SOURCE OF MONOCHROMATIC PHOTONS DRIVEN BY POSITRON IN-FLIGHT ANNIHILATION USING INTERNAL TARGET OF THE STORAGE RING VEPP-3 L.Z. Dzhilavyan 1, S.I. Mishnev 2, V.G. Nedorezov 1, D.M. Nikolenko 2, I.A. Rachek 2, D.K. Toporkov 2,3 1 Institute for Nuclear Research RAS, Moscow, Russia 2 Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia 3 Novosibirsk State University, Novosibirsk, Russia

The project combines two main components: A. The method to produce quasimonochromatic photons using for positron beams with narrow energy spreads and small transverse emittances annihilation in flight in relatively thin targets with low Z. Method "A" was realized on the extracted beams of accelerators in a number of scientific centers, including the INR RAS (Moscow). B. The method to use in positron (electron) storage rings internal super-thin targets with possibility to achieve their effective thicknesses up to (0.1  0.2)X 0 and with keeping very high qualities of beams, circulated through these targets. Method "B" is under development and realization in the INP SB RAS (Novosibirsk).

In LS for two-photon annihilation and bremsstrahlung there are presented below: designations for used values: r 0 -”classical radius” of electron E  E pos - total positron energy;  - electron rest energy;  E/  ; E  - photon energy; k a  E  /  ; (k a - photon energy in  - units for annihilation photons, k a1 - and k a2 - for “hard” and “soft” annihilation photons, emitted in the same act of two-photon annihilation); k b - photon energy in  - units for bremsstrahlung photons;   - (and d  -) angle (and element of solid angle) of photon emission with respect to positron direction;  a, (  a1,  a2 ) angles of annihilation photon emission;  b - angle of bremsstrahlung photon emission;   ch  (1/  )- characteristic angle for both processes of photon emission; relations between energies and angles of annihilation photons: k a  /[    0.5 cos  a ] ; (1) {so it is possible to change continuously k a, changing  (or  a )} k a max  1/{1  [(  1)/(  1)] 0.5 } .  k b max  1.5; k a min  1/{1  [(  1)/(  1)] 0.5 } .  It is possible to tag the event, caused by the “hard” photon, using registration of the “soft” photon k a1  k a2 ; (k a1  k a2 )  (   1)  2k a symm ;  a1   a2  ; cross sections for annihilation in comparison with those for bremsstrahlung: d  a /d  a )  [(r 0 ) 2 /2][(  )/(  )] 0.5 {  [2/(  (  2  ) 0.5 cos  a ) 2 ]  [(  )/((  )(  (    ) 0.5 cos  a ))]   [(  (    ) 0.5 cos  a )/((  )(  (    ) 0.5 cos  a ))] 2 } ; (2) or approximately: at  a  ~(  /  ) (d  a /d  a )  (r 0 ) 2  /(  2 (  a ) 2 ), (2) at  a  b  0 (d  a /d  a )~ , while (d  b /d  b ) grows steeper than  2 (2  ) at (1/  )  (2/  ) 0.5 : (d  a /d  a )  [(r 0 ) 2 /  (  a ) 2 )], while (  b /d  b )~1/(  b ) 4 ), (2  ) {so off-axial collimation helps to reduce relatively background from bremsstrahlung}.

for annihilation cross sections: (d  a /dk a )  [(  (r 0 ) 2 )/(    )]{  2  [(  )/(k a  k a  )]  [  /(k a  k a  )] 2 }. (3) Photons with k a  k a max  0.5 are emitted at  a  1/ , when (d  a /dk a ){k a  k a max  0.5}  0.5(d  a /dk a ){k a max } and this sets the level of “monochromaticity” of the method: at any E  intrinsic width ~ 250 keV, and, for example, for photon energies of ~600 MeV, this width is better in about an order of magnitude than the energy spreads in competitive methods of "photon monochromatization". in LS at k a symm  k'  k a1  k''  k a max, we have integral annihilation cross section:  a ( ,k',k'')  [(  (r 0 ) 2 )/(    )]{  (k''  k')[2  (1/k'k'')  (1/((  k')(  k'')))]   ((    )/(  ))ln[(k''(  k'))/(k'(  k''))]}; (4) at axial collimation, i.e. if k''  k a max and k'  k a max  k, we have:  a ( ,k'  k a max  k,k''  k a max )  [(  (r 0 ) 2 )/(    )]   {  2·  k[  [(    0.5 )/(  k(  2  0.5 ))]  [(    0.5 )/(  k(  2  0.5 ))]]   ((    )/(  ))ln[(  k(  2  0.5 ))/(  k(  2  0.5 ))]}; (4') at  1 and  k  (~1):  a ( ,k'  k a max  k,k''  k a max )  [(  (r 0 ) 2 )/k a max ]ln[  2(k a max  k')]; (4  ) at  k  (1/2):  a ( ,k'  k a max  (1/2),k''  k a max )  [(  (r 0 ) 2 ln2)/  ]. (4  ) All presented above cross sections for positron annihilation are for a single electron. The cross sections per atom are proportional to Z, while the cross sections of positron bremsstrahlung per atom are roughly proportional to Z(Z  ). So the best element for annihilation targets is hydrogen.

For annihilation and bremsstrahlung photons

d  a /dk a ){k a }  solid lines. For curves 1; 2 and 3, respectively,   20; 40 and 60. For   60 it is shown also non-uniform axis of abscissae – the axis  a. Dashed curve – (d  a /dk a ){k a max }  2  (r 0 ) 2 /k a max

There are radiation losses in magnetic fields in a storage ring. To compensate radiation energy losses and (if necessary) to accelerate additionally positrons (electrons) HF resonator, operating at a frequency f R  k R f 0 (f 0  frequency of beam revolution, k R – integer), is installed. In a storage ring with super-thin target positrons (electrons) perform damped oscillations with their times T D >>T 0  (1/ f 0 ). This leads to contraction of a beam (having its lifetime T L >>T D ) in a rather small region of trajectory space. In many nuclear physics experiments it is possible to consider such a beam as continuous one. Usually for precision studies in nuclear physics with positrons (electrons) it is necessary to have small thicknesses of targets (typically ~10  3 X 0 ). With supper-thin targets it is possible to win up to two orders of magnitude for target thicknesses, keeping very high quality of beams. It is known about studies of electron scattering on atomic nuclei with inclusive (e,e') and exclusive (e,e'X) reactions carried out at storage rings with internal super-thin targets (INP SB RAS (Novosibirsk)). These experiments were permanently in the spotlight of the Seminar EMIN (see, e.g., report by Dr. I.A.Rachek in the present Seminar). It is possible to study also electronuclear reactions, in which only emitted in such reactions particles X are registered. It is important that internal super-thin targets are very "transparent" for low-energy charged particles X, if it is necessary to detect them in electronuclear or (e,e'X) reactions. Moreover, on the electron beam in storage ring ADONE in Frascati with the internal argon-jet target there has been realized one of additional directions of internal super- thin target applications, namely, production of tagged bremsstrahlung photons with usage of nearest to internal target one of the bending magnets of this storage ring. It should also be pointed out that at least in one work, performed in the INP (Novosibirsk), the part of the accumulated “statistics” was received in this way.

But until now using of internal super-thin targets in storage rings for production of annihilation photons was not carried out, although it is very interesting to examine the possibilities of such direction of their applications. Expected advantages: If the transverse and energy acceptances of the used circular accelerator-injector for a storage ring are sufficiently large (as for the new positron source at VEPP-3), then due to "radiative cooling", providing growth of the positron density in the trajectory space, the current of injected positrons with the desired geometrical and energy parameters, can be significantly increased in comparison with the current from the injector, where there is no such a growth. Due to the "radiative cooling" in the storage ring itself effective thicknesses of annihilation targets and coefficients of conversion from positrons to quasimonochromatic annihilation photons increase significantly. The produced beam of annihilation photons acquires a quasicontinuous character, which can facilitate the work of the used "electronics". Strongly improved background conditions are provided. The most "attractive" targets for the best relationship between annihilation and bremsstrahlung, namely, pure hydrogen targets become possible. In this method expected profits are several orders of magnitude in intensity of the quasimonochromatic photons and several times in ratio of the intensities of annihilation and bremsstrahlung photons produced in targets with different Z. According to estimations the VEPP-3 answers in a great extent to demands for such a source. The table lists some important parameters for this project

central orbit perimeter , m frequency of beam revolution f 0, MHz4.03 period of beam revolution T 0, ns frequency of HF resonator f R1, MHz8.06 frequency of HF resonator f R2, MHz72.54 time of injection, s12 stored positron current I, mA~60~60 energy of positrons E, MeV~600 damping time of beam oscillations T D, s for vertical betatron oscillations; for radial betatron oscillations; for phase oscillations 0.161; 0.175; H - target thickness X H, atoms/cm positron beam lifetime T L, s~30 average luminosity L, cm  2 ~6  flux of annihilation photons N , s  1 at    3 mrad at 5   10 mrad ~2  10 5 In one sub-cycle of injection with duration ~2 s there will be injected in VEPP-3 ~2  positrons with energy ~500 MeV, what will give current ~10 mA in VEPP-3. So it will take 12 s for the full cycle of injection to reach the positron current I~60 mA. Positrons will be additionally accelerated up to energies ( ) MeV during time about 10 s. The positron lifetime in the VEPP-3 with H-target, having the thickness atoms/cm 2, is ~30 s. In these conditions the optimal time for the data measurement is ~33 s, taking into account also the time ~5 s for returning to the injection energy. After that the total cycle of operation starts again. But even in these conditions “macroscopic” duty factor is on the level of tens percents. At injection of positrons, which already have necessary energies, this duty factor can be essentially increased. In pointed out conditions the average over the total cycle of operation positron current will be ~13.3 mA, and the average luminosity L  8  cm  2. In the table the values of N  are given for the positron energy E  600 MeV. Table. Parameters of VEPP-3, target and beams

Arrangement of equipment for production of quasimonochromatic annihilation photons at VEPP-3: the dipole (D1-D3) and quadrupole (Q1-Q3) magnets; the elements of the vacuum chamber; several turbo-molecular pumps with pumping speed (1  2)  10 3 l/s; the elements of beam diagnostics; two high-vacuum valves; etc.; hatched region may be used for targets and detectors in carried out physics researches; at off-axial collimation targets for physics researches having central holes may be useful to exclude main part of background from bremsstrahlung, produced by positrons in internal target itself

Estimated spectrum of bremsstrahlung and annihilation photons at   <3 mrad for: positron energy E  600 MeV; luminosity L  6  cm -2 s -1 ; binning width keV

Such a spectrum may be essential for solving tasks, connected with searches for narrow resonances in photonuclear processes (see, e.g., report by professor L.V.Fil’kov in the present Seminar). At estimations of energy spreads of real annihilation photon beams, except for the pointed out above intrinsic spread ~  /2, it is necessary from different possible spreads for super-thin target to take into account, first of all, energy (~3  10  4 E) and angular (horizontal  ~7  10  4 rad and vertical  ~1.5  10  4 rad) spreads of circulating in VEPP-3 positron beams. Taking into account expression of interdependence (1) (from which at  ~(1/  ) we have  k  [  2 (  ) 2 /2]) we receive in these estimations instead of the intrinsic spread ~250 keV at E  600 MeV the resulting characteristic energy spread of real annihilation photon beam in VEPP-3 ~350 keV (still rather good!).

Estimated flux of bremsstrahlung and annihilation photons at   <3 mrad for: positron energy E  600 MeV; luminosity L  6  cm -2 s -1 ; binning width keV

The grave shortcomings of the shown the spectrum and the flux of the expected photons are connected with the relatively high level of bremsstrahlung background. However, influence of this background may be essentially decreased at off-axial collimation.

Estimated spectrum of bremsstrahlung and annihilation photons for: off- axal collimation with   from 5 to 10 mrad; positron energy E  600 MeV; luminosity L  6  cm -2 s -1 ; binning width keV

Estimated flux of bremsstrahlung and annihilation photons for: off-axal collimation with   from 5 to 10 mrad; positron energy E  600 MeV; luminosity L  6  cm -2 s -1 ; binning width keV

Presented in figures and in table beam characteristics are suitable for solving different problems: Searches for narrow structures in cross sections of photo-processes. Researches of  -mesic nuclei, described, in particular, in the present Seminar in report by Valeriy Polyanskiy. Studies of the virtual photon spectra, produced in processes with relatively small transfers of energy and impulse and accompanied by reactions of photofission, for example, 238 U( ,F), at energies of incident photons in the region of hundreds MeV, where for fission of nuclei-actinides there was found in joint experiment of INR RAS (Moscow) and INP SB RAS (Novosibirsk), that total cross sections are essentially higher than, what are predicted by so-called “universal curve”, in result, as it is supposed, of influence of unlinear effects of quantum electrodynamics in photonuclear processes, what promises good perspectives for interesting investigations. In such investigations acts of fission in relatively thin detector of fission fragments are registered in coincidence with signals from spectrometer of total absorption from photons (or, maybe, together with electrons and/or positrons), emitted in the forward direction. For detector of fission fragments it is suggested to use parallel-plate gas detectors. Measurements of total photofission cross sections of nuclei-actinides, for example, 238 U. In these cases it is possible to use detectors of fission fragments on the base of thin polycarbonate films with automatized counting of tracks from fission fragments. ___________________________________________________________________ It is important to point out that the proposed scheme for a photon source is almost completely identical to the sceme of installation for search of a new vector boson А' in the experiment, which was also proposed for the VEPP-3.

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