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Published in the Nuclear Instruments LBL-381 1 and Methods .!_2.2, 575 - 585 (1.974/ (" .'
A BREMSSTRAHLUNG-IDENTIFICATION TECHNIQUE FOR COSMIC-RAY ELECTRONS AND POSITRONS
Andrew Buffington, Charles D. Orth, and George F. Smoot
September 19, 1974
Prepared for the U. S. Atomic Energy Commission under Contract W -7405-ENG-48
For Reference
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This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. ~\, u u \,) 0 u u .j 6 0 "'"' 2
NUCLEAR INSTRUMENTS AND METHODS 122 (1974) 575-585; © NORTH-HOLLAND PUBLISHING CO.
A BREMSSTRAHLUNG-IDENTIFICATION TECHNIQUE FOR COSMIC-RAY ELECTRONS AND POSITRONS
ANDREW BUFFINGTON, CHARLES D. ORTH and GEORGE F. SMOOT
S"pace Sciences Laboratory and Lawrence BerkeleJ!. Received 19 September 1974 •. We introduce a new technique to separate, energetic\+ and e using this,.ltechnique f()r cosrn'ic-ray e± spectral measurements in from a background of other particles. The technique utilizes a the energy range froffi:5 to 50 GeV. Calibrations of the apparatus selective trigger, radiation of bremsstrahlung photons by inci at particle accelerators and the method of flight data analysis dent e±, magnetic deflection of the e± after photon emission, and are also described. Our balloon-borne apparatus provides an the separate detection of both thee± and the photons in a shower efficiency of about 50% for the detection of e± and a proton detector. Descriptions are given of a balloon-borne apparatus rejection of about lQ-5. 1. Introduction ground particles is not much greater than 10-3. Placing In this paper we 'introduce a new technique for iden several e ± identification techniques in series can of tifying and separating energetic e + (positrons) and e cours~ provide bette~ 1 background rejection. A single (negatrons, electrons) in the primary cosmic rays while application of our new technique results ,in rejections 5 providing powerful rejection of the plentiful back of about 10~ • ground of heavier particles. Many other techniques for Our technique is based on the simultaneous detection identifying e ± already exist. Cherenkov counters, for both of a particle and its accompanying bremsstrahlung 1 radiation. This radiation is characteristic of e ± but not example, are commonly employed both at accelerators ) 2 of heavier background particles. Although other tech and in cosmic-ray measurements ). •Visual examination 3 niques" also rely .on bremsstrahlung radiation through of shower development ), energy measurement in a 4 detection of the resulting electromagnetic showers, we total absorption device ), arid identification of shower 5 use a, mag!}etic, field)o separate the e± from its radi development profiles ) ha~e also been employed. Recently, experimenters have had success identifying ation before any further shower development has 6 e ± through detection of transition radiation X-rays ). occurred. Two separate showers are then observed at lt has even been suggested that e ± in high energy beams the proper locations in a detector employing heavy could be individwi.lly identified by detecti~g the syn absorbers. It is this unique signature which gives our chrotron radiation they emit while passing through technique its great background rejection. 7 We describe the basic method and theory in more bending magnets ). Some of these techniques have been used to measure detail in section 2. In sections 3 and 4 we describe a .primary cosmic-ray e ± fluxes froni balloons or satel balloon-borne apparatus for the measurement of lites. These measurements should provide a vital probe primary e+ and e- cosmic-ray spectra from 5 to of cosmic-ray origins and history. Unfortunately, the 50 GeV. Related accelerator calibrations and the 8 detailed methods of data analysis are also described. ' reported fluxes span a factor of four at 5 GeV ), presumably because e ± efficiency and background Efficiencies and background are investigated in sections rejection have often been poofly assessed. Accurate 5 anp 6. Limitations of the technique are discussed in assessment of background is particularly important for section 7. The astrophysical results are presented else where13). I.Jf' e + measurements since cosmic-ray protons outnumber e + by a factor of I 000. Because of this large background "' 2. Basic method and theory and the low e + flux, separate e + measurements have 9 10 been reported only below 5 GeV : ). Attempts have Fig. I shows the essential features of our new been made to infer the e +/e- ratio· above this energy "bremss'trahlung-identification" technique. An incident 11 12 from the east-west asymmetry in arrival direction • ) •. e ± emits b~emsstrahlung photo~s while passing Although a single application of any of the previously through a thin lead radiator. A magnetic spectrometer used techniques provides high efficiency for e ± iden separates the e ± from its bremsstrahlung photons, tification, the rejection of strongly interacting back- measures its momentum, and qetermines the sign of its 575 576 A. BUFFINGTON et al. charge. Finally the photons and e ± enter a shower (mefm) 2 factor in eq. (I). This factor is 10- 4 for muons 7 detector where both are observed visually. The spatial and 3 X 10- for protons. This inhibited bremsstrahlung separation between the two showers and their absolute photon emission is what suggested to us that energetic locations in the shower detector are predicted from the e ± could be uniquely identified, .. even in a large back measurements in the magnetic spectrometer. The ground of heavier particles, by simultaneously dete~ting appearance of this characteristic two-shower signature both the e ± and at least one accompanying brems- in the shower detector permits very great rejection of strahlung photon. ~- other particles. The rejection is achieved at some cost Bremsstrahlung photons are emitted at a mean angl~ . in e± efficiency, however, since not all e± have brems qln y strahlung photons meeting the required topological (}rms::::::; --, criteria. We now turn to a detailed discussion of the )' processes involved. The probability that a singly charged particle of mass where y = E/mec2 and q is a quantity of order unity m and total energy E emits a bremsstrahlung photon which depends weakly upon f and Z. For energies with fractional energy fE upon passing through a above 4 GeV, erms is always less than a milliradian. As 14 radiator of thickness dx radiation length is given by ) a result the two trajectories are very close together and separate detection of an e ± and its photons is ordinarily This result ignores the small degradation of E and hence the changing value of /min which would occur in passing through a finite thickness AX of radiator. For Bremsstrahlung radiator rough calculations, one may use the approximate' relation N ~ 1.3 photons radiated per radiation length '"~- Region of high magnetic field ( -5 kG -m) per natural logarithmic interval in E. The efficiency 8 for identifying e ± with such an Magnetic infinitely thick detector is given simply by: -1m spectrometer spark chambers 8 = 1 - exp [ -N(fmin)]. (5) For 90% efficiency, NCfmin) must be about 2. This l implies that a thickness AX of about t radiation length 3 yields N(fmin) = 2 for /min= 10- . Shower detector In practice, the shower detector cannot be infinitely (lead-plate spark chamber) . thick so some bremsstrahlung photons are lost out the rear. There are other processes which also contribute to a Joss of e ± events, such as the interaction of brems Fig. I. Schematic diagram for the technique. strahlung photons before they leave the radiator. BREMSSTRAHLUNG-IDENTIFICATION TECHNIQUE 577 Proper evaluation of e ± efficiency therefore requires a but follows to better approximation merely from the detailed analysis of the particular apparatus used. absence in eq. (2) of any explicit mention of the e ± The radiator degrades the energy of an e ± in produ energy E. cing .bremsstrahlung photons. By definition the average Very great background rejection for our technique energy degradation is just e -.1x for a given incident arises from the unique double-shower signature. The energy E and a radiator of thickness L1X radiation small value for erms means that the photon shower lengths. On the other hand, if we ask from what E on occurs at an accurately predictable place in the shower . tl_le average did a given degraded energy E* arise, we detector. To utilize this fact, it is imperative that the must resort to an analysis dependent on the incident magnetic spectrometer include spatial detectors which spectrum. Consider, for example, the incident power- can determine the precise trajectory of the e ± after its ·-! •. law spectrum bremsstrahlung emission. The e ± shower must occur along this trajectory where it enters the shower detector, dN(E) = AE- j (6) while the bremsstrahlung photon shower must occur dE ' along the tangent defined where the trajectory passes and the quantities through the radiator. It is a comparison of the observed shower locations with these expected locations which Q (E*) = fw dE dN(E) P(E, E*, LIX) (E*)", (7) provides the final background rejection. n E• dE E For practical application, to reduce the number of unwanted background events recorded, a selective where P(E, E*, L1X) is the probability for degradation trigger may be employed. This trigger can require that in the radiator from primary energy E to within dE* a ·single particle traverse the spectrometer and then of energy E*. Using eq. (6) and P(E, E*, L1X):::::: deposit a large amount of energy in a scintillator placed ::::::{(In (EjE*))(.dX/InZ)-l/F(L1Xjln 2)}/E, where F is the 15 behind several radiation lengths of heavy absorber. gamma function ), we obtain Since such a trigger accepts only those events in which significant interaction occurs in the absorber, it thereby eliminates practically all of the muons incident, If we now choose a given E* and ask what the and a large fraction of the pions and protons. If the average energy degradation is for events producing absorber thickness and scintillator threshold are pro an e ± at (or above) E*, the answer is perly chosen, this initial background rejection occurs ~ MAGNET VENT STOCK MAGNET BREMSSTRAHLUNG RADIATOR,.] r. L. SCINTILLATORS S 1 AND S2 ?> o:; c >rj CAMERAS I '"rj -z ....,Cl z0 ~ ~ LEAD PLATE SPARK CHAMBER S3 AND S4 AND ELECTRONICS PHOTOMULTIPLIER TUBES PRESSURE SHELL IN MAGNETIC SHIELDS 0 .5 I M l.5 2 METERS SCALE ELECTRON POSITRON EXPERIMENT Fig. 2. Schematic diagram for the balloon flight apparatus. The figure shows the outer pressure shell, main frame, electronics locations, and detector layout. '-•~ "'f ·-~ BREMSSTRAHLUNG-IDENTIFICATION TECHNIQUE 579 TABLE 1 Specifications of the balloon flight apparatus. Pressure shell (fiberglass glued to aluminium flange) length 4.0 m diameter 2.1 m Gondola weight 1800 kg Magnet coil inner radius . 17.5 em outer radius 31.7cm width 8.0cm turns 14 886 material Ni-Ti, Cu-clad filaments 180 copper to superconducting ratio 1.8:1 maximum operational current 117 A apparent current density 14 600 A/cm2 average magnetic field integral (11 0 A) 5.1 kG m Trigger scintil/ators (all are Pilot Y, 1 em thick) S169 cmx 89 em S2 33 em x 75 em S361 cmx81 em S4 71 em x 76 em Geometry factor (0.084±0.003) m2 sr 1.4 gfcm2 scattering material within spectrometer {0.04 radiation lengths spark chamber gap spacing 1.5 em in each of 4 gaps/chamber spark reconstruction accuracy 0.1 mm/spark chamber Resulting specific curvature accuracy (for Z= I) (80 GV/c)-1 Bremsstrahlung radiator lead 0.32 radiation lengths other gondola material above spectrometer 0.07 radiation lengths Lead-plate spark chamber Active area 69 x 74 em gap spacing 0.5 em 6 aluminum plate gaps 0.02 radiation lengths each 18 lead gaps 0.16 radiation lengths each The first six plates of this chamber were constructed of scintillator. For most of the flight, we also required that · 0.04 radiation length aluminum to delay photon con there be no count in an anti-coincidence scintillator and version and thus aid in the rejection of multiprong no greater than 2.5 times minimum ionization response background (i.e. multiple charged-particle entry). The in the first scintillator. About 10% of the data were other plates were 0.16 radiation length lead, and the recorded without the anticoincidence scintillator \~ total thickness of the chamber was about 3 radiation included in the trigger, but operating a light photo lengths. Four trigger scintillators were used, three in graphed with the spark chambers. Out of 69 good the spectrometer, and one below the lead-plate chamber. events in this sample, only one would have been vetoed We calibrated the lead-plate chamber at accelerators by the anticoincidence scintillat6r. This puts the loss of to verify the feasibility of the basic technique and to data due to the anticoincidence scintillator at about measure the rejection and acceptance efficiencies to be 2%. The loss of data due to the 2.5 times minimum~ described i '1 sections 5 and 6. The chamber was then ionizing upper bound on the first scintillator is expected assembled with our magnetic spectrometer in a gondola to have been even smaller. for balloon flight and flown to an altitude of about Figs. 3 and 4 show respectively an e+ event and a 35 km. Particles meeting our trigger scheme had to proton event from the flight. These pictures clearly traverse the first three scintillators and then deposit at show how the bremsstrahlung-identification technique least 11 times minimum ionization in the bottom achieves its great rejection against proton events. 580 A. BUFFINGTON et al. PRIMARY COSMIC-RAY POSITRON BREMSSTRAHLUNG RADIATOR .3 r.L. POSITRON INCIDENT NO 1 S C t ======:====~Lr-~--:.~-;-·------N02SC I NO 3 SC • 0 :..' ~';08=~ LEAD SPARK CHAMBER I. .. .I - :U 3r.L. J! )>~ BREMSSTRAHLUNG PHOTON POSITRON SHOWER SHOWER Side Front View Fig. 3. Cosmic-ray positron recorded in the,apparatus. The spark chamber format has been re-arranged here and in fig. 4, to put both front and side views of the sparks in the correct spatial relationship. Measured energy is about 5 GeV. PRIMARY COSMIC-RAY PROJON IN SAME APPARATUS NO IS C NO 2 sc NO 3 SC ~ L ~ LEAD SPARK CHAMBER 3 r.l. ( .2Coll.l.J PRONGS CHARACTERISTIC of INTERACTION Side Front View Fig. 4. Cosmic-ray proton recorded in the apparatus. BREMSSTRAHLUNG-IDENTIFICATION TECHNIQUE 581 4. Data analysis About 20% of the protons incident downward on the Film from the flight was scanned using the following apparatus in flight met the trigger criterion. About 25% criteria for acceptable e ±: more triggers were caused by interactions of wide angle particles above the lead-plate chamber. To elim 1) In the lead-plate chamber inate the latter, we rejected any e ± event whose photon (a) only one charged particle incident from above; conversions started before gap 3 of the lead-plate (b) ~o obvious prongs (like those of fig. 4); chamber. Such conversions could occur within the •.J (c) at least one photon conversion. bremsstrahlung radiator, in the spectrometer, or in the 2) In the momentum spectrometer first two gaps of the lead-plate chamber. Our chosen (a) a single trajectory passing through all three cham radiator thickness was a compromise between too few bers; bremsstrahlung photons to get a good identification (b) no more than one extra track visible in the chambers. efficiency, and so many that conversions before the third gap of the lead chamber would cause excessive A prong in the lead-plate chamber was defined as a rejection of the data. roughly co-linear sequence of at least 5 sparks pointing For flight data, typically one event in thirty satisfied back towards the charged particle trajectory. Brems the above selection criteria. These were 10- 2 of the strahlung showers were also required to have sparks in background events, and about half of thee± events. All at least 5 gaps but to be approximately parallel to the of the data were double scanned, and nearly half of the incident trajectory. This 5-gap criterion limited the data triple scanned, for a total scanning efficiency of lead-plate chamber to photons of energy greater than 99%. ';::j E(n; 0 10 MeV, and to a fiducial volume of total thick After the events were selected, their trajectories and ness 2.15 radiation lengths since no bremsstrahlu'JJ.g shower locations were measured, and our computer shower could begin after gap 20. program determined the sign of the particle's charge, its momentum, and the expected locations for the e ± 19 and photon showers in the lead chamber ). Two thirds of the background events failed to have a good fit. The expected double shower locations were then compared with the observed locations on an event-by event basis. Any event whose bremsstrahlung shower was mislocated by more than 1 em and by more than 30% of the double-shower spacing was rejected as background. This final requirement gave at least forty > u times more rejection against the background. c .~ -~ 5. e± acceptance efficiency ~ 40 To determine the e ± efficiency of the apparatus we ! Trigger • exposed the lead-plate chamber and bremsstrahlung radiator (without the momentum spectrometer and 20 £ Trigger + all select ion criteria 0.07 radiation length of gondola material) to 5 and to 15 GeV e- beams at the Stanford Linear Accelerator Center (SLAC). For these calibrations a bending .magriet was located between the bremsstrahlung Incident energy (GeV) radiator and the lead chamber. Its current was set to Fig. 5. Triggering and identification efficiency, as measured at simulate the average bend of the flight magnetic spec SLAC. The hatched curves indicate expected values, as calculated trometer. Two additional sets .of data were taken with by our Monte Carlo program. Top curve (Trigger): trigger efficiency alone. Middle curve (T+ B): expected efficiency for the current in this magnet reduced to bring brems- · events meeting all bremsstrahlung identification criteria; dashed strahlung and e-. showers closer together and thereby portion indicates efficiency for simulated higher energy runs. simulate 30 and 50 Ge V incident e-. Since early Bottom curve (TOTAL): expected efficiency, when the effects shower development depends only logarithmically on of track merging in the lead-plate spark chamber have be<;n the incident e ± energy, these data provided a good taken into account. The higher momenta (30 and 50 GeVjc points) were simulated at SLAC by reducing the current in the approximation to the way real higher energy events spectrometer magnet. would appear in the apparatus. 582 A. BUFFINGTON et al. The e- events recorded at SLAC were scanned for was ascribed to track merging even though other effects e--like topologies to determine the efficiency of the might contribute. To parametrize the departure we bremsstrahlung-identification technique for the various selected the following simple function energies recorded. The data points in fig. 5 show the Probability of m~rging = exp [ -aj(d-b)J, (13) resulting measured efficiencies as a function of incident energy. Most of the e- events without the double where d is the distance in the lead-plate chamber shower topology were ones in which either no photon between the e ± and the bremsstrahlung photon;-tra conversion showed within the fiducial volume of the jectories (proportional to I IE* of the degraded e ±): lead-plate chamber or a photon converted before the Fixed constants a and b are presumably determined by third gap of the lead-plate chamber and caused the the detailed construction and powering scheme of the event to appear as two or more incoming particles. lead-plate chamber. A fit to the data gave a= ( 1.4 ± r' The drop in efficiency at high energies in fig. 5 is ± 0.2) mm and b = (0.3 ± 0.3) mm. Fig. 5 presents the qualitatively what we expect from a phenomenon called results of the Monte Carlo calculations (hatched track merging. Track merging can be described as curves). The bottom curve includes the effects of track follows. When two tracks in our spark chamber are merging. close together, typically within a millimeter, the sparks As shown in fig. 5, the expected efficiency as a func merge, appearing on the photographic record as a tion of energy ignoring track merging was approx single track. When the energy of the incident e ± is high imately constant at SLAC except for triggering effi enough, a bremsstrahlung photon and a degraded e ± ciency. The constancy above 15 GeV reflects the are not sufficiently separated by the magnetic field to assumption that higher energy points were really appear as two independent showers. Thus, for our 15 GeV points with reduced magnetic fields. The spark chamber, track merging sets an upper limit to merging was therefore the only contributer to the drop the incident energy for which the bremsstrahlung in efficiency observed for the simulated high energies identification technique is useful. The gradual fall-off at SLAC. in efficiency which sets this limit is a direct result of fluctuations in the depth at which photon conversion occurs in the chamber, fluctuations in the lateral development of an e ± shower with increasing depth, and variations in e ±-photon separations. To determine what part of the observed drop in efficiency in fig. 5 was due to track merging, we con structed a Monte Carlo computer program to simulate the bremsstrahlung radiation and detection process for our apparatus. It calculated bremsstrahlung radiation according to eqs. (1) and (2) with the known radiation lengths of material in our apparatus, and photon con 20 versions according to the standard cross-sections ) for photon energies greater than E/nin =:' I 0 MeV. Various photon loss mechanisms were considered: photon conversion in the radiator; conversion in the lead chamber prior to the third gap; no photon conversion above 10 MeV in the lead chamber; and. Energy after Bremsstrahlung bremsstrahlung involving such large energy transfer to degradation (GeV) the photon that the degraded e ± was enough deflected by the magnet to miss. the lead chamber. These loss 5 10 20 30 50 100 -mechanisms were responsible, respectively, for 10%, Average incident energy ( GeV) 10%, 20% and 2% loss of incident e ± events. Trigger Fig. 6. Expected efficiencies for the flight apparatus. Top curve ing efficiency was also inserted as a function of incident (T): trigger efficiency alone. Middle curve (T + S): events satis energy according to the predictions of another com fying both trigger and spectrometer criteria, the proper efficiency 21 curve for e-. Bottom curve (TOTAL): events satisfying all puter program ). bremsstrahlung-identification criteria, the proper efficiency curve The departure of the observed data from the effi for e+. The lower energy scale indicates the average incident ciency predicted using the above Monte Carlo program energy at the top of the atmosphere, for a spectral index ofj = 2.8. 0 0 0 BREMSSTRAHLUNG-IDENTIFICATION TECHNIQUE 583 We made a cross-check to verify the validity of the (bottom efficiency curve). Our average efficiency fore track-merging prescription in the slightly different is 77 ± 2%, and our average efficiency for e + is 46 ± 2%. geometry used in flight. This was done by making a We have included an incident-energy scale in fig. 6. separ.ate scan of flight data to select negatively charged This is based on the assumed scaling ratio of 1.4 events which showered in the lead chamber and obtained from eq.(lO) for an incident e ± spectral satisfied all scanning criteria except that a brems index of j = 2.8 and our flight value of LIX = 0.54. This strahl'ing conversion [l(c) of section 4] was not re scaling ratio typically decreases by 0.05 as j is increased quired. Since a negligible number of muons ancf few by 0.2. Such fixed power-law behavior is altered by pions can meet our trigger and prongless criteria, the geoma~netic cutoff effects, which cause rapid departure 673 negatively charged events tltus f6und were pre- from the power-law den~hdence. For our flight data .,sumably almost exclusively e-. 58±3% of these had with a mean 4 GV/c cutoff, fixed scaling cannot be identifiable bremsstrahlung conversions. This compares used for Ji* ;;55 GeV. favorably with the Monte Carlo prediction of 59%. Fig. 6 presents the Monte Carlo efficiency calculations 6. Backgroun~ for the flight geometry as a function of degraded energy In order to measure,'the background from protons measured by the spectrometer. The widths of the hat directly, we exposed the basic apparatus to 1.5 x 10 5 ched curves indicate the possible errors in the predic protons of 4.5 GeV/c from the Lawrence Berkeley tions. The e- events described in the preceding para Laboratory Bevatron. 1.1 x 104 events met the trigger graph are displayed in fig. 7 as a function of degraded criterion and were photographed. A double scan of energy through the measured ratio for e-: f these events disclosed 23 with bremsstrahlung-photon like ,shower activity in the lead chamber. No more than events satisfying all bremsstrahlung-identification one or two of these events could possibly have had criteria R= ----,-..,...... ,------showers properly 'rbcated to be identified as true e ± events satisfying trigger and spectrometer topology events. This calibration therefore established the tech } criteria nique's rejection against protons to be about w- 5 at The associated Monte Ca,rlo,prediction is also shown. 4.5 GeV/c. Again, the agreement b'gtw~en prediction and obser jfo obtain the final measure of background rejection vation is good, thereby providing a flight cross-check for our flight data, we plotted the differences between of the curves in fig. 6. the predicted and the observed locations of the brems Since negative-charged background in the primary strallltmg:ffihow(}rs in the lead chamber. Fig. 8 shows cosmic rays is low, we regard the middle curve of fig.6 the results~ for our data. The spread of points on the as the proper efficiency for e-. This applies only to e plot, due mostly to multiple Coulomb scattering in the selected on the basis of good topology in the spectro bremsstrahlung radiator, is about 4 mm. Also shown meter and prongless showers in the lead-plate chamber. are the projected distributions expected on the basis of However, because of the large proton background, we Monte Carlo-'calculations. Some background contam require the full bremsstrahlung identification for e + ination in the form of showers occurring in the wrong places is apparently present. If all the events outside the box in fig. 8 form a smooth background, we would conclude that no more than one or two events from proton interactions could be expected to lie within the (/) selection box shown. This is consistent with the ~ 1 + 1- 0.6 event that we would expect from the Bevatron cali 0 bration. Since there are several hundred events inside ';§ 0.4 the selection box, background contamination must II 0:: indeed be negligibly small, even for our e + sample. 0.2 The above measurements are consistent with an .. · order-of-magnitude calculation using particle inter action·properties. We have already seen how the factor Energy after Bremsstrahlung (me/m) 2 in eq.(l) makes direct bremsstrahlung degradation (GeV) radiation negligibly small for heavy particles. A proton Fig. 7. Flight data cross-check of the ratio R of curves TOTAL/ or other hadron could undergo a nuclear interaction (T+ S) in fig. 6, using the background-free e- data sample. in the radiator or subsequent apparatus, producing 584 A. BUFFINGTON et al. one or more neutral pions which then decay into 7. Discussion photons. However, for this to satisfy our requirements, The previous section shows that the bremsstrahlung no extra charged prongs can appear in the spectro identification technique is capable of very great meter, and one of the photons must travel in the right rejectioJ?. against particles othe.r than e ±. The selt;.ction direction to within a few milliradians. We can estimate efficiency for e ± is very dependent on the particular the probability for this neutral-pion contribution by choice of shower detector. The spark-chamber tech computing the average angle e characteristic of nique we have used has an average efficiency o! 46% secondary particles leaving a high-energy interaction: and our particular apparatus is limited to e ± with. energies less than about 50 GeV. This high-energy ~ (0.3 GeV/c transverse momentum) e _...:...... ______.:. ______:____ (14) limit is not fundamental to the technique and could be (inelasticity* incident energy /multiplicity) raised by increasing the bending power of the magnet.' • If the incident proton has energy from 10 to 100 GeV, or the drift space between magnet and shower detector. the multiplicity is roughly 4 produced particles. The It could also be increased by using a shower detector inelasticitity in such interactions is generally 0.5, thus with much finer spatial resolution for detecting photon giving pion emission angles of typically 100 milli showers very close to the e ± track. radians. The decay of neutral pions into photons The bremsstrahlung-identification technique could increases this typical angle. The chance that a photon conceivably be valuable in a particle physics experiment lies within the proper 20 mrad square bin is therefore where e ± must be separated from a large background several percent, with the probability increasing with of other particles. Many particle physics experiments, increasing energy. The chance of a proton interacting however, would find the energy degradation a serious in the radiator in the first place is about 0.01. The detriment. This limitation could be overcome by follow chance of no charged prongs in the spectrometer is ing the shower detector with a total absorption device perhaps 0.2. Combining these factors, we expect a to measure residual shower energy, and designing the rejection between 10-4 and 10- 5 over our range, as we shower detector itself to measure or at least sample have observed. energy. On the other hand, a shower detector could be employed which has a much smaller least detectable photon energy E:.,in than the lead-plate chamber we used. The thickness L1 X of the bremsstrahlung radiator could then be diminished without harm to the overall efficiency, thereby decreasing the amount of energy degradation suffered by the e ± upon passing through the radiator. We would like the thank Prof. D. H. Frisch at the 'E .s Laboratory for Nuclear Science, M.I.T., for the loan ..., 4 ~ of the lead-plate spark chamber used in this work. The "' G> II> .D accelerator tests, vital to the successful application of this technique, would have been impossible without the 0 ~ :~. assistance of the Bevatron staff, and of Dr R. Gearhart .. " ]l -2 0 and the operations personnel at SLAC. We are grateful 't5 -4 E 0. for the fine service and hospitality offered us by the >- -6 staff of the National Center for Atmospheric Research L-~6~-~4~-~2~~0~2~~4~~6~0~~2f0~~4~0~6~0~~8~0~10Q Balloon Facility at Palestine, Texas. Numerous ideas • X predicted- X observed (em) Number of events and much support for this work came from the other Fig. 8. Comparison of bremsstrahlung photon real space location members of this group, particularly L. H. Smith and as predicted by the spectrometer measurement, and as actually R. A. Muller. W. Mountanos helped with the early observed for a portion of our e± data. We have included events accelerator tests of this technique. It is a pleasure to only above 2 GeV in the spectrometer, since multiple Coulomb acknowledge the continual encouragement of L. W. scattering broadens the distribution below this energy. Events Alvarez. This work was supported by N.A.S.A. outside of the 2 em box are rejected as "background" even though five of them apparently have negative charge. The last contract no. NAS 9-7801 and by the Lawrence Berkeley bins on the projection histograms include events oft' scale. Laboratory. BRE~SS~RAHLUNG-IDENTIFICATION TECHNIQUE 585 References Conf. on Cosmic rays Budapest, Hungary (1969) vol. I, p. 235. • .• , . 1) J. Litt and R. Meunier, Ann. Rev. Nucl. Sci. 23 (1973) I. 13) A. Buffington, C. Orth and G. F. Smoot, Phys. Rev. 2) D. Hovestadt, P. Meyer and P. J. Schmidt, Nucl. Instr. and ·o. Letters 34 (1974) 38; more detailed articles will be submitted Meth. 85 (1970) 93. to Astrophys. J. 3) '•:B. Agrinier et a!., Nucl. Instr. and Meth. 88 (1970) 109. 14) For a review of electromagnetic interactions, see sections 4) R. Hofstadter, Science 164 (1969) 1471. 2-11 through 2-13, B. Rossi, High energy particles (Prentice 5) R. F. Silverberg et a!., 12th Intern. Conf. on Cosmic rays, Hall, Englewood Cliffs, New Jersey, 1952). f.fobart (1971) vol. I, p. 122; and D. Miiller, Phys. Rev. 15) Op. cit., pp. 60 and 244. D5 (1972) 2677. ' 6) A. I. Alikhanian, K. A. lspirian, A. G. Oganessian and A. G. 16) P. J. Schmidt, J. Geophys. Res. 77 (1972) 3295. Tamanian, Nucl. Instr. and Meth. 89 (1970) 147; also M. L. 17) A preliminary report of this work was given at the Denver · Cherry, D. Miiller and T. A. Prince, Nucl. Instr. and Meth. Cosmic Ray Conference: A. Buffington, G. F. Smoot, ,., '\ 115 (1974) 141. L. H. Smith and C. D. Orth, 13th Intern. Conf. on Cosmic 7) W. Busza, M. Chen and D. Luckey, Nucl. lnstr. and Meth. rays, Denver, Colo. (1973) vol. I, p. 318. 105 (1972) 613. 18) L. H. Smith, A. Buffington, M. A. Wahlig and P. Dauber, 8) W. R. Webber, 13th Intern. Conf. on Cosmic rays, Denver, Rev. Sci. Instr. ·43 (1972) I. Colo. (1973) vol. 5, p. 3568. 19) We have previously described our momentum analysis 9) J. L. Fanselow, R. C. Hartman, R. H. Hildebrand and P. procedure in considerable detail: L. H. Smith, A. Buffington, Meyer, Astrophys. J. 158 (1969) 771. G. F. Smoot, L. W. Alvarez and M.A. Wahlig, Astrophys. 10) J. K. Daugherty, Goddard Space Flight Center Report J. 180 (1973) 987. No. X-660-74-16 (January 1974) to be published. ' 11) B. Agrinier et a!., Lettre a! Nuovo Cimento 1 (1969) 53. 20) Ref. 14. section 2-19. 12) K. C. Anand, R. R. Daniel and S. A. Stephens, II th Intern. 21) C. D. drth and A . .Buffington, to be submitted to Astrophys. J. ~ .. u J t '' ~------LEGAL NOTICE------, This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. • I • ~ TECHNICAL INFORMATION. .. DIVISION LAWRENCE BERKELEY LABORATORY UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA 94720