The “BOL” nuclear research project

NUCLEAR

INSTRUMENTS

AND

METHODS

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(I97I) 157-I6o;

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NORTH-HOLLAND

PUBLISHING

CO.

THE "BOL" NUCLEAR RESEARCH PROJECT L. A. C H . K O E R T S * , K. M U L D E R , J. E. J. O B E R S K I a n d R. V A N D A N T Z I G

Institute for Nuclear Physics Research (IKO), Amsterdam, The Netherlands Received 12 N o v e m b e r 1970

A multidetector system has been built,for detailed investigations of nuclear reactions. The equipment is designed to perform kinematically complete coincidence experiments. A survey of the system and its main characteristics is given.

1. Introduction This article is intended to give a general background for a series of papers describing the recently developed nuclear detection system BOL. In kinematically complete nuclear reaction experiments, one wants to measure type, energy, flight direction of, and coincidence relation between the secondary particles induced by a sufficiently intense beam with good energy and angular resolution. For this purpose a multi-detector measuring system has been developed in this Institute. The system described below may contain 64 detection units, measuring all the types of parameters, mentioned above with the very large solid angle of 13°/o of 4~z, an angular resolution of l ° and an energy resolution better than 0.5%. The system was built around a 20 years old synchro-cyclotron which yields external beams of variable energy (48-58 MeV protons, 23-28 MeV deuterons, 62-75 MeV 3He and 48-58 MeV alpha particles) with intensities of the order of only about 0.1 nA for the required 100 keV energy resolution and 1 (mm degree) 2 phasespace. Thanks to the large geometrical efficiency and because accidental coincidence rates must be kept small anyway, experiments with BOL hardly suffer from the low beam intensity. Modern accelerators can produce polarized beams of an intensity comparable with the unpolarized beams now used in BOL. Extrapolating the experience obtained with the BOL-system to coincidence experiments with such polarized projectiles would suggest an exciting new tool in various studies of nuclear states. In this design provisions have also been made for the measurement of gamma ray spectra in coincidence with emitted charged particles. This particular extension might contribute to widen the scope of today's in-beam spectroscopy. The BOL-project started in 1965, had its first check runs in 1967 and came into operation in 1969. Already * Present address: Philips Research L a b o r a t o r y , T h e Netherlands. MARCH 1971

Eindhoven,

the first measurements showed the presence of nuclear processes that would have been very difficult to observe in any other way.

2. Design philosophy Generally formulated, nuclear reaction studies amount to obtaining estimates of probability densities for emission of single particles, pairs of particles, etc. in nuclear collisions, in terms of physically relevant parameters. Simultaneous measurement of such probability functions carl be realized best using a polyhedric arrangement of a sufficient number of solid state detection telescopes, each of which measures energy loss and total energy of an incident particle. The particle type and energy can mostly be deduced from these two quantities. Timing pulses marking the occurrence of impact can be obtained and used to determine coincidence relations. For a given number of detection telescopes (N), other parameters of the system follow through optimalization. The cost is nearly linear with N. The most efficient geometrical arrangement is a sphere with a radius (R) roughly determined by the relation NA = f R z where f is the optimal packing fraction (0 < f < 47z) and A is the detection sensitive area per detector. The linear angular resolution fl = (A/RZ) ", though, would turn out to be unacceptably large for any reasonable N. Inversely, by choosing (on physical grounds) /3 ~ 1/60 (l°), N = f / / 3 2 would be of the order of 104. For these reasons it was tried to partition each detector telescope into n subfields. This turned out to be possible by providing the AEdetectors of the telescopes with perpendicular sets of electrode strips on front- and rear side. Using the electrodes as a coordinate system the resulting "Checkerboard" detector allows, apart from A E measurement, the localization of particle impacts; yet its price is comparable to that of a conventional AE detector. As far as 2-fold coincidence experiments are concerned, the simultaneous use of N Checkerboard telescopes provides effectively about ½N2nZ-pairs of l 57

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L . A . CH. K O E R T S

Fig. I. Model o f the experimental area a r o u n d the synchro-cyclotron at I K O . In one of the beam r o o m s (upperside), the spherical scattering c h a m b e r of the BOL system is situated.

Fig. 2. A view of the BOL system. U p p e r picture: scattering c h a m b e r with nearly all detection units in position. Lower left: control room with console and a PDP8 data processor. Lower right: EL-X8 computer.

THE "BOL"

Be~ipar%icles Energy (MeV) Current extern I(nA) Current in BOL:

5 Quadrupole-,

Ix

NUCLEAR

: : :

d 23-28 200

RESEARCH

p 48-58 60

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PROJECT

4}le 48-58 20

3He 62-75 20

10-3; burst: ,., = 1500s -I

3 horizontal and 3 vertical bending magnets,

6 Fluorescence screens, optical alignment,

3 slits

communicating liquid level system

3 Concentric spheres: aligning and cooling detectors-, vacuum-, aircool, electron.64 holes for charged part. detection units: 13.2% of 411, angular resolution I°; 10 holes for Y detection units Vacuum

(sublimator ion getter pumps): 10 -6 Tort

10-target system; freon expansion cooling: -25 ° 64 Checkerboard detectors: 0.3 mm thick, 88 fields, 20-55 keV noise ~64 Si(Li) p-i-n detectors: ~128 Si(Li) p-i-n detectors:

5 mm thick, < 50 keV noise, 0.2 mm backside window 3.6 mm thick, 0.03 mm backside window

(thin surface barrier detectors: 0.010 mm thick) (Ge (Li) detectors) Faraday cup Position signals 30 ns; Charge-sensitive ampl., clip 1.25 Us ~]~CtFOD0 I

LOG]C

Coincidence system: 80 ns, remote control multiplicity I

Logic: signal * CU, position sign., start ADC, store 72-bit register a

start ~ sequencer o u(0.3 ~s tper channel, ) 2.5 ~s re

] ~l~CtFOn.] I

64 ADC's:

[ ~

72-bits register

~

~

PDP 8 I [ (2)

8192 channels,

100 Mc, 100 ~:s; 192 ADC's:

512 chan., 6 Mc.

~ PDP8 (I) : 4 k, 12 bits, 1.5 Bs memory; ~e~i2h~r~l~: 'DP O _~---q2 DAT~MAC] 7070 tape units . , ~ / ]M-unit N~: 4 k, 18 bits, 20 ~s memory-scope (I) l ~ i n t e r f a c e s I to PDP8(2) and EL-XS, etc. PDP8(2):

[

MON IKOR : Mu i tiprog ramming system with modular asyn~hronous programs runnlng icroscoplcal±y zn para±iel

k_4_k~__~12bits, 1.5 ~s memory; Peripherals: 4 ]Storaeeldisplay . . . . ]Interfaces] to PDP8(1) and EL-X8, etc.

EL-X8

: 48 k, 27 bits, 2.5 l;s memory; Pe~i~h~r~l~: ~-~: 512 k 27 bit words ]Line printer I (10/20 lines per second ~ ]4 Magnetic ta?e]units (CDC) ~-_____.~---------~Papertape punch/reader] --------~Interfaces] to PDPS(1) and PDP8(2)

] [ L-X 8

, N ME

WAMMES: Time sharing system. Multiple access, file oriented, swapping mechanism and flexible memory allocation system for large memory.

Data analysis with WINDOW-processing: Independent I/O programming/Transformation/Counting on drumstore upon selection/Representation on lineprinter and storage display/Level structured programming.

Fig. 3. Schematic survey of the complete BOL system. detectors, able to establish an equivalent number of energy correlations. With the somewhat high, though yet realistic values: N~n~ 10 2, an angular resolution - now given by [ ~ = ( A / n R 2 ) " - near the required value (1 °) can be obtained. As to the data reduction side and its requirements on the electronics of the system, the following can be said. In order to increase reliability and to ease data handling, the obtained data are digitized in the earliest possible stage.

Since energies (up to 90 MeV) have to be measured with a precision of about 1 : 104, 13 bits are necessary for this purpose. An additional 9 bits are used for the A E measurement. For the registration o f the flight direction o f detected particles 6 bits are necessary to mark the detector and 12 bits to specify the position of incidence on the Checkerboard detector. Another 6 bits are necessary to possibly indicate coincidence relations between different events and some more bits to check and control the system during operation. The system has been set up in such a way that it

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may be run routinely by technicians and that the physicist can have easy access to the produced information. In view of the many parameters, this could only be done by incorporating computers. To this purpose two small processors (PDP8) connected directly to a medium size computer (EL-X8) have been coupled to the detection system. 3. Realization

Fig. 1 shows an outline of the experimental area where the BOL detection system, developed along the principles discussed above, is placed; fig. 2 shows pictures of the actual system. A schematic survey of the system is given in fig. 3. In each of the 64 detector telescopes the 300 pm Checkerboard detector gives both analogue (AE)and logic (position) signals. For cases where the Checkerboard detectors are too thick, even thinner AE-detectors (0.0l mm) can be placed before them. Reversely, for particles with larger ranges than the thickness of the 5 mm Si(Li) detectors, these can be stacked with

one or two 3.6 mm Si(Li) detectors with very thin ( < 0.03 ram) backside windows. Each detection channel is a fully independent hardware data conversion channel including all electronics necessary for processing fast timing and analogue pulses, analogue to digital conversion, logical control and buffering of the final digitized information. The computer system allows recording of all data (or those obeying preset criteria) on magnetic tape for later off-line analysis. Fig. 3 also mentions several software packages developed for on-line and off-line data handling and for real time control. An extensive list of specifications for the complete system is available on request. The following set of papers, describing the complete system BOL, shows clearly that this project could only be realized thanks to the outstanding effort of the technical staff and students of the Institute. The authors are very much indebted to Prof. A. H. Wapstra for his encouraging interest and invaluable suggestions during the preparation of this series of papers.