Conceptual Design Report A Compact Photon Source

Transcription

1 Cnceptual Design Reprt A Cmpact Phtn Surce arxiv:7.0649v [physics.acc-ph] 8 Dec 07 Cntents B. Wjtsekhwski Thmas Jeffersn Natinal Acceleratr Facility, Newprt News, VA 3606 G. Niculescu James Madisn University, Harrisnburg, VA 807 June, 05 Intrductin Magnet-Dump as a Cmpact Phtn Surce 3. Magnet Diffuser-Absrber Radiatin issue and its mitigatin 8 3. Radiatin frm a thick target Hermeticity f shielding Mnte Carl simulatin 4. Mnte Carl cde cnfiguratin Event generatr validatin MC calculatin f the particle prductin in the DINREG cde MC calculatin f the particle prductin in the GEANT4 cde Gemetry f the magnet-dump mdel in MC Radiatin calculatin results 3 5. The dse rates at key lcatins Radiatin at the magnet-dump cils Radiatin at a surce shielding surface Radiatin at a 5-meter distance Radiatin at the plarized target

2 5. Phtn distributin in the frward directin Radiatin in the hall induced by the phtn beam n the plarized target 3 7 TOSCA calculatins f the field 4 8 The plan fr further testing and develpment 5 Abstract We prpse t build a shielded magnet that will act as an untagged bremsstrahlung phtn surce suitable fr deplyment in Hall A/C at Jeffersn Lab. The gal f the prject is a cmpact surce which prvides a narrw -mm diameter phtn beam at the target with the intensity adequate t the plarized target peratin. The prpsed slutin is based n the absrptin f the electrn beam in the magnet. The cmpactness is achieved by means f a shallw channel which leads t an electrmagnetic shwer in the Cu-W bdy f the channel and surrunding material after a - mm deflectin f the beam electrns by a hrizntal magnetic field. This surce will allw us t achieve ten times the luminsity f the mixed electrn phtn beam cnfiguratin while keeping the heat lad n the (plarized) target as well as the radiatin level in the experimental hall at acceptable levels. The cncept fr this device was develped fr the experimental prpsal PR ( WACS-ALL ) but, if built, it wuld becme a general purpse surce that wuld be usable fr a variety f phtn induced reactins f interest in the -GeV era. Intrductin The luminsity f an experiment is always a paramunt parameter which needs t be cnsidered. In -GeV era experiments we ften deal with large energy final state particles, s the detectr trigger rate-impsed limitatins are nt critical. At the same time, in the plarized target experiments the limit due t the plarized target heat lad limitatin n the luminsity becmes a primarily parameter. This is especially true fr the plarized prtn/deutern target, which is based n super lw temperatures belw K and lses its perfrmance quickly with an increase in heat lad. The typical NH 3 target culd perate at an average electrn beam current f 90 na r a pwer depsitin in the plarized material f 500 mw. A large grup f Jeffersn Lab (JLab) -GeV era physics prpsals (including PR []) cnsiders real-phtn-induced prcesses. T study these reactins f interest, ne needs a pure beam f real phtns. At JLab, nly Halls B and D have built-in real phtn capabilities. This dcument presents a device which will act as a cmbinatin f a sweeper magnet and a beam dump, prducing a narrw intense untagged pure phtn beam. Because f its relatively mdest size, the device can be deplyed in either Hall A r Hall C at JLab, thus pening the pssibility f carrying ut real-phtn type experiments

3 with s-called vertical bend spectrmeters and making accurate measurements f the angular crrelatins. There is als interest in such a surce fr Hall D due t the LOI-5-00 []. This dcument presents a preliminary CDR fr the Cmpact Phtn Surce. We plan t cntinue the analysis f every element f the device using MC simulatin. After apprval f the physics prpsal and cmpletin f the CDR, we plan t start the design and prttyping f the magnet insert (diffuser-absrber). The fllwing sectins address the layut f the device, the TOSCA mdel and results f magnetic analysis, and the GEANT4-based simulatin cde and its validatin. We als analyzed the radiatin dse rate at key lcatins arund the device: at the magnet cils and the plarized target slenid, utside the shielding (.5 m frm the surce), and in Hall A (an average f 5 m frm the surce). Distributin f the radiatin in the frward directin is cnsidered in detail. The heat lad n the plarized target was fund via an analytical estimate and frm the MC f the full experimental setup. Magnet-Dump as a Cmpact Phtn Surce A traditinal surce f bremsstrahlung phtns includes a radiatr, a deflectin magnet with large mmentum acceptance and a dump fr the used electrn beam. Such a cnfiguratin requires significant space and shielding. In additin, it leads t a large size f the phtn beam at the target due t the natural divergence ( /γ) f the phtn beam. T vercme these prblems we have prpsed the Cmpact Phtn Surce []. We prpse t take advantage f the narrwness f the phtn beam. Withut a lss f intensity, a narrw channel culd be made arund the phtn beam. Such a channel made f heavy metal (Cpper-Tungsten ally, a radiatin length f a few mm) will serve as a diffuser fr used electrns, an absrber fr the shwer, and a cllimatr f secndary particles prduced in the electrmagnetic and hadrnic shwer. There is a slw raster f the beam which mves the beam acrss the area f the plarized target with a typical frequency f 50 Hz. In the prpsed device it will be synchrnized with the slw mtin f the channel, and the electrn beam will be turned ff briefly (fr a few µs) when the transitin frm ne narrw channel t anther is needed. The principal cmpnents f the prpsed device are listed belw and will be discussed in detail in the remainder f this dcument: A.3-mm cpper radiatr (% radiatin length) lcated inside the shielded area. A nrmal cnducting diple magnet ( amagnet ) prviding a strng hrizntal field t sweep dwn the beam electrns. A 50-cm lng W/Cu blck with a set f -mm channels lcated in the diple field which serves as a diffuser-absrber f an electrmagnetic shwer. The diffuser-absrber mvement, synchrnized with the beam psitin raster, is required fr the plarized target. A ne meter thick layer f heavy radiatin shielding t ensure that the radiatin level in the Hall des nt exceed allwable limits. 3

4 . Magnet Figures -5 shw the TOSCA mdel f the prpsed magnet (0x60x50 cm 3 ) in several different prjectins partly pen (sme parts are remved) fr clarity. Figure : Schematic f the sweeper magnet. The Cu inserts (blue green) act as shielding in the immediate vicinity f the beam. The central area f the magnet bre is filled by a W/Cu diffuser-absrber. Figure : The sweeper magnet withut the cpper inserts. Figure 3: The sweeper magnet withut the tp irn plate. Figure 4: The sweeper magnet frnt view. The transverse field alng the beam directin n the magnet axis is shwn in Fig. 6. The resulting trajectries fr 8.8, 4.4, and. GeV electrns are shwn in Figs. 7 and 8. 4

5 Figure 5: The sweeper magnet side view (with the cpper insert). Figure 6: The vertical field in the magnet alng the beam line. Figure 7: The trajectries f electrns with mmenta 8.8, 4.4, and. GeV. 5

6 Figure 8: Zmed versin f the previus figure. 6

7 . Diffuser-Absrber There are tw ptins fr the diffuser-absrber. The first ne is a rtating cylinder with a set f -mm diameter hles. This is the best fr beam cllimatin but cmplicates the cling water arrangement. The secnd is a vertically scillating blck with hrizntal slts fr the beam as shwn in Fig. 9. The MC result shws n increase in radiatin in the secnd ptin cmpared with the first ne in spite f a wider channel, s we will discuss belw nly the secnd ptin. The large length f the insert (.5 meters) allws almst cmplete blcking f the radiatin created inside the insert mstly at a distance f cm frm the entrance. The radiatr will be munted at a distance f 5- cm frm the entrance inside the insert. The scillating ptin has the advantage f design simplicity, especially fr the cling system. At the same time, the slts will help t arrange unifrm distributin f the heat lad in the diffuser-absrber. Per ur calculatin, f the kw ttal pwer f the beam, abut 8 kw will be depsited in the diffuser-absrber and the rest in the cpper wedges between the magnet ples, see Fig. 9. The absrber-diffuser will be made frm a heavy metal ally (CuW80) whse shrt radiatin length and high density will reduce the size f the radiatin surce. A water flw will be used t cl ff the inserts with a clsed lp chiller. The water line will use stainless steel bellws which are capable f hlding 9 cycles f defrmatin. A 3-4 m lng metal shaft will be used t cnnect the absrber-diffuser t a step-mtr t activate the mtin. Figure 9: Frnt view f the magnet gap filled with cpper blcks and the CuW80 insert with hrizntal slts. All fur slts fr the beam are shwn. The magnetic field is hrizntal. 7

8 3 Radiatin issue and its mitigatin The radiatin aspects f the prpsed device culd be fully analyzed via a GEANT4-based MC simulatin as it is presented in Sectin 5. Hwever, the analysis presented in this sectin helps t clarify the frmulatin f the cncept and supprts the validatin f the MC results. 3. Radiatin frm a thick target Radiatin generatin by a high energy electrn beam has been investigated since the 960s after the cnstructin f the SLAC and DESY acceleratrs. The variatin f radiatin intensity per kw f beam pwer (mstly neutrn) is very small fr beam energies abve 500 MeV. The PDG review [3] shws a plt with energies up t 0 MeV, and the riginal SLAC reprt [4] discussed 500 and 00 MeV beam energies. Figure shws the neutrn yield and related radiatin dse (at a ne-meter distance frm the target). In this prject we will use a beam f. µa at 8.8 GeV energy with Figure : Neutrn yield frm a thick target induced by a high energy electrn beam (frm the SLAC reprt [4]). a ttal pwer f.5 kw. An unshielded target f large thickness irradiated with such a beam will release a ttal flux f 3 neutrns per secnd, which leads t a radiatin level f 0 krad/hur at a distance f ne meter frm the target, as ne can see frm the right scale in the same Figure. It is als easy t find that the irn yke f the magnet and external shielding shuld be capable f reducing this radiatin level by 3-4 rders f magnitude. Indeed, as it is shwn in Fig., taken frm the PDG reprt, a cncrete slab f cm thickness reduces the neutrn flux by a factr f e. A reductin factr f 00 is achievable with a cncrete slab f 75 cm. High energy neutrns with energies abve 00 MeV have a 4 times lnger attenuatin length up t 0 g/cm. Hwever, even fr such neutrns, 8

9 50 Attenuatin length (g cm ) High energy limit Cncrete!=.4 g cm 3 Figure : Attenuatin length f the neutrn flux vs. neutrn energy (frm the PDG reprt [3]) Neutrn Energy (MeV) with the prpsed shielding thickness f 0 cm f irn, the reductin is expected t be by a factr f 700. Additinal insight int the shielding f radiatin and the neutrn rate utside the shielding culd be btained frm Fig., which shws the spectra f neutrns utside f the shielding fr a 5 GeV electrn beam striking a thick cpper target. A cmbinatin f irn E dφ/de (cm - per primary) cm cncrete, electrns x 0 80cm cncrete, prtns 40cm irn, electrns x 0 40cm irn, prtns Energy (GeV) Figure : The spectrum f neutrns frm a thick cpper target bmbarded by a 5 GeV electrn beam (frm the PDG reprt [3]). The rates are utside the 80 cm cncrete shielding r 40 cm irn shielding at 90 degrees frm the beam directin. and cncrete shielding shuld prvide the best result. The neutrn intensity integrated ver the energy spectrum is cnsistent with the infrmatin in Fig.. 3. Hermeticity f shielding In the design f the shielding, sme attentin shuld als be n the hermeticity f the shielding structure. Fr example, the penetratin f a 5-cm diameter is acceptable fr a 9

10 0-cm lng tunnel in a 90 degree directin. Diffusin f the neutrns thrugh such a channel is strngly suppressed because f the large number f the neutrn reflectins required (45 fr such a tunnel) fr the diffusive transprt and the lw albed factr f the reflectin (typically between 0.3 and 0.5). The slid angle f such a large pening is f the ttal 4π, s the channel shuld nt riginate in the httest spt f the diffuser-absrber and shuld use a chicane if pssible. Hwever, in the frward directin (alng the beam), the pening in the shielding needs t be much smaller than the discussed 5-cm diameter channel. It must be minimized because the high energy neutrn distributin has a small pening angle f m π /E γ. Fr the prpsed phtn beam channel ( mm x 0 mm), the slid angle is f 3 5, which is sufficiently small t blck 98% f the mst energetic neutrns. Using the cnsideratins abve, we find that at a typical distance f 5 meters in Hall A between the surce and the sensitive electrnics, the neutrn dse wuld be n the rder f 0 mrad per hur, which is acceptable accrding t previusly perfrmed experiments in Hall A with an pen gemetry detectr package. In spite f the ptimistic results frm the estimates and cnsideratins shwn abve, already in the prpsal stage we decided t develp a full MC simulatin f the radiatin using a MC cde in the GEANT4 framewrk, which is described in the next sectins. 4 Mnte Carl simulatin While the simplified beam deflectin calculatin shwn abve indicates that the design is capable f deflecting the primary electrn beam int the shielding part f the device, substantially mre insight can be gained frm a detailed Mnte Carl simulatin. In rder t gauge the expected perfrmance f the prpsed setup and t prvide a slid basis fr the expected dse rate calculatin needed t ensure safety, the whle sweeper magnet/target dump (and its shielding) were implemented in a GEANT4 based (g4) simulatin and extensively tested. 4. Mnte Carl cde cnfiguratin A list f the cde design chices made in implementing the sweeper magnet/target dump mdel int the simulatin are listed (and justified, where apprpriate) belw: Versin: Geant4, v.. (release date Dec. 04) implemented n CentOS Linux. Multithread: Given the relative simplicity f the mdel, multithreading, thugh available, was nt used. Magnetic Field Mdel: The magnetic field map (with a cm mesh size) prduced by TOSCA was read at the beginning f the prgram and used in the simulatin. A 3D linear interplatin functin was implemented and used in the cde. Fr cnvenience, a field strength variable stred in a separate text file was read at runtime, allwing fr the scaling f the magnetic field withut recmpilatin/extensive changes in the field map. All results quted here were btained using the nminal value f the magnetic field.

11 Gemetry: In rder t keep the cmplete definitin f the mdels gemetry (and materials) cmpletely separated frm the surce cde f the simulatin, the GDML [5] extensin f GEANT4 was used. As all f the gemetry is cntained in a stand-alne xml file read at runtime, the verhead and ptential pitfalls (usually) assciated with gemetry and/r material changes in the simulatin were greatly diminished. Materials: The cde makes extensive use f the NIST defined materials already defined in g4. In a few cases, materials were defined in the gdml file. Fr example, the JLab develped brn rich cncrete was cnsidered fr use as a shielding material (as a cst saving measure, thugh we decided nt t use it). Randm Number Generatr: CLHEP s RanEcuEngine [6]. Starting frm the same pair f seeds, this randm number generatr has a perid f 8, which was deemed sufficient fr this simulatin. Furthermre, the pairs f randm seeds were changed after every 5 generated beam electrns. Physics List: The physics list (i.e. a list f all the particle types and prcesses that they might underg, fr all energy scales, that are enabled in g4) used was FTFP BERT. This mdel cmbines the Bertini Cascade Mdel [7, 9], knwn t reprduce detailed crss sectin data fr nuclens as well as pins and kans in the GeV range (nminally 0 t 5 GeV) with the Fritif Mdel [8, 9], expected t be valid fr larger (>5 GeV) energies. Input/Output: Besides the gdml gemetry file and the field map mentined abve, the cde als reads a plain text file that defines the energy, lcatin, directin, and spread f the initial electrn beam. The utput cnsists f diagnstic messages, plain text reprts (dse rates, particle flux thrugh varius parts f the mdel, etc.) and a ROOT tree cntaining the initial gun particle (8.8 GeV electrns fr prpsal PR ) as well as a list f all particles (fur mmenta, psitin, PDG ID, etc.) exiting the magnet area, fr further analysis. Scring vlumes: Fr dse rate calculatins, the vacuum ghst vlumes placed 5 m frm the center f the magnet were defined and mnitred. Additinal vlumes were used at the external surface f the shielding. 4. Event generatr validatin In rder t gain cnfidence that, as implemented, the Mnte Carl simulatin f the magnet generates particles at rates that are similar t accepted standards in the field, the fllwing simple exercise was carried ut: All the gemetry f the magnet and its shielding was stripped away. The use f gdml greatly simplified this prcess. A -mm Carbn target was placed in the center f the g4 wrld with a spherical bunding envelpe arund it, as seen in Figure 3.

12 Figure 3: The GEANT mdel fr calculatin f the particle yields frm a -mm thick C target. T match the existing [] particle prductin crss sectin, the initial electrn beam energy was set t GeV. A substantial number (. 9 ) f electrns were fired at the carbn target. Emerging particles were saved in ROOT trees as explained earlier. Particle prductin rates as a functin f the kinetic energy and angle (nrmalized t the number f electrns simulated) were btained. Results fr e + C n + X are shwn in Figure 5. The particle rates we btained frm ur GEANT4-generated events cmpare well with standard particle prductin rates [] in bth the magnitude and shape. This gives us cnfidence that the Mnte Carl simulatin des nt underestimate prductin rates and, therefre, that the dse rate calculatin based n this mdel shuld be a reasnable estimate. The key step in the validatin f the cde is the test f the particle prductin rate, which is cntrlled by a number f switches and ften culd be incrrect. Belw we cmpare the results f ur cde with the well-tested DINREG results. 4.. MC calculatin f the particle prductin in the DINREG cde The DINREG cde in the Geant-3 framewrk was develped by P. Degtiarenk fr the calculatin f particle yields and radiatin dses in the experimental halls at JLab. The results f this cde were cmpared with numerus measurements at JLab and published data. A systematic difference was bserved at JLab: DINREG verpredicts the radiatin

13 level measured in the hall by a factr f 3. The rigin f this factr is nt cmpletely knwn but culd be due t the calibratin methd used fr the radiatin mnitrs. 4.. MC calculatin f the particle prductin in the GEANT4 cde Figure 4 shws the neutrn yield fr a -mm Carbn target with a -GeV electrn beam. This surce yield was als fund t be in gd agreement with FLUKA calculatins. Because DINREG uses the GEANT package fr the gemetry and the particle interactins and the tracing f particles thrugh the material, it is very much suitable fr experiment simulatins. Hwever, the currently mre cmmn package, GEANT4, was used fr the MC simulatin f the prpsed device. We are using the result shwn in Fig. 4 t validate ur MC cde based n the GEANT4 framewrk. The MC was perfrmed using the MC cde develped fr the Cmpact Phtn Surce simulatin but with all the device gemetry replaced by a -mm Carbn plate. The resulting rates are shwn in Fig. 5. The spectra are very similar t DINREG s except fr very large neutrn energy where the DINREG statistical apprach t the particle energy is unlikely t be applicable. In the angular range belw 45 degrees and the energy up t a few hundreds f MeV, ur MC cde predicts a lwer flux f neutrns by a factr f than DINREG des. Such agreement is sufficiently gd fr the purpse f ur preliminary study. 4.3 Gemetry f the magnet-dump mdel in MC The GEANT4 mdel f the Cmpact Phtn Surce is shwn in the set f pictures belw. 5 Radiatin calculatin results 5. The dse rates at key lcatins This sectin presents the cunting rates, energy spectra, and induced radiatin dse rates fr the neutrns and gamma-rays in several key lcatins: The cil f the magnet-dump. The cil f the plarized target magnet. 5 meters frm the surce. The plarized target cell. As shwn in the previus sectin, the event generatr used in the Mnte Carl simulatin f the sweeper magnet-beam dump cmbinatin matches particle prductin rates accepted by the cmmunity fr the energy range f interest. One can then prceed t simulate the prpsed magnet gemetry t assess the suitability f the design fr prviding the clean, narrw phtn beam the experimental prgram calls fr and als t determine the size and 3

14 e + C! n + X at E e = GeV (0. cm target) 00/04/ N e - dn/dt/d# (electrn - MeV - sr - ) Pints and slid line: GDINR M.C. calculatin T (MeV) 0.0 < $ < < $ <.0.0 < $ < < $ < < $ < < $ < < $ < < $ < e + C! n + X at E e = GeV (0. cm target) 00/04/ N e - dn/d# (electrn - sr - ) T > 0. MeV T > 0.3 MeV T >.0 MeV T > 3. MeV T >.0 MeV T > 3.6 MeV T > 0.0 MeV T > 36. MeV T > 00.0 MeV $ (degrees) Right scale: Detectr Lad (events/sec) Assuming beam current µa and detectr slid angle 0.0 sr Figure 4: The intensity f the neutrns prduced in a -mm Carbn target by a -GeV electrn beam []. 4

15 - e + C --> n + X sr - ] - MeV - [el /dt/n e < < 5 5 < < < < 0 0 < < < < < < < < < < 80 dn/d - N e e + C --> n + X T [MeV] 3 4 sr - ] - [el /N e dn/d - N e T > 0. MeV T > 0.3 MeV T >.0 MeV T > 3. MeV T >.0 MeV T > 3.6 MeV T > 0.0 MeV T > 36. MeV T > 00.0 MeV theta [deg] Figure 5: The intensity f the neutrns prduced in a -mm Carbn target by a -GeV electrn beam frm ur GEANT4-based cde. materials that need t be used as shielding arund the magnet in rder t bring dwn the radiatin levels t acceptable JLab and DOE levels. 5

16 Figure 6: The external view f the shielding (the verall size is abut. m x. m x.6 m). Figure 7: The view f the partially pen shielding. The cncrete blcks arund the magnet are shwn in yellw. The dwnstream shielding has a 0.66 m thickness. Figure 8: The view f the magnet yke (shwn in blue) and the cncrete shielding (shwn in yellw). Cpper blcks fill the space inside the magnet. Figure 9: The view f the magnet with the tp and side cncrete plates remved. 6

17 Figure 0: The view f the cils and the ples (shwn in blue). The grey blcks are the cpper inserts with the same side prfile as the magnet ples. Figure : The frnt view f the magnet and the cncrete shielding. The insert between the ples is the diffuser-absrber with the beam rastering area shwn in dark red. 5.. Radiatin at the magnet-dump cils The dse rate n the cils was fund t be 80 krad/hur, which leads t a ttal dse f 0 MRad ver the duratin f the WACS experiment. Such a high dse is nt unusual fr an extractin line in prtn acceleratrs. Fr the prpsed device, it wuld be sufficient t replace the epxy insulatr f the cil winding used in a lw radiatin envirnment with a kaptn tape. The results f the kaptn film investigatin are shwn in Fig Radiatin at a surce shielding surface The neutrn induced dse rate was fund t be. rem/hur at the tp,.3 rem/hur at the bttm,. rem/hur n the left,.7 rem/hur n the right, and 3.5 rem/hur at the frnt f the shielding. Such a level f radiatin shuld nt lead t significant residual radiatin after the beam is switched ff Radiatin at a 5-meter distance The neutrn induced dse was evaluated by means f the large detectrs lcated 5 meters frm the radiatr. We bserved average dses f 5-0 mrem/hur in 90-degree directins, abut 90 mrem/hur in the backward directin, and 70 mrem/hur in the frward directin. In the frward directin the phtn distributin is strngly peaked at the beam line as shwn belw, s it is discussed in detail later. 7

18 Figure : The test results f the kaptn tape prperties vs radiatin dse []. Figure 3 shws the hit distributin fr phtns f all energies. Figure 4 shws the same hit distributin in an area meter by meter. Figure 5 shws the hit distributin in an area 0 cm by 0 cm. Figure 6 shws the same in an area 4 cm by 4 cm (the ftprint f the slts is recgnizable). The energy flw frm these phtns is shwn in Fig. 7, which shws the distributin in an area 0 cm by 0 cm. Fig. 8 shws the distributin in an area 4 cm by 4 cm Radiatin at the plarized target The radiatin dse rate was fund t be 3.9 rem/hur n the cil f the plarized target slenid. The crrespnding integral ver the experiment duratin is krad, which presents n prblem fr the equipment peratin. The intense phtn beam leads t sme pwer depsitin in the lw temperature cell as well as inducing radiatin defects in the material. These radical mlecules, at sufficient cncentratin, reduce the efficiency f the prtn plarizing mechanism. Heat lad n the plarized cell The full list f materials in the plarized target shwn in Fig. 9 frm Ref. [] was implemented in the GEANT4 mdel. The energy depsitin btained frm MC nrmalized t the incident beam intensity f. µa was fund t be 90 mw. The same pwer wuld be depsited by an electrn beam f 35 na. 8

19 x:y {vl==4 && ID==} x y Figure 3: The hit distributin in the frward directin at a distance f 5 m frm the radiatr fr phtn energies abve kev. The crdinates X and Y are in millimeters. x:y {vl==4 && ID== && abs(x)<00. && abs(y)<00.} x y Figure 4: The first zmed hit distributin in the frward directin at a distance f 5 m frm the radiatr fr phtn energies abve kev. The crdinates X and Y are in millimeters. 9

20 y:x {vl==4 && ID== && abs(x)<0. && abs(y)<0.} y x Figure 5: The secnd zmed hit distributin in the frward directin at a distance f 5 m frm the radiatr fr phtn energies abve kev. The crdinates X and Y are in millimeters. y:x {ID== && vl==4 && abs(x)<0 && abs(y)<0} y x Figure 6: The third zmed hit distributin in the frward directin at a distance f 5 m frm the radiatr fr phtn energies abve kev. The crdinates X and Y are in millimeters. 0

21 y:x {KE*(vl==4 && ID== && abs(x)<0. && abs(y)<0.)} y x Figure 7: The energy flw distributin in the frward directin in an area 0 cm by 0 cm at a distance f 5 m frm the radiatr. The crdinates X and Y are in millimeters. y:x {KE*(ID== && vl==4 && abs(x)<0 && abs(y)<0)} y x Figure 8: The energy flw distributin in the frward directin in an area 4 cm by 4 cm at a distance f 5 m frm the radiatr. The crdinates X and Y are in millimeters.

22 Figure 9: The list f materials fr the NH3 target frm Ref. []. Radiatin damage t the NH 3 material The radiatin damage t the material is due t the creatin f radical mlecules, e.g. NH 3 e. These mlecules have a very lng lifetime at the lw temperature f the plarized target. Under the beam irradiatin they accumulate and cmprmise the rate f the prtn plarizing prcess. Annealing f the plarized material needs t be perfrmed (warming up t 77 K) after 5 /cm electrns have passed thrugh the cld target [3]. The prductin rate f radicals by a phtn beam as well as by an electrn beam is prprtinal t the head lad induced by the inizatin f the same material. Using the result f the heat lad analysis (see the paragraph abve), the required rate f annealing cycles was fund t be every 40 hurs. 5. Phtn distributin in the frward directin The energy and crdinate distributin f the phtns were analyzed at a distance f 5 meters frm the radiatr. Figures 30 and 3 shw the energy spectrum in the lgarithmic and linear scales. Figures 3 and 33 shw the radial distributin f the energy flw and its integral. The ttal energy f the phtns in the frward directin (integrated up t a 5 meter distance frm the beam) is 770 MeV per incident electrn (r 0.9 kw fr the. µa beam intensity), which is clse t the expected value fr a % radiatin length radiatr and 8.8 GeV beam energy. An imprtant bservatin made frm this MC study is that abut 99% f the ttal energy flw f phtns is cncentrated inside a -cm radius circle. The beam pipe t the Hall A beam dump and the beam dump itself have apertures fr the ±0.5 degree cne which, at a distance f 5 meters frm the radiatr, translates t a radius f 3 cm. The ttal pwer f phtns utside a radius cm was fund t be belw.5 W. This means that almst all

23 Phtn Energy Figure 30: The phtn energy spectrum in the lgarithmic scale. 3 4 E [MeV] Phtn Energy E [MeV]*Cunts/electrn Figure 3: The phtn energy spectrum in the linear scale E [MeV] phtn pwer is ging t the main Hall A beam dump. Phtn Energy Density vs /el.] r dr)/electrn [MeV/cm E/( Figure 3: The phtn energy density at 5 meters frm the radiatr r [cm] 6 Radiatin in the hall induced by the phtn beam n the plarized target In the presence f the plarized cell in the beam line, a significant fractin f phtns (3.7%) will interact with the target material and cnvert int electrn-psitrn pairs as well 3

24 Integral R Phtn Energy Density dr vs 0 E/electrn [MeV/el.] Figure 33: The phtn energy flw integral R [cm] as prducing hadrns. Thse pairs are als very well aligned with the directin f incident phtns. Hwever, after they leave the slenid, their trajectries deflect frm the beam line directin. Sme part f that pwer (less than 35 W) will cntribute t the radiatin level in the hall. The radiatin level in the hall will have a cntributin frm the particle prductin by the phtn beam n the plarized target. Instead f a direct calculatin f such a backgrund, we will cmpare the γ-nuclen luminsity f the prpsed setup with the effective γ-nuclen luminsity f the GEP experiment which is based n the same SBS spectrmeter. The prpsed WACS experiment luminsity is abut 36 Hz/cm equivalent gamma-nuclen. The GEP experiment electrn-nuclen luminsity is 8 38 Hz/cm with a 40-cm lng LH target, which is equivalent t the luminsity 4 37 Hz/cm equivalent gamma-nuclen. It is easy t see that backgrund rates wuld be 40 times lwer in the prpsed experiment than in the GEP experiment. There is an interesting experimental bservatin which supprts the cnsideratin abve: The 006 GEN and 008 dn experiments with pen gemetry detectrs in Hall A (the BigBite spectrmeter and the neutrn detectrs) perated at electrn-nuclen luminsity f 37 Hz/cm, r at the luminsity Hz/cm equivalent gamma-nuclen, which is cmparable t the equivalent gamma-nuclen luminsity we plan t use in the prpsed setup ( 36 Hz/cm equivalent gamma-nuclen). At that time, the BigBite tracker used drift chambers, whse hit rate capability was much lwer than the hit rate at which the GEM chamber f SBS can perate. 7 TOSCA calculatins f the field There are tw aspects f this device which are cncerned with the magnetic field utside f the diple. They are the field gradient in the plarized cell regin in the prpsed cnfiguratin and the frce between the slenid f the plarized target and the irn structure f the magnet-dump. The relative field gradient was fund t be belw 4, which is sufficiently lw fr the target peratin. The calculated frce in the z directin n all cils was fund t be 500 kg (five times abve the recmmended limit f 0 kg [4]). T reslve this issue we added a 4

25 5-cm thick steel cmpensatin plate, see Fig. 34, after which the frce became belw 0 kg. In the actual experiment there are additinal blcks f irn in similar prximity t the target. They are the SBS spectrmeter magnet and its supprt structure. This suggests that the enclsure f the target in a fur-sided steel cage wuld be a mre reliable scheme. It wuld als help t cntrl the frce in the x directin. Figure 34: The view f the magnet-dump shielding with the plarized target cils and the cmpensating plates (left t right). 8 The plan fr further testing and develpment There are several imprvements f the design which culd be made, fr example a step prfile f the narrw channels in the CuW80 insert (fr reductin f the phtn leakage) and nn-magnetic shielding material (fr reductin f the frce between the surce and the slenid). The analysis presented in this dcument culd be be cnfirmed by the measurement f the radiatin level and detectr cunting rates with a very lw intensity beam f high energy in a thick target surrunded by the prpsed shielding. The target wuld be made f a heavy metal bar with a channel riented at ne degree with respect t the beam directin t imitate the gemetry f the shwer in the case f the magnetic field, s n magnet wuld be used in such a test. 5

26 References [] S. Abrahamyan, G. Niculescu, B. Wjtsekhwski, Prpsal PR t PAC43. [] M. Amaryan and P. Degtiarenk, private cmmunicatin, 05. [3] K.A. Olive et al.(particle Data Grup), Chin. Phys. C, 38, (04). [4] W.P. Swansn, SLAC-PUB 04, 977. [5] [6] F. James, Cmp. Phys. Cmm. 60, (990). [7] M.P. Guthrie, R.G. Alsmiller and H.W. Bertini, Nucl. Instr. Meth. A 66, 9 (968); H.W. Bertini and M.P. Guthrie, Nucl. Phys. A 69, (97). [8] B. Anderssn et al., Nucl. Phys. B 8 89 (987); B. Nilssn-Almquist, E. Stenlund, Cmp. Phys. Cmm (987). [9] Geant4 cllabratin, Geant4 general paper (t be published), Nucl. Instr. Meth. A, (003). [] P. Degtiarenk, private cmmunicatin, 00. [] V.V. Petrv, Yu.A. Pupkv, a reprt BINP TESTING OF RADIATION RESIS- TANCE OF THE MATERIALS USED FOR PRODUCTION OF ACCELERATOR MAGNETIC SYSTEMS, Nvsibirsk, 0. [] D. Day, private cmmunicatin, 05. [3] J. Pierce et al., NIM A 738 (04) 54. [4] D. Meekins, private cmmunicatin, 05. 6