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there are new possibilities for strong cp problem solution , which allow the existence of axions with a large mass (  1 mev ) , while their interaction with ordinary particles remain at the level of the invisible axions . the models rely on the hypothesis of mirror particles @xcite and susy at the tev scale @xcite . the existence of these heavy axions is not forbidden by the laboratory experiments or astrophysical data . this article describes the experimental search for 5.5 mev solar axions , which can be produced by @xmath4 reaction . axion flux should be proportional to the @xmath5-neutrino flux , which has been estimated with high accuracy @xcite . the searches have been performed with the use of bismuth orthogermanate @xmath6 ( bgo ) scintillation and bolometric detectors . the solar axions are supposed to interact with atoms via the reaction of axioelectric effect @xmath7 . this kind of interaction is governed by @xmath8-constant and the cross section depends on the charge as @xmath9 . from this point of view the bgo detector is a very suitable target , because of the high @xmath10 of bismuth nucleus . the high energy solar axions and axions from a nuclear reactor have been looked by the borexino @xcite , the cast @xcite and the texono @xcite collaborations . this paper is based on the results of obtained with bgo scintillation @xcite and bgo bolometer detectors @xcite . the 5.5 mev axion production probability ratio @xmath11 depends only on the isovector axion - nucleon coupling constant @xmath12 ( see @xcite and refs therein ) : @xmath13 ^ 2\left(\frac{p_a}{p_\gamma}\right)^3 = 0.54(g_{an}^{3})^2   \left(\frac{p_a}{p_\gamma}\right)^3.\ ] ] where @xmath14 and @xmath15 are , respectively , the photon and axion momenta and @xmath16 is isovector nuclear magnetic momenta . at the earth s surface the axion flux is : @xmath17 where @xmath18 is the @xmath5-neutrino flux . the cross section for the a.e . effect was calculated in @xcite . bgo crystal with mass 2.46 kg contains 1.65 kg of bi . the crystal was shaped as a cylinder , 76 mm in diameter and 76 mm in height . the detector signal was measured by an r1307 photoelectron multiplier , which had an optical contact with a crystal end surface . the external @xmath19 activity was suppressed using a passive shield that consisted of layers of lead and bismuth ( @xmath20 ) with the total thickness @xmath21 110 @xmath22 . the setup was located on the earth s surface . in order to suppress the cosmic - ray background we used an active veto , which consisted of five @xmath23 cm plastic scintillators . the measurements were performed over 29.8 days . the energy spectrum of the bgo detector in the range of ( 011 ) mev is shown in fig . 1 . in inset the calibration spectrum measured with pu - be neutron source is shown .    in the spectrum , one can identify two pronounced peaks at 1.460 mev and 2.614 mev ; these are due to the natural radioactivity of the @xmath24 and of @xmath25 from the @xmath26 family . the positions and intensities of these peaks were used for monitoring of time stability of the detector . four cubic ( @xmath27 @xmath28 ) bgo bolometers , containing 1.65 kg of bi , were arranged in a four - plex module , one single plane set - up . the scintillation light was monitored with an auxiliary bolometer made of high - purity germanium @xcite . the detector was installed in the @xmath29 dilution refrigerator in the underground laboratory of l.n.g.s . and operated at a temperature of few mk . the four crystals were housed in a highly pure copper structure , the same described in @xcite . neutron transmutation doped ( ntd ) germanium thermistor was coupled to each bolometer , ntd acts as a thermometer : recording the temperature rises produced by particle interaction and producing voltage pulses proportional to the energy deposition . details on electronics and on the cryogenic set - up can be found in @xcite . the detector was operated for a total live time of 151.7 days . the different light yield of interacting particles was used to discriminate @xmath30events from @xmath31 ones . this allowed to strongly increase the sensitivity , due to rejection of all @xmath30events in the region of interest . the total statistics in the range of ( 0.3 - 4 ) mev for @xmath31 events are in shown in fig.[muratova_valentina_fig2.pdf ] . the most intense gamma lines are produced by @xmath32 decays . in first approximation , the energy resolution of large mass bolometric detector is independent of the energy - the fwhm is @xmath33 kev at 2614 kev ( @xmath34 ) and @xmath35 kev at 570 kev ( @xmath32 ) . the expected number of axioelectric absorption events are : @xmath36 where @xmath37 is the axioelectric effect cross section ; @xmath38 is the axion flux ( [ fluxa ] ) ; @xmath39 is the number of bi atoms ; @xmath40 is the measurement time ; and @xmath41 is the detection efficiency for 5.5 mev electrons . the axion flux @xmath38 is proportional to the constant @xmath42 , and the cross section @xmath37 is proportional to the constant @xmath43 . as a result , the @xmath44 value depends on the product @xmath45 . the experimentally found condition @xmath46 imposes some constraints on the range of possible @xmath47 and @xmath48 values .    the range of excluded @xmath49 values is shown in fig .  3 , at @xmath50 the limits @xmath51 @xmath52 were obtained for bgo scintillating and bgo bolometer detectors , correspondingly . these constraints are completely model - independent and valid for any pseudoscalar particle with coupling @xmath53 less than @xmath54 .    for hadronic axion model with concrete relation between @xmath1 and @xmath48 one can obtain a constraint on the @xmath8 constant , depending on the axion mass ( fig . 3 ) . for @xmath48 = 1 mev , this limit corresponds to @xmath55 at @xmath56 . figure 3 also shows the constraints which were obtained in the borexino experiment for 478 kev @xmath57 solar axions @xcite and in the texono reactor experiment for 2.2 mev axions produced in the @xmath58 reaction @xcite . borexino coll . reported new more stringent limits on @xmath8 coupling for 5.5 mev solar axions @xcite . unlike our work , these limits on @xmath8 coupling were obtained in assumption that the axion interacts with electron through the compton conversion process .    in the model of the mirror axion @xcite an allowed parameter window is found within the p - q scale @xmath59 gev and the axion mass @xmath60 1 mev . the limit ( [ limgaegan ] ) may be represented as a limit on the value @xmath61 by taking the following relations into account : @xmath62 and @xmath63 . for axion masses about 1 mev , the limit is @xmath64 , which is close to the lower bound of mirror axion window . our results set constraints on the parameter space of the cp - odd higgs @xmath65 , which arise in the next - to - minimal supersymmetric standard model due to the spontaneous breaking of approximate symmetries such as pq - symmetry . the corresponding exclusion region can be obtained from fig.[muratova_valentina_fig3.pdf ] using the conversion @xmath66 where @xmath67 is the coupling of the cp - odd higgs to fermions and @xmath68 is the gauge coupling . the limit ( [ limgaegan ] ) translates into @xmath69 mev for @xmath70 mev , which is compatible with the limits obtained in reactor experiments exploring compton conversion . z.  berezhiani et al . , phys . b500 , 286 ( 2001 ) . l.j .  hall and t. watari , phys . d70 , 115001 ( 2004 ) . g.  bellini et al . , ( borexino coll . ) nature , 512 , 7515 , 383 ( 2014 ) g.  bellini et al . , ( borexino coll . ) epj , c54 , 61 ( 2008 ) . g.  bellini et al . , ( borexino coll . ) , phys . rev . d 85 , 092003 ( 2012 ) . s.  andriamonje et al . , ( cast coll . ) jcap 1003 , 032 , ( 2010 ) . arxiv:0904.2103 h.m . chang et al . , ( texono coll . ) phys .   rev . d75 , 052004 ( 2007 ) . a.v .  derbin et al . , europ .  phys . j. c73 , 2490 ( 2013 ) . arxiv:1306.4574 a.v . derbin et al . , europ . j. c74 , 3035 ( 2014 ) . arxiv:1405.3782 a.r . zhitnitskii and yu.i . skovpen , yad . fiz . , 29b , 995 ( 1979 ) . l.  cardani , s.di  domizio , l.  gironi , jinst 7 , p10022 ( 2012 ) . beeman et al . , jinst 8 , p05021 ( 2013 ) . f.  alessandria et al . , astropart . 35 , 839849 ( 2012 ) . s.  pirro et al . , nucl . methods a 444 , 331 ( 2000 ) . c.  arnaboldi et al . methods a 559 , 826 ( 2006 ) .
a search for axioelectric absorption of 5.5 mev solar axions produced in the @xmath0 reaction has been performed with a bgo detectors . 
 a model - independent limit on the product of axion - nucleon @xmath1 and axion - electron @xmath2 coupling constants has been obtained : @xmath3 for 90% c.l ..