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‑39 ‑

Guiding Field for The Neutron Life Measurement

Masayoshi Wake and Keisuke Maehata

KEK National Laboratory for High Energy Physics

Introduction

The life time of neutron can be measured by counting electrons or protons produced by the decay of neutrons in neutron beam n i t h Known linear density. The major problem in this measurement is the definition of the decay volume and the accurate counting of decayed particle which is not too large compared to the background signals. The low energy electron makes small circular orbit around the flux line and. as a result, travels along the flux line. Therefore one can gather electrons in one direction by magnetic field. By this, counting rate of the decayed electron for the same dimension of the detector, which means the same back ground, can be significantly increased. If the guiding field line crosses the beam and the decayed electron follows the field line the decay volume can be defined by the field line. The ambiguity of the decay volume become less than the diameter of the electron orbit. The guiding field in the neutron life experiment is effective both for the definition of the decay volume and the counting efficiency.

Electron Trapping by The Inhomogeneous Field

If the electrons simply follow the field line, the most efficient guiding field would be a long solenoid coil because all the electrons produced in the neutron beam line are gathered at the both ends of the solenoid.

Fig. 1 shows the magnetic field in such an arrangement. The electron counters can be small and h'ince the back ground for this measurement is very small. This type of magnet was used in the experiment in ref. 1. The problem in this type of magnet is the definition of the decay volume.

Since the decay volume is defined by the curved field at both ends, the magnet has to be very Ion? to have accuracy In the decay volume. If the magnetic field is perpendicular to the beam line. the definition of the decay volume become much easyer. Although. the gathering efficiency of the decayed electron can not be so large as a long solenoid. Even if the magnetic field is perpendicular to the beam line. there is another confusion in the decay volume. Low energy electrons are trapped in a bulged field. This is a well known effect applied for mirror fusion facilities. As a matter of fact, more than 10% of the decayed electron was lost in the usual magnetic field between pole pieces which has slight

Inevitable bulge of the field. (ref2)

Superconducting Split Solenoid

The magnet for the guidir.7 field of the neutron life measurement should not have buldging field. It is very difficult to reduce the buldge of the

field for a conventional magnet to the order of less than 10~ . But if the magnet is composed of a pair of split superconducting solenoid coil, the magnetic field in the experimental area is rather spreded at the outside of the coil which means there is no buldging in the field at all.

. ， .  

o 

Guiding Field  for The Neutron Life Measure田ent

lIasayoshi胃ake and Kel5uke Maehata 

KEK National Laboratory for High Energ吉PhYsics 

Introduction 

官he llfe  tl皿e of  neutron  can  be  mea5ured  by  countlng  electron5 or  proton5 produced by the  decay  of  neutrons  1n  neutron  bea皿withKnawn  linear densit~ The major problem  1n th1s皿easurement 15  the definltion 

。

f the decay volu皿e and色he accurate counti~ of  decayed partic1e wb1cb 19  not  too  1arge  compared  to  the background signals.  Th. low energy e1ectron 

血akes small  clrcular orbit around  the  flux l1ne  and.  as a result.色ravels a1005 the  flux 11ne.  Therefore one ca且gather e1ectro且s ln one dlrectlon  by mag冨etlc fleld.  By this.  cou且tlngrate of  the decayed electron for the  same dlmenslo且 。f the  detector.  whlch皿eans the  same back grou且d. ca且be slgnlficantly increased.  If  the  guldlng fleld  l1ne  crosses  the bea皿 and

色he decayed  electron  fol1oW5  the  fleld  line  the  decay votu皿.e can be  deflned by the  fleld l1ne.  The a皿bigultyof the decay volume becollle  le5s  tban the dlameter of tbe electron orbit.  The guldlO5 field  ln the oeutron  life experiment  Is effectlve botb  for  the deflnltlon oC the  decay volttne  and  the  counting efficiency. 

Electron Trapplng by The Ir，hOmogeneous Fleld 

1f the  eleotrons simply  follow  the  fleld 1Ioe.  the回ost errlclent guldlng  field would bc a long soleooid ooil  because all  thc electroo r.produced  in  the neutron bea皿 line are  gathered  at  thc  b。色h ends of the soleoold. 

Fig.l shows the magnetic  field  in  such  sn  arrnnge田.ot. The electron  cou且ters can be  smal1 and  h'Jnce  the  back  ground  for  this皿easurementis  very s田al1. Th15  type  of m~gnet was used  ln  the  experlment  10 ref. 1.  The  proble田 10 this  type  of  mnonet  is  the  defioltion  of  the decay volu血e. Sloce the decay volume  1.  d号fioed by  the  curved field at  both ends.  the  magnet haa to be very  100耳 to have  accuracy  10  the  decay volume.  If the 

皿agnetic field  18 perpcod.c111ar  to  the  beam 110e.  the deflnltlon of  the  decay volu皿e become mucb  edsyer.  Althoug~ the gatbering efflcleocy of  the decayed electron can not be 80 large as a long Bolenold.  Even if色he magnetic  Cleld  15  perpendicular  to  the  bea皿 llne. there  Is  ano色her confuslon 1n  the  decay  volume.  Low  energy  electrons are  trapped  ln a  bulged f leld.  Thls  15  a  well 孟nown effect  applled for mirror !u810n  fac111tles.  As a matter  of fact， more  than  10兎 of thc decayed clectron  was lost  in  the usual皿agr.etlc Cleld  between pole pieces which has s11ght 

lnevitable bulge of the field. (refZ) 

Superconducting Spl1t Soleロoid

The lIIagnet  for the guidlr."  field oC  the  neutron J lf" 血easure~ent should  not have buldging Cleld.  1t  Is very dlcricult  to  reduce  the bu1dge of  the 

field for a conventional magnet  to  the  order of  less  than 10・ B uttf the magnet  is  composed of  a palr oC  spl1t  Buperconducting 801eoold coll.  the皿agneticf1e Id 且 the exPerlmental  area  18  rather spreded at  the  outslde of  the  col1

・

hichmeans  there  18  no bUldglng ln the fleld at  all. 

The flux line of the designed magnet is shown in Fig. 2. The central field of the magnet is 1. 5 T. The inner diameter of the coil is 500 mm and the gap between two solenoid coils is 80 mm. The operation of the magnet is made with the current of ISO A. The stored energy and the inductance of the magnet are 162 kJ and 14. 4 H. respectively. The conductor of the magnet is planed to have 1. 5 mm diameter pure copper matrix with 50 % of its cross section filled by 46. 5%Nb-Ti fine filaments.

The coil has 2559 turns in each block of the split solenoid winding.

Magnet Construction

The real construction and assemble of the magnet has to be designed after a careful consideration of liquid helium consumption. Figure 3 is the cross sectional drawing of the magnet. The support structure of the coil is the inner vessel of the cryostat itself. the coil is mounted in the inner vessel of the cryostat by shrink fitting. The mechanical support of the inner vessel in the cryostat can be made at the window for tit beam line. A kind of bellow structure can keep the inner vessel and the coil at the center allowing the thermal contraction in the radial direction.

The field map of this coil is shown in Fig. 4. The largest magnetic field on the conductor exceeds 3 T even the central field of the magnet is as low field as 1. 5 T. Since the operation current of this magnet is 150 A, it is possible to hold the magnetic field in persistent current mode with a superconducting switch. By the elimination of the excess heat load, the helium consumption of the cryostat may be minimized down to a hundred litters per week. The protection of the magnet against a quench should be made by the protection resistance in the cryostat. The size of the protection resistance can be rather small because the large gas flow due to the evaporation of the liquid helium will cool the resistance if the

resistance is located in the proper position. The main difficulty of the protection of the magnet is in the normal resistance of the superconducting switch. The protection resistance of 3 0 requires at least 300. off resistance in the superconducting switch which is very large compared to the standard size. Figure S is the simulated quench result assuming large enough switch resistance. In this case, the magnet can survive through quenches with its maximum temperature of 44 K.

Conclusion

the neutron life measurement can be made with the aid of superconducting magnet field. The magnet which produces a good field can be constructed by use of present superconducting magnet technology. The preliminary design of the magnet was shown in this report.

References

1. P. Bopp et. al.. Phys. Rev. Lett. 56(1986)919 Z. C J. Christensen et. al.. Phys. Rev. D5( 1972] 1628

" " "  

h岨

The flux line  of  tbe  designed  magnet  Is  SbOWO ln Flg.  2.  Tbe  ceotral  fleld of  the皿agnet Is  1.  5 T.  The  ln且er diameter of  tbe col1  15  500皿皿

aOd  the gap between two  solenold  coi18  18 80 ~ The operation of the  mag百et i9 made with the  current  of  150 ~ The  stored  energy and  the  induclance of  the  magnel  are  162 kJ  and  1~4 払 respectlvelY. The  conductor of tbe血ag百et 18  planed  to  have  1.  5 m田diameterpure copper  matrix with 50包 。f lt8  crQSS sectloo fll1ed by 46. 5%Nb‑Ti  fine fila皿ents. The col1  has 2559 turos  10 each block of tbe  spl1t  solenoid曹inding.

Uag百et Constructio且

The real  constructl。且 a且d assemble of  the magnet has to be deslgned after  a careful  consideratlon of  liquld  hellu血 con5umptio~ Flgure 3 15  the 

C~oss sectlonal drawlog of  the血agnet. The support  s.ruc色ure of the 0011  18  the  lnner vessel  of  the  cr~ostat ltself.  the  coil  18 mouoted  in the  lnner vessel of tbe  oryostat by sh.rlnk fltting.  The mechanlcal  support of  the  ln且ervessel  10 the  cryostat  can  be made at  the window for t~~ beam  l1ne.  A klnd of bel10w structure  ca且keep tbe  lnner vessel  and  tbe col1  at  the ceoter al10wing the  thermal  contraction  ln tbe  radlal directlo~

The field皿apof thls coil  18  shown ln Fl~ ~ The largest magnetlc field 

。

n the  conductor exceeds 

a 

T  even  the  central  field of  tbe皿agnet ls  as  low fleld as 1.  5 ~ Slnce  the  operatioo curreot of this血agnet Is  150 A  it  Is possible to hold the magnetlc  field  in persistent  currcnt mode wlth  a superconductlog s曹itch. By the el1田lnatlonof tbe excess heat  load.  the  helium consumptio且 。f the  cr~ostat ma~ be  miol田lzed down to  a bundred  11tters per wee~ The protectlon of the magnet agalnst a quench should be  made by  the  protection  resistance 且 tbe cryostat.  The slze of  tbe  protectlon resistance can be rather  small  because  the  large gas flow due  to tbe evaporatlon of  tbe  liquid  heliu血 wl11 cool  the  resistance  lf  the 

resistance  is  located  In ¥he proper posltlo~τhe malo dl!flculty of the  protectlon  o(  tbe  magnet  18  10  the  oormal  res18tence  ot  the  superconductilllf  switch.  The  protectlon  resistance  of 3D requires at  least 80D.  off resl8ta!¥ce  in the  superconduc色Ing switch whlch Is very  large  compared to  the  sta且dard 81ze.  Figure  5  ls  the 81皿ulatedquench  resul t assum11llf large eoough swltch res 1staoce.  10 this case.  the皿agnet can survive through queoches wltb its田axlmu皿 temperature of 44 K 

Conclusion 

the neu色ron l1fe me&surewent  can be 皿ade with the ald of supercondu~tlng

magnet  fleld.  The田agnet which  produces a good  fleld can be coostructed  by use of  preseot  5upercooductJog皿agoet tecbnology.  The prelimlnary  deslgn of  tbe magnet .as sho

・

o ln this report. 

Refere且ces

1.  P. Bopp et. al..  Ph1s. Rev. Lett.  56! 1986)919  2.  C. J.

ロ

lrIste且seoet.  al..  Phys. Ruv.  D5{ 197211628 

IN3

Fig-. 1 Long Solenoid Coil Arrangement for Neutron Life Measurement The flux line at both end of the solenoid can be bent to the radial direotion by placing compensation ooils.

Fig. 2 Flux Lines In The Split Solenoid

The flux lines are actually the equal vector potential lines calculated by POISSON group programs.

Fig. 3 Split Solenoid Magnet for Neutron Life Measurement

The coil is assembled in the oryostat prior to the welding of the cryostat. The dimensions are subject to chage for the reduction of heat load.

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cb 7 o C J . O O ' « . 0 0 ' C 00 £ 0 0 ,Z Cy 12.00 U 0 0 : 6 . ( < 0 I B tit 2C.C0 <*.,00 ^'t 00

Fig. 4 Field Map of The Magnet Lines are at every 0. 05 tesla.

01>. 

~

Fig.  1 Long So1eno1d Co1l Arrangement  for Neutron u1fe Measurement  The flux 11ne at both end  of  the  solenoid  can  be bent  to  the radial  dire。色lonby placing compensation coils. 

Fig.  2 Flux Lines  10 The Split Solenold 

The flux lines are actually the  equal vector potent1al  llnes  calculated  by FOISSON group progra皿s.

Flg.  a Split Solenoid Magoet  for Neutron Llfe Measure皿ent

The col1  Is  assembled  in  the  cryostat  prlor  to  the weldlng of the  cryostat.  The dlmenslons  arの sUbject to  cbage fOr the  reduct10n of heat  load. 

fluOI 

Fig.  4 Field Map of The Magnet  Llnes are at  every ~05 tesl~

ドキュメント内 KEK Report January 1988 H PROCEEDINGS OF THE FIRST WORKSHOP ON THE NEUTRON LIFETIME KEK, Tsukuba, November 19, 1987 Edited by S. YASUMI NATIONAL (ページ 44-48)