S O V I E T S C I E N T I S T S V I S I T B R I T I S H R E S E A R C H C E N T E R S

Yu. A b o v

In accordance with an agreement between the USSR State Commit tee on the Uti l izat ion of Atomic Energy and the British Atomlc Energy Authority on the exchange of scient i f ic delegations, a group of Soviet physicists visi ted British research centers in December, 1965. The purpose of the visit was to become acquainted with the operation and construction of accelerators and with the research program in e lementa ry par t ic le physics and in high- and low-energy nuclear physics. The delegat ion visi ted the nuclear physics division at Harwe11, the Rutherford High- Energy Laboratory, the nuclear physics laboratory at Oxford University, the nuclear physics laboratory at Darsbury, Manchester University, Cambridge University, an astrophysical observatory, and also several companies which man- ufacture instruments for physics laboratories.

The research work in the low-energy field which has been carried out on "small" accelerators can be divided into two principal groups: 1) research whose main purpose is the obtaining of constants needed for reactor construc- tion; 2) research having no prac t ica l s ignif icance at the present t ime,

One ought to point out the growing interest in the obtaining of da ta needed for fast reactor calculations. The pulsed Van de Graaf generator (IBIS) and the t inear electron accelera tor at Harwell are very suitabte for these studies,

The pulsed Van de Graaf generator, which accelerates particles 3 MeV, produces 10 nsec ~ deuteron or proton pulses with 1 gsec spacing. Current per pulse is 10 mA, Using various targets in this device, one can obtain mono- energet ic neutrons over a wide range of energies. It is used to carry out experiments for the study of angular and energy distribution of scat tered neutrons and y-rays.

The Hurry, e l l l inear e lectron accelera tor can be ca l led an effect ive neutron cross section factory, A U 235 mul t ip l ier , the so -ca l l ed booster, which is instal led behind a mercury target, has a mul t ip l ica t ion coeff icient of 10, The energy of the acce le ra ted electrons is 42 MeV: With a 400 mA current, the number of neutrons per pulse is 1028. The pulse length can be reduced to 1-3 nsec. In addit ion to the booster, there are other targets to which the electron beam can be directed by a pulsed magnet. The pulse length at each target can be different, making i t possibte to perform several different experiments simultaneously. In measurements of effect ive neutron cross sec- tions by t ime of flight, various fl ight paths and mult ichannel t ime analyzers are used. Data put out on tape is anaiyzed with e lectronic computers. Measurements of total effect ive neutron cross sections, of radia t ive capture cross sections, of fission, and of scattering over a wide range of energies make i t possibte to de termine the pa rame- ters of resonance levels,

Measurements of neutron cross sections are atso carried out on the Van de Graaf generator. Recently, a beam of slow neutrons was obtained with HarwelI synchrocyelotron, which accelerates protons to 166 MeV, making i t possible to use this accelera tor also for the study of effect ive neutron cross sections,

The construction of a 70-inch, var iab le -energy cyclotron intended for the study of radiat ion effects has been comple ted at Harwe11. This cyclotron can acce le ra te ions with masses up to 40 mass units. An internal 50 MeV proton beam wii1 have a current of 1 mA (100 /iA in the extracted beam). There are nine targets (one in temat) which are at separate locations. This cyclotron has advantages when compared with MTR-type reactors: i t pro- vides a reduction in exper iment t ime, makes it possible to study effects at doses unat ta inable with the MTIL per- mits one to study effects produced only by charged particles, etc. With this accelerator , i t is planned to produce heavy elemenrs for radiochemistry and for t racer atom techniques. There are plans for the study of-fission fragment mass spectra, of fission fragment angular distributions, of isomer yields in fission, and for other studies of the fission process in heavy nuctei.

Of the work not having pract ica l s ignif icance and performed on "small" accelerators, one should first mention the work of J. Freeman's group, In this work, carried out with a tandem which acce lera ted part icles to 12 MeV,

*Because of supplementary focussing, pulse length at the target is reduced to 1 nsec.

Translated from Atomnaya ~nergiya, Vol. 20, N'0. 6, pp. 530-532, June, 1966. : .............

610

exact ft values were measured for a number of purely Fermi B decays: O +, T = 1 --, O +, T = 1. In this case, the :natr ix e lement for the transition is known and is M F = 4-2 for any nucleus. In other words, the relat ion

a~r M~ 2 It =- coast,

must be fulfi l led, and from that i t is possible to determine the B-decay vector coupling constant G v. To de te rmine f, i t is necessary to measure precisely the max imum energy of the B-spectrum. The max imum energy of S-spect ra is found from the threshold value of the corresponding (p, n) reaction, It is necessary to emphasize an advantage of the tandems: these accelerators make i t possible to obtain h igh-energy particles with high monochromat ic i ty of the acce le ra ted par t ic le beam. The use of a tandem made i t possible for I. Freeman to make measurements of (p, n)- react ion thresholds, and therefore of maximum t3-spectra energies, with an accuracy of 3-4 keV. The ft values for 014, C1 s4, Sc 42, V 4s, Mn 5~ and Co 54 were in good agreement with one another within the l imits of exper imenta l

error. The ft value for A1 z6 was below the overal l average by approximate ly 1-1.5%, which is beyond the l imits of s ta t i s t ica l error. The average ft value was 3123 ~ 7 sec. This va lue was changed by the inclusion of the necessary corrections. From the average ft value, the B-decay vector coupling constant was de termined:

a v = (t,4034 -F 0,0016) �9 10 -49

which gives Gv/G p = 0.978 • 0.001; OCabibb o = 0.21 in comple te agreement with the expected values.

A great deal of attention is being given to the study of isobaric analog states. The existence of isobaric ana- log states has been established even in the heaviest nuclei , which is a very noteworth fact. Analog states may arise in (p, n), (d, n), and (d, p) reactions, in (p, p) and (d, p) reactions. In the case of the (d, n) reaction, the angular distribution of the neutrons is determined by the angular momentum and parity of the state, i .e . , the quantum characterist ics of the state are determined in addition to the Q-value of the reaction. The study of analog states is

being carr ied out in Great Britain on many "small" accelerators.

One can use the (He s, n) react ion to study the energy states of proton-r ich nuclei. Such studies are being ca r - r ied out with the IBIS accelera tor at Harwell. The analysis of angular distributions is being made on the basis of dis torted-wave theory. On t h e Van de Gram generator at Manchester University, which accelerates part icles to 6 MeV, the (He 3, n) reaction is being used for measurements of the Q-value of reactions leading to the formation of mirror nuclei. This same acce lera tor is being used for the study of(HeS; c~) and (d, P7) reactions for the purpose of invest igat ing cross sections and angular correlations.

With the heavy- ion l inear accelera tor at Manchester University, the isomeric state Re 18sin (I = 25/2) has been observed. The isomer was synthesized in the react ion Ta Isl (c~, 2n) Re 183m. The ha l f - l i fe of this isomer is 0.95 msee. The decay scheme was studied with a germanium ?'-spectrometer and with a g-spectrometer . This same ac - ce lera tor is being used for the study of ro ta t ional bands in neutron-def ic ient nuclei in the 110 < 1 < 122 region, par t icular ly in Xe ns-lzz. As yet, rotat ional band have not been observed. If the negative result is confirmed, that wil l be an interesting fact. The work is supposed to be compla ted within the next 3-6 months. In addition, other experiments are being prepared: on coulomb exci ta t ion of h igh- ly ing nuclear levels in various nuclei through the act ion of mul t ip ly-charge ions, on the dependence of r n / F f on the N/A ration for fixed values of the parameter Z2/A, on the dependence of F n / F f on angular momentum, and on elast ic and inelast ic scattering of heavy ions,

12 12 - part icular ly the study of C - C systems.

The research program for the 166 MeV proton synchrocyclotron at Harwell is concentrated mainly on studies of (p, p) and (n, p) scattering, and of proton scat ter ing by nuclei; studies are also being made of the changes in pro- ton polar izat ion during the scattering of protons by hydrogen targets (the synchrocyclotron extracted proton beam can be polar ized to 47.1 ~ 0.49% by scat ter ing from an internal a luminum target; in addition, they have a polar ized hydrogen target) , there is a study of bremsstrahlung in proton scattering, etc. As ind ica ted above, the synchrocyclo- tron is also used as an intense source of low-energy neutrons. Neutrons in a broad energy range are obtained by evaporat ing them from nuclei which are bombarded by high energy protons. The neutron beam intensity per pulse (pulse length 10 nsee) is 5 �9 10 is. It is proposed that the synchrocyclotron be modernized for the purpose of increas- ing the intensity of the internal proton beam from 2 to 10 gA, reducing the pulse length from 10 to 2 nsec, and providing the capabi l i ty of acce lera t ing cx-partieles and He a nuclei in addit ion to protons.

On the proton l inear accelera tor which can acce le ra t e particles to 50 MeV (Rutherford Laboratory), po la r iza - t ion of the protons can be produced in the ion source i tself so that acce lera t ion of a polar ized proton beam takes p lace in the accelerator . Using a polar ized hydrogen target in the polar ized proton beam, they are studying the asymetry in the scattering of 50 MeV protons for the purpose of invest igat ing the spin dependence of p -p in te rac - tions. In addition, they are studying the changes in proton polar izat ion during scattering. They are also studying

611

the scattering of protons by nuclei on this accelerator in order to check the optical m o d e l For the purpose of eliminating the effect of Coulomb interactions, they are studying the scattering of protons by various nickel isotopes. A stack of silicon detectors 13.5 mm thick is being used as a proton detector in this experiment. The instrumental line width of such a detector is 150 keV.

Research in the field of elementary particle physics and high-energy nuclear physics is being carried on main- ly at the 7 GeV proton accelerator of the Rutherford Laboratory (NIMROD). Joint groups from various research c e n - ters in the country take part in setting up experiments. A French 80-centimeter bubble chamber, presently filled with tiquid deuterium, has been installed at the accelerator, and is operating. The interactions of ~r + mesons with deuterium are being studied. A bubble chamber of English manufacture, 150 • 40 • 40 cm in size, is supposed to go into operation in the summer of 1966. The chamber can be filled with hydrogen or deuterium. The Oxford University group has completed preparation of a chamber 80 • 40 • 40 cm in size to be filled with liquid helium. They are planning to install it in the K meson beam to study the production of the hyperfragments AHe a, AHe 3, AH 3.

They are proposing to begin a study of the rr- + p ~ 4 ~ + n reaction for the purpose of measuring the spin of the 4~ ~ meson decay products. Neutron energies can be measured by the t ime-of-f l ight method over a 3-m path with a resolution of ~ 10 -9 sec. Anatysis of the results wi l lbe made by electronic computer.

In Darsbury, they are building a 5.3 GeV electron accelerator. With this accelerator, the following studies are being planned: photoproduction of rr mesons, pair production at large angles to the direction of the y-ray mo- mentum, measurements of recoil proton polarization in electron scattering, production of a K ~ beam for checking Drell's calculations, and studies using spark chambers located in a magnetic field for investigating processes of the y ~ e + 1 y-type.

It is impossible not to point out the value of visits to industrial firms manufacturing equipment for physics instrumentation. The Vickers company demonstrated its electron accelerator, which is used not only in physics laboratories but which also finds wide application in medicine and industry. The Nuclear Enterprises Company manufactures varioustypesofscintillators, semiconductor detectors, and electronic equipment. The company is car- rying on development of a 5 �9 5 cm germanium detector. The E/vII company manufactures television equipment, photomultipliers, and computers. The company is completing development of a photomultiplier with a signat 0.5 �9 10 -9 sec long. The firm also manufactures electronic equipment of the so-called Harwe11 series. This equip- ment, developed and tested at Harwe11, is today the standard equipment of its kind in physics laboratories in Great Britain. The firm is also completing development of a pulse height analyzer with a million channels. The Elliot Company manufactures semiconductor detectors for recording y-rays and charged particles.

The Soviet scientists departed from hospitable England with a sense of deep gratitude to their English col- leagues and to the people who accompanied the delegation.

612