see figure 1. These differ-ent origins result in large differences in boththe abundances and the charge-state distribu-tion.

The interaction of multiply charged solar-wind ions with the neutral gas from the cometis dominated by electron capture. Whenthe incoming solar-wind ions capture an elec-tron, the electron typically ends up in an ex-cited state. When it decays to the groundstate, a photon is emitted. The popula-tion of specific states depends strongly onthe charge of the projectile, on the colli-sional velocity and on the (electronic) struc-ture of the target, especially its ionisationpotential. Using this light, it is possible toanalyse the charge-exchange processes that

Figure 1: The Sun as seen in X-Ray. Coronalholes can be seen as large, dark spots.

take place either in a comet, or in the lab-oratory — as is done in our AGORA setup. With this setup, we can study state-selective charge-transfer processes at energiesof direct relevance for astrophysical observa-tions [3]. The method combines the tech-nique of RF ion guiding with crossed-beamphoton-emission spectroscopy. Briefly, ions ex-tracted from our electron cyclotron resonanceion source are injected via a five-element lenssystem into an RF multipole ion guide, in ourcase an 8-pole system. The photon emissionfollowing charge-transfer reactions can be ob-served by different monochromators.

Because helium is very abundant in thesolar wind, we started with experiments us-ing fully stripped helium ions. An alpha par-ticle can capture either one or two electronsfrom a target. This results in two differentpeaks in the Far-UV spectrum: at 30.4 nmfrom the He+(2p → 1s) transition followingsingle-electron capture and at 58.4 nm fromthe He(1s2p → 1s2) transition. It is the com-petition between single- and double-electroncapture and thus between those two peaks thatmake helium so interesting.

From figure 2, it can clearly be seen thatdominance of either of the peaks swaps frompeak HeII(2p → 1s) at high velocities to peakHeI(1s2p → 1s2) at low velocities. Given that

14

the two helium lines (at 30.4 and 58.4 nm)characteristic for single- and double-electroncapture, respectively, have been observed invarious comets, one wonders whether their lineratio could be used as a solar-wind probe.

The cometary nucleus mainly produces wa-ter and CO, but especially the water is rapidlybroken down into atomic oxygen and hydro-gen by interaction with sun light. Beside themolecular composition, other cometary param-eters that play a role in the interaction betweencomets and the solar wind are the amount ofgas produced by the nucleus and the comet’sdistance to the Sun.

It is therefore clear that it is not possibleto directly apply the relation between line ratioand velocity; one has to carefully model the in-teraction between comet and solar-wind ions.The ratio is also affected by sequential electroncapture; an ion that has already captured oneelectron, might be involved in a second charge-exchange reaction.

In the model we developed, helium ions arefollowed as they fly through a cometary envi-ronment. For each point of their trajectorythrough the cometary gas, the relative frac-tions He2+, He+ and neutral helium are calcu-lated. Once this is done, the number of pho-tons emitted by all the relevant processes can

0 250 500 750 1000 1250

0.1

1

10

100

line

ratio

(30

.4 n

m /

58.4

nm

)

velocity (km/s)

Figure 2: Velocity dependence of the ratio betweenemission due to single and double electron captureby He2+ ions colliding on H2 and CO.

Solar Wind Velocity (km/s)

0 200 400 600 800 1000

30

.4/5

8.4

nm

lin

era

tio

0.001

0.01

0.1

1

10

Figure 3: Hale-Bopp, line emission ratio for dif-ferent solar-wind velocities. The observed line ratioof 0.6 is indicated.

be found. The results for comet Hale-Bopp areshown in figure 3. A minimum initial velocityof 220 km/s is found for the solar-wind heliumions, corresponding to slow wind velocities.

This analysis is only one of the ways thatexperimental knowledge of charge-exchangeprocesses can be used to analyse energeticcometary emission. In the case of helium,only the first excited state is involved. Whenone considers heavier elements (O, Ne), higherstates will be excited leading to distinct spec-tra in both X-Ray and Far UV.

Using the two new observatories CHAN-DRA and XMM, a new field of astrophysics hasopened; from the cometary observations men-tioned above, it has been discovered last yearthat besides comets also most of the planetsare visible in X-Ray. In some of these cases,charge exchange is again the driving force be-hind the emission providing a clear demand foratomic-physics data, as measured at KVI.

Dennis BodewitsZoltan Juhasz

Xander Tielens (Univ. Groningen)Ronnie Hoekstra

More detailed information is available on ourwebsite: http://www.astro.rug.nl/∼dennis

[1] T.E. Cravens, Science 296 (2002) 1042.

[2] C.M. Lisse et al., Science 292 (2001)1343.

[3] G. Lubinski, Z. Juhasz, R. Morgenstern,R. Hoekstra, Phys. Rev. Lett. 86 (2001)616.

15

Recent developments at NGD

The activities of the KVI Nuclear GeophysicsDivision (NGD) in the past period have fo-cused on Monte Carlo simulations and on de-veloping a methodology to optimise and mon-itor road constructions. Monte Carlo simula-tions are an integral part of an EU project aspart of the programme GROWTH and of aUN project to improve identification of land-mines. The road construction work is con-ducted in close collaboration with Heijmans,a large Dutch firm working on roads andother infrastructures in the Netherlands, Bel-gium and Germany. In the EU project ‘Anon-destructive Pulse Neutron Multiple Detec-tor Tool for use in environmental, hydrocar-bon and mineral exploration work’ (NuPulse),work is carried out to design, test and build de-tectors for gamma radiation. In this work theMonte Carlo simulations of the radiation trans-

port are crucial. At present, we have designeda stopper to shield the detector from directirradiation by neutrons, build a large watertank to carry out test measurements and havetested a BGO detector, specially equipped toprovide information on the slow-neutron flux.The Monte Carlo calculations proved to bevery helpful: not only for getting high preci-sion results, but also to be able to investigatethe contributions to the neutron flux and thegamma-ray spectrum from the stopper mate-rial and the geological matrix. Based on thesecalculations a stopper has been built and isready to be tested after the pulsed-neutronsource has become operable. Moreover, MonteCarlo based techniques are being developedto advance the interpretation of gamma-raydetector responses in combination with full-spectrum analysis.

Figure 1: Test experiments on radiometric evaluation of asphalt-layer thickness in road construction: NGD’sPANDORA detector in an isolated box mounted on a double-layer asphalt-spreading machine.

16

Monte Carlo simulations are also the back-bone of a UN project to assess a method foridentifying explosives in packages as well assmall, plastic landmines. The method is alsobased on a pulsed-neutron source and the ex-plosives and/or landmines are detected fromthe specific gamma-rays due to interactions,such as inelastic scattering by neutrons fromcarbon and nitrogen nuclei in the explosives incomparison to those from the main geologicalmatrix elements. With Monte Carlo simula-tions the response function of the detector forvarious conditions will be calculated. These re-sponse functions in turn will be used to analysethe measured detector response.

A number of successful experiments hasbeen conducted with respect to the analysis ofasphalt and concrete layers on roads (see figure1). The methodology is based on the principle

that each material has its characteristic set ofnatural radionuclide concentrations. A certainlayer acts on the one hand as a source of natu-ral gamma radiation and on the other hand itwill act as an absorber for the gamma radiationfrom the layers below. The initial experimentswere carried out to investigate the validity of asource-absorber model and to apply it to checkthe thickness of asphalt layers in situ. The re-sults of these measurements were input to theproof of principle for a patent application andconfirmed the validity of the source-absorbermodel.

In conclusion, the combination of our ex-pertise on Monte Carlo simulations and detec-tor technology has been very fruitful. It is ex-pected that this combination will further in-tensify.

Rob de Meijer

17

The KVI Programme Advisory Committee meeting,

13–14 December 2002

This year, the composition of the PAC has re-mained unchanged:

Juha Aysto (Jyvaskyla),Philippe Chomaz (GANIL),Volker Metag (Gießen/GSI),Leif Nilsson, chair (Uppsala),Hide Sakai (Tokyo),Hans Stroher (Julich).

Ex officio:Muhsin Harakeh, Director KVI,Olaf Scholten, Secretary PAC,Ad van den Berg, Facility Coordinator.

For a report of the PAC on the status of theKVI laboratory and its activities, drawn up inconnection with the 2002 PAC meeting, see be-low. The following proposals were accepted bythe PAC:

S34 Search for the GT strength in 14C via the14N(d,2He)14C reaction.Spokespersons: A. Negret, D. De Frenne.

S35 Study of Gamow-Teller transitions in64Co using the 64Ni(d,2He)64Co reaction.Spokespersons: L. Popescu, D. De Frenne.

S36 Study of the Gamow Teller transitions tointermediate states in double-beta decay:48Ti(d,2He)48Sc, 100Ru(d,2He)100Tc and116Sn(d,2He)116In.Spokespersons: S. Rakers, D. Frekers.

S37 The structure of 7He and the strengthof the spin-orbit force through7Li(d,2He)7He.Spokespersons: D. Frekers, P. von Neu-mann Cosel.

S38 Structure of electric dipole strength belowthe neutron threshold.Spokesperson: A. Zilges.

S39 Investigation of the decay of the third 0+

state in 12C.Spokesperson: A.M. van den Berg.

S40 Search for molecular states in lightneutron-rich nuclei.Spokesperson: A.M. van den Berg.

S41 Branching ratios for decay by protonemission or α-emission from states in18Ne and 26Si.Spokesperson: A.M. van den Berg.

S42 Pionic fusion: comparison of differentspin/isospin channels.Spokespeson: H. Lohner.

S43 Investigation of isovector giant reso-nances in 12C, 90Zr and 208Pb nucleiwith the (t,3He) reaction at 43 MeV/A.Spokesperson: J. Guillot.

F13 Break-up studies of proton-deuteron sys-tem with a 4π detector.Spokespersons: N. Kalantar-Nayesta-naki, J. Messchendorp.

F14 Bremsstrahlung studies on the p + p sys-tem; p + p → 2He + γ.Spokespersons: J.C.S. Bacelar, N. Ka-lantar-Nayestanaki.

T14 Measurement of double-differential dis-tribution of neutrons produced by 100MeV p on 13C.Spokesperson: G. Prete.

T15 Test of scintillating GEM dosimeter.Spokespersons: J.M. Schippers, R. Kreu-ger.

T16 Radiation damage to parallel organisedorgans after high-precision proton irra-diation.Spokesperson: H. Meertens.

Support by the EU/IHP Access to ResearchInfrastructures programme in the frameworkof LIFE (Light-Ion Facility Europe), was re-quested for proposal numbers S34, S35, S36,S37, T14 and T15.

The next PAC is scheduled for 21 and 22November 2003. The deadline for submittingproposals is 20 October 2003. Further infor-mation concerning beam scheduling and beam-time request forms can be obtained from theWeb via the KVI homepage.

Olaf Scholten, PAC Secretary

18

Report of the Programme Advisory Committee

In connection with the 2002 Programme Ad-visory Committee meeting at the KernfysischVersneller Instituut held in Groningen Decem-ber 13 - 14, the committee decided to give thefollowing comments on the status of the labo-ratory and its activities:

The activities around the AGOR cyclotronhave developed in a very positive way in thatthe amount of beam time to experiments hasincreased progressively, beams of various kindshave been developed and detector arrange-ments installed in a remarkable pace. Oneresult of this progress is that the laboratoryhas now developed a few specialities, in someof which it is unique in the world.

For example, the high-resolution (d,2He) ex-periments using the BBS and ESN detectorshave improved the knowledge of spin-isospinexcitations in nuclei considerably. Furtherprogress in this field can be foreseen for thenear future.

Similarly, the experimental studies of few-nucleon systems have contributed substan-tially to the understanding of these systemsand will certainly continue to do so.

The development of the tritium beam has pro-gressed quite well and the use of that beampromises to give interesting results shortly,e.g. regarding isovector multipole resonances.

In addition to the programme in basic nuclearphysics, the laboratory has attracted a highlyinteresting activity in radiation biology beingof considerable importance for improving thequality of radiation-therapy treatments.

The Programme Advisory Committee wantsto convey its congratulations to the labora-tory on the recent considerable progress bothregarding technical matters and scientific ac-tivities.

Leif Nilsson, PAC Chairman

19

KVI Newsletternumber 10August 2003

Kernfysisch Versneller InstituutZernikelaan 25NL–9747 AA GroningenThe Netherlands

Phone: +31 50 363 3600

Fax: +31 50 363 4003

E-mail: news@kvi.nl

Website: www.kvi.nl