302 Philips tech. Rev. 39, 302-307, 1980, No. II

BOL

R. van Dantzig

The 'BOL' measurement system for nuclear physics(fig. 1) [1] [*] was built by IKO during the period 1965to 1969; it was called BOL because of its sphericalshape ('Bol' is the Dutch word for sphere). The equip-ment and instruments were designed for studyingnuclear reactions induced by the external particlebeam of the synchrocyclotron [2]. The system wasparticularly suitable for studying nuclear collisions inwhich two or more of the escaping nuclear particles

had to be detected so as to reconstruct the collisionprocess.

BOL was the brainchild of Dr L. A. C. Koerts- who was later to join the Philips staff; the system

Fig. 1. The 'BOL' measurement system for nuclear reactions. Par-ticles accelerated in the synchrocyclotron are guided through apipe - on the left - into the spherical scattering chamber, wherethe particles cause nuclear reactions in a target. Reaction productsare detected with 64 detector telescopes; the connecting wires' andcables for these can be seen in the figure. The black hoses are partof the air-cooling system for the electronic circuits outside thevacuum.

Dr R. van Dantzig is with [KO (Institute for Nuclear PhysicsResearch), Amsterdam.

Philips tech. Rev. 39, No. 11

was also built under his supervision. The Institute andPhilips worked closely together on this project, par-ticularly with regard to the particle detectors [3],

which formed an essential part of the system. 64detector telescopes (fig. 2) - each containing one ofthe 0.3 mm thick checkerboard detectors developedfor BOL and one or more Si(Li) detectors of 4 to5 mm thickness - were mounted in a spherical scat-tering chamber, at about 10 cm from the centre,where a target was located. DOJo of the surface of thesphere was 'covered' by detector devices, thus en-suring a particularly high detection yield. The detec-tion system has been compared by Koerts to a giant,inward-turned insect's eye.

BOL 303

incidence on the checkerboard detector, coincidencerelationships, and dead time) for each detected par-ticle were stored on magnetic tape with the aid of aDEC POP8 process computer. For complicated 'on-line' calculations the data could also be passed directlyto an Electrologica EL-X8 computer and presented ona POP8 display. The interface equipment between thecomputers was developed in close cooperation withPhilips and Electrologica.

This collection of almost autonomous pieces ofequipment could operate automatically as a singleunit because of the development of suitable software.For example, for asynchronous processing of dataoriginating from the detection equipment and for

Fig. 2. A BOL detector telescope. The checkerboard detector and Si(Li) detector in the head ofthe telescope can be seen on the extreme left, with the charge amplifiers connecied io them. Thedetectors and charge amplifiers were located inside the vacuum of the scattering chamber.

The detector telescopes were held in place by aspherical copper shell about 20 cm thick, in which thepositioning holes had been very accurately located(fig. 3) [4]. The purpose of this 'inner sphere' was tocool the detectors ( - 20°C) and hence reduce the elec-trical noise. Amplifiers were mounted as close to thedetectors as possible. They amplified the weak detec-tor signals to make them suitable for transmissionover more than 1000 coaxial cables to the electronicmeasurement and control circuits, which containedabout 100000 discrete components. (The electroniccircuits were developed at the lnstitute under thesupervision of R. F. Rumphorst who was also later tojoin the Philips staff.) The inner sphere, the detectortelescopes and the preamplifiers were located in avacuum (10-4 Pa), enclosed by the outer sphere. Themeasurement and control circuits received data fromthe telescopes relating to: the energy, the point of inci-dence (and hence the direction of movement) and thenature of every particle detected, and coincidenceswith events detected in other telescopes.

The energy of a detected particle could be rapidlydigitized, to an accuracy of 0.01 %, in each of the 64electronic detector channels. 72 bits of in formation(relating to aspects such as energy loss and point of

interfacing the computers the POP8 was providedwith a multi-task monitor and 'drivers' for the variousdata streams. A high-level programming language(SIMPLEX) was designed and implemented for theEL-X8. This language met the requirements of thedata processing and was based on a standardizationof the data streams ('window-processing'). Processeddata, which often had to be studied in multi-dimen-sional spaces, could be presented on a purpose-builtgraphic display; here again Philips Research Labora-tories and Electrologica had an important part toplay.

Much of the work associated with the building ofBOL lay on the fringe of what was technically feasible.This was a great stimulus for the many people whoapplied themselves so enthusiastically to making theinstrument a practical reality.

[.] The author would be pleased to supply a complete list ofreferences relating to the construction and the research.

[I] L. A. Ch. Koerts, K. Mulder, J. E. J. Oberski and R. vanDantzig, The "BOL" nuclear research project, Nucl. lnstr.Meth. 92, 157-160, 1971.

[2] See for example G. Luijckx, The cyclotron, this issue,page 290.

[3] See for example W. K. Hofker, Semiconductor detectors, thisissue, page 298.

[4] K. Mulder, R. van Dantzig, J. E. J. Oberski and L. A. Ch.Koens, Mikroniek 9, 240, 1969 '(in Dutch).

304 R. VAN DANTZIG Philips tech. Rev. 39, No. 11

Nuclear-physics research

For studying nuclear processes particles acceleratedin the synchrocyclotron were allowed to collide withstationary atomic nuclei in a target. In such a collisionvarious processes can occur. The simplest of these iselastic scattering, rather like a collision between two

if the colliding atomic nuclei consist of many nucleons(Le. protons and neutrons). In the research carriedout using BOL, attention was directed mainly towardsreactions in which only a few nucleons have a part toplay or in which a group of nucleons could be con-sidered in practice as a single entity, a 'cluster'. For

Fig. 3. Diagram of the cross-section of the BOL scattering chamber. Holes have been accuratelymachined in the spherical copper shell 1 for the detector telescopes 2. The target transportsystem 3 could be used to introduce different targets into the beam, which was guided through theholes 4. Checkerboard detector 5, Si(Li) detector 6 and charge amplifiers 7 were mounted in thevacuum 8. The spherical shell and detector telescopes were cooled by Freon 9.

billiard balls. Inelastic collisions can also occur. Inthese collisions kinetic energy may be converted intomass or into internal excitation energy, or vice versa;one of the colliding particles mayalso become un-stable. After some time such a particle may return toa stable state by emitting electromagnetic radiationor, if the energy of impact is high enough, it may emitone or more of the nuclear particles from which it isbuilt. It is also possible that during the actual colli-sion one of the colliding particles may break up im-mediately into fragments. The dynamics of all theseprocesses is extraordinarily complicated, particularly

such reactions it was possible to determine how energyand momentum were distributed among the variousescaping particles. Because of the large number ofdetectors and the ability of the checkerboard detectorsto determine direction it was possible to cover thegreat majority of all the possible final momentumstates - determined by four independent degrees offreedom in the four-dimensional kinematic phasespace. For a given nuclear reaction this makes it pos-sible to determine the probability, expressed as the'differential cross-section', for different kinematicfinal states.

Philips tech. Rev. 39, No. 11

Theoretical interpretation

Some examples of experimental results obtainedwith BOL will be discussed later. Two important aidsto analysis and interpretation are firstly, the 'exact'theory of three-particle processes introduced by theRussian mathematician L. D. Faddeev and secondlytwo simplified models that can be used to obtain anexplanation of the most important collision phe-nomena that is surprisingly good in view of the sim-plifications.Quasi-Free Scattering (QFS), for example, is one of

these phenomena that can be described with the aidof a simplified model, the 'impulse approximation',as a collision between one of the two atomic nucleias a single entity and one of the constituents of theother atomic nucleus. In this model the remainingconstituents only betray their presence throughthe way they affect the momentum of the collidingparticle.The second model is that of Final-State Interaction

(FSI), in which some particles that emerge from theprimary region of the collision remain so closetogether for an appreciable time that they can be con-sidered as a single unit in a description of the primarycollision process. A mathematical description of thismodel has been given by K. M. Watson and A. B.Migdal.

Complete measurements

In the measurements made using BOL special em-phasis was placed on the processes in which the mo-mentum, energy and identity of all escaping particlescould be reconstructed. Such measurements are saidto be kinematically complete. The checkerboard struc-ture was absolutely essential if we were to combineaccurate determination of the momentum directionwith a high detection yield. Almost all the final statesthat occur in a collision were recorded simultaneouslywith BOL; in this respect again the measurements cantherefore be designated as virtually complete. Becauseof the large number of detector telescopes, informa-tion could be obtained on processes occurring inpractically the entire kinematic phase space, sothat the measurements were also complete in thisaspect.

Analysis of the data

In a typical experiment several hundred magnetictapes were filled with data in about a hundred hours.The data was then analysed from various points ofview. The number of detected events as a function ofthe energies of detected particles - energy spectra -was determined for specific angles, while on the otherhand the angular dependence was studied for specific

BOL 305

values of the energy. The results of such an analysiswere compared with theoretical calculations in whichMonte Carlo simulation techniques had been used.This enabled an experiment to be simulated on thecomputer in accordance with a theoretical model, inwhich due allowance was taken of the particular cir-cumstances of the experiment, including the experi-mental limitations. The measured results, in con-junction with the theoretical analysis, often led toconclusions that would have escaped if we had used aconventional experimental arrangement with only afew detectors.The studies made with BOL have so far led to six

dissertations, fifteen theses and dozens of publica-tions and conference contributions. Other publica-tions are being prepared.

Examples of research results

Reactions with three nucleons

The collision process between a proton (p) and adeuteron (d), in which two protons and a neutron (n)emerge (p + d - ppn), was studied for two collisionenergies. First, the regions of the kinematic phasespace in which the quasi two-particle processes QFSand FSI predominate were analysed. This means thatin the computer analysis of the data kinematic con-ditions were selected in which mainly QFS and FSIcould be observed (fig. 4) [5]. Secondly, as theanalysis developed further, the process was studied inthe kinematic regions that are remote from the regionswhere QFS and FSI predominate. Very few measuredresults were available at the time for these regions,where in comparisons with nurrierical calculations thegreatest sensitivity to oversimplification of thenucleon-nucleon potential might in fact be expected.Finally, we made an analysis for almost the entirephase space, with measured results reproduced infour-dimensional 'arrays' corresponding to the fourkinematic degrees of freedom of the reaction.There was some surprisingly good agreement be-

tween the measured results obtained with BOL andthe theoretical computations based on Faddeev'sscheme, carried out by Utrecht theoreticians [6], butthere were also some very marked differences. It is notyet clear whether these differences are primarily a con-sequence of terms in the nucleon-nucleon potentialthat were neglected in the calculations, or whether thedifferences are more fundamental. We hope thatfurther calculations will provide a more detailedanswer to this question.

[5] B. J. Wielinga et al., Lelt. Nuovo Cim. 11, 655, 1974, andNud. Phys. A261, 13, 1976. '

[6] W. M. Kloet and J. A. Tjon, Nucl. Phys. A210, 380, 1973.

306 R. VAN DANTZIG Philips tech. Rev. 39, No. 11

H+d-P1P2nTd=26MeV

o

Fig. 4. Isometric representation of the results of a measurementmade with BOL. Protons were bombarded with deuterons at anenergy of 26 MeV. Two protons PI' P2 and a neutron n emergedfrom the nuclear reaction. Both protons were observed in coinci-dence over almost the entire range of angle and energy; the angleand energy of the neutron could then be calculated. The observa-tions are represented here as a 'Dalitz piot': the number of eventsas a function of the relative energy between one proton and theneutron (Tpln) and between the other proton and the neutron(Tp2n), integrated over the two remaining degrees of freedom inphase space. The high peaks in the figure correspond to FSI proces-ses: final-state interaction for a low relative energy between protonand neutron (Tpln '" 0 or Tp2n "" 0) and between proton and proton(Tp1n + Tp2n "" maximum, i.e. TpIP2 '" 0).QFS processes appear inthe figure as raised bands along lines of constant relative energy(Tp1n = Tp2n = TplP2 = 4.5 MeV).

Reactions with four nucleons

In the case of three nucleons it was possible to usethe 'exact' method of calculation based on Faddeev'sscheme to make a quantitative comparison between atheoretical prediction and an experimental result, butthis cannot yet be done in the case where there arefour nucleons. For the present, analyses of reactionswith four nucleons are therefore mainly intended togive a qualitative grasp of the phenomena, preferablyindependently of any theoretical models. In addition,phenomenological models are used, such as the QFS,FSI and 'transfer' models. The transfer models de-scribe the exchange of a constituent between collidingnuclei: 'pick-up' (from target nucleus to incident par-ticle) and 'stripping' (vice versa).In the reaction p + 3He - dpp, for example, in

which p-p and p-d pairs were detected with BOL incoincidence, it was found that some of the effectscould be explained by assuming that the process takesplace in two stages (a sequential model). On a moredetailed examination, however, it was found thatthere were substantial deviations from this model asthe result of contributions from other reaction mech-anisms. We therefore investigated the relationshipbetween different reaction mechanisms as well as theirrelative importance. In particular we tried to find outwhere these processes were located in the kinematicphase space and how far they extended.

In the reaction d + d - dpn we observed that quasi-free scattering between one deuteron and one nucleonof the other deuteron is an important feature overalmost the entire phase space. The measured resultswere compared with theoretical calculations in whichthe most important reaction mechanisms were takeninto account.

Another study of four nucleons was made with theaid of reactions on various atomic nuclei, which wereinduced by 3He. This gave indications of a neutronpick-up process, in which an unstable excited state of4He nucleus is formed, which then decays into aproton and a triton or into two deuterons. The twoend-products were detected in coincidence.

Disintegrating 3He

If a 3He nucleus is allowed to collide with an atomicnucleus, the 3He may disintegrate into a deuteron anda proton or into two protons and one neutron. Withthe BOL system the charged particles could be de-tected in coincidence. Correlations in energy anddirection of the 3He disintegration products were usedto study the nature of the disintegration process andthe state of the (heavy) residual nucleus after the col-lision, for gold, nickel, aluminium, carbon and beryl-lium nuclei. If the residual nucleus continued toremain in the ground state or in a low excited state thedirectional correlations showed that the 3He hadalready disintegrated at the outer edge of the nucleus.

In this study proton pairs were also detected withstrong FSI. These showed up in one detector telescopeas 'double hits' and could be considered as a di-proton - i.e. an unstable 2He nucleus - but becauseof the structure of the checkerboard detector theycould still be analysed as a two-particle coincidence- but one with a small relative angle.

Reactions with three clusters

The description of reactions involving many nu-cleons can be simplified if most of the nucleons occurin clusters, which - during the process - can be con-sidered to a good approximation as single particles.If moreover the reaction process takes place in twostages (the first stage is the formation of a metastablenuclear state with well-defined quantum numbers, thesecond is the decay of this state), detailed informationcan be obtained about the primary reaction process byobserving the directional correlations of the decayproducts.

For example, inelastic scattering of deuterons wasused to bring 12C nuclei into an excited state at9.3 MeV and the decay from this state into 4He and8Be nuclei was then studied. The results were ap-preciably different from a theoretical analysis, using

Philips tech. Rev. 39, No. 11

Fig. 5. Results of measurements made with BOL, in which 9Benuclei were bombarded with deuterons. The nuclear reaction mayproceed via intermediate states (resonances) in SBe or via states inLi, in both cases giving a triton (t) and two 4He nuclei (a) as end-product clusters. a) Dalitz plot (for explanation see caption tofigure 4). b) The Dalitz plot in (a) viewed from above. Here thebands A-E correspond to resonances of the intermediate nucleussBe and bands I-IV correspond to resonances of the intermediatenucleus 7Li. Where resonance bands intersect, the nuclear reactionproceeds simultaneously via different intermediate states. In theoverlapping areas the interference of these different processes canbe studied. In the areas of the Dalitz plot where there is no overlapa process via one of the two intermediate nuclei can be studiedseparately. c) Energy spectrum for the process taking place via thesBe intermediate nucleus (n number of events as a function of therelative energy Taa). A similar spectrum can be given for theprocess that takes place via the 7 Li intermediate nucleus.

the 'Distorted Wave Model', made in a joint studywith the Eindhoven University of Technology [7].Studies were also made of reactions in which a deu-

teron collides with a 9Be nucleus and can form, withthe emission of a triton, an BBe nucleus in variousexcited states (fig. 5), which decay into two 4Henuclei [B]. From the relations between the energiesand directions of the triton and the two 4He nucleiwe obtained a great deal of information about the ex-cited BBe intermediate states. The contributions fromvarious reaction mechanisms were analysed, with par-. ticular attention to the relationship between them.For example, we made measurements on the (d,t)reaction, in which the deuteron picks up a neutronfrom the 9Be nucleus, the residual nucleus BBe thendecaying into two 4He nuclei, and compared it withthe (d,4He) reaction, in which the incident deuteronpicks up a deuteron from the 9Be nucleus, forming anexcited intermediate state in 7Li that decays into atriton and a 4He nucleus. The end-products are thesame in both cases, so that the two processes arequantum-mechanically indistinguishable and there-fore interfere. So as to compare the results of thisinvestigation, corresponding reaction processes werealso studied using reactions of protons with 9Be.Many other reaction processes have been studied

with the help of BOL. One example is the reaction ofdeuterons with 9Be, which produces a proton, a4He nucleus and a 5He nucleus. This particular studyrevealed a previously unknown state in lOBe.

[7( B. J. Verhaar, W. C. Hermans and J. E. J. Oberski, Nucl.Phys. A195, 379, 1972.

[s( L. R. Dodd, M. A. A. Sonnemans and R. van Dantzig, Lett.Nuovo Cim. 12, 597, 1975; see also M. A. A. Sonnemans,J. C. Waal and R. van Dantzig, Phys. Rev. Lett. 31, 1359,1973.

BOL 307

9Be+d_taaTd=26:3MeV

o

o

a

b

..o

30MeV

n

l00r--------------------------.98e+d-t +B8e-taa

Td =26.3MeV

t

5 10 15 20MeV-Tct.Cf. c

308 Philips tech. Rev. 39, No. 11

Some pictures taken during the con-struction of the Philips synchrocyclo-tron:Winding the magnel coils,Delivering some of the magnet iron,The dees,Pari of the radio-frequency system.

Philips tech. Rev. 39, No. II 309

310 Philips tech. Rev. 39, No. 11

Philips tech. Rev. 39, No. 11 311

312 Philips tech. Rev. 39, 312-314,1980, No. 11

Assembling the wiring in the cylinder. There are 2 x 64printed-circuit boards with electronic circuits for the BOL system.

Electronics for nuclear physics

W. K. Hofker

As a result of their work on radiation detectors, thePhilips group at IKO found that they had to pay moreand more attention to 'electronics for nuclear physics',i.e. the design of the electronic circuits for processingthe signals originating from these radiation detectors.

In about 1950, when the signals to be processedcame from Geiger-Müller counters, the electronic cir-cuits were still quite simple, even for that time. The

D,. I,. W. K. Hofket is with Philips Research Laboratories, Amster-dam Department.

pulses only had to be counted [1); their magnitudesdid not have to be measured. Even then, however, itwas important to ensure the smallest possible deadtime, i.e. the time immediately following a signalduring which the instrument cannot handle anothersignal. However, the dead time is determined both bythe dead time of the counter tube and by the deadtime of the electronic circuit, and optimization ofthese dead times was another important subject ofstudy at the time [21.