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Nhng cu hi thng gp trong m phng vn chuyn htbng phng php Monte Carlo

ng Nguyn Phng

[email protected]

Ngy 5 thng 5 nm 2014

Danh sch cu hi

Cu hi chung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nhng chng trnh no c dng m phng vn chuyn ht bng phng php Monte Carlo? . . . . . . S lch s ht l g? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Trong m phng ph gamma, v sao chng trnh thng hay chun xc sut pht v 1? . . . . . . . . . . . . . V sao kt qu m phng thng hay c chun v dng xc sut (tnh trn 1 ht ngun)? . . . . . . . . . . . Khi m phng ph gamma a nng (c nhiu nh nng lng), lm th no chuyn i gia kt qu m

phng v thc nghim? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Trong trng hp c b qua mt s nh nng lng do nm ngoi ngng o ca detector, phi tnh xc sut

pht nh th no chuyn i qua li gia m phng v thc nghim?. . . . . . . . . . . . . . . . . . . . . V sao phi cung cp thng tin nng lng theo knh trong m phng ph gamma? . . . . . . . . . . . . . . . . V sao phi p dng xp x n rng Gaussian trong m phng ph gamma?. . . . . . . . . . . . . . . . . . . . . MCNP5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ti c th tham kho ti liu hng dn MCNP5 bng ting Vit u? . . . . . . . . . . . . . . . . . . . . . . quan trng ca cell l g? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Trng s ht l g? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lm sao khi phng trnh gii tch ca mt elliptic paraboloid khng ging vi trong MCNP? . . . . . . . . . Cch khai bo ngun hn hp?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cch tnh tally tng ca cc cell? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gi tr bn knh R0trong khai bo tally F5? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chng trnh MCNP5 ang chy t nhin b ngng v l do cp in, liu c th chy tip c khng? . . Ti sao khi dng tally F6 th b li tally volumes or areas were not input or nor calculated m dng tally F4

th khng b? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GEANT4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ti c th tham kho ti liu hng dn GEANT4 bng ting Vit u? . . . . . . . . . . . . . . . . . . . . .

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ng Nguyn Phng Ti liu ni b NM

1 Cu hi chung

Nhng chng trnh no c dng m phng vn chuyn ht bng phng php Monte Carlo

Hin nay c rt nhiu chng trnh m phng vn chuyn ht bng phng php Monte Carlo, mt s chng thng dng c th k n nh l:

MCNP (Monte Carlo N-Particle) chng trnh m phng a mc ch dng m phng vn chuyn cht neutron, photon v electron. Ngoi ra, phin bn m rng MCNPX cn c th m phng vn chuyn ca cnng v cc ht c bn. Chng trnh MCNP ln u tin ra i vo nm 1977 ti Phng Th nghim Quc giAlamos (Hoa K).

Homepage: http://mcnp-green.lanl.gov/Gii thiu: http://mcnp-green.lanl.gov/pdf/MCNP5_Manual_Volume_I_LA-UR-03-1987.pdfDin n: http://mcnp-green.lanl.gov/forum.htmlTi liu: http://mcnp-green.lanl.gov/resources.html,http://mcnp-green.lanl.gov/documentation.ht

GEANT (GEometry ANd Tracking) chng trnh dng m phng vn chuyn ht tng t nh Mthng tp trung vo lnh vc vt l nng lng cao, ht nhn v my gia tc. Hai phin bn ph bin hin naGEANT3 c xy dng trn ngn ng lp trnh Fortran v GEANT4 vi ngn ng lp trnh C++. Chng trnh

c pht trin ti Phng Th nghim CERN (Chu u).

GEANT4: http://geant4.web.cern.ch/geant4/Download: http://geant4.web.cern.ch/geant4/support/download.shtml

Trong trng hp ch cn m phng cc ht in t (electron/photon), ta c th s dng cc chng trnh sau:

EGS (Electron Gamma Shower) chng trnh m phng Monte Carlo cho vn chuyn photon v electron, pht trin u tin ti SLAC (National Accelerator Laboratory, US) vi mc ch ban u cho tnh ton vt l lng cao, sau ny c pht trin tip di s h tr ca NRC (National Research Council, Canada) v KEK (Energy Accelerator Research Organization, Japan) nhm s dng gii quyt cc bi ton vi mc nng lng hn.

EGS4: http://www.irs.inms.nrc.ca/EGS4/get_egs4.htmlEGSnrc: http://www.irs.inms.nrc.ca/EGSnrc/EGSnrc.html

PENELOPE (PENetration and Energy LOss of Positrons and Electrons) chng trnh m phng MCarlo cho vn chuyn cp electron/photon, c pht trin u tin ti i hc Barcelona.

Gii thiu: http://www.nea.fr/html/science/pubs/2009/nea6416-penelope.pdf

S lch s ht l g?

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http://mcnp-green.lanl.gov/http://mcnp-green.lanl.gov/pdf/MCNP5_Manual_Volume_I_LA-UR-03-1987.pdfhttp://mcnp-green.lanl.gov/forum.htmlhttp://mcnp-green.lanl.gov/resources.html%20,%20http://mcnp-green.lanl.gov/documentation.htmlhttp://geant4.web.cern.ch/geant4/http://geant4.web.cern.ch/geant4/support/download.shtmlhttp://www.irs.inms.nrc.ca/EGS4/get_egs4.htmlhttp://www.irs.inms.nrc.ca/EGSnrc/EGSnrc.htmlhttp://www.nea.fr/html/science/pubs/2009/nea6416-penelope.pdfhttp://www.nea.fr/html/science/pubs/2009/nea6416-penelope.pdfhttp://www.irs.inms.nrc.ca/EGSnrc/EGSnrc.htmlhttp://www.irs.inms.nrc.ca/EGS4/get_egs4.htmlhttp://geant4.web.cern.ch/geant4/support/download.shtmlhttp://geant4.web.cern.ch/geant4/http://mcnp-green.lanl.gov/resources.html%20,%20http://mcnp-green.lanl.gov/documentation.htmlhttp://mcnp-green.lanl.gov/forum.htmlhttp://mcnp-green.lanl.gov/pdf/MCNP5_Manual_Volume_I_LA-UR-03-1987.pdfhttp://mcnp-green.lanl.gov/



S lch s ht (number of particle histories) theo nh ngha th chnh l s cc lch s ca cc ht c m phnchng trnh (nghe rc ri qu nh ). Nhng trong thc t th hu nh n chnh l s ht pht ra t ngun (nuof source particles).

Trong m phng ph gamma, v sao chng trnh thng hay chun xc sut pht v 1?

Khi bt u m phng mt ht pht ra t ngun, chng trnh s gieo ngu nhin quyt nh cc thng s nc m phng da vo danh sch cc thng s (nng lng, v tr, hng,...) ngi dng a vo chng trnh vi xc sut tng ng ca n. Tng cc xc sut ny khng nht thit phi bng 1.

Tuy nhin, trong hu ht cc chng trnh, dy s ngu nhin thng thng c gieo trong khong (0,1) nn th gieo ngu nhin xc nh cc thng s, chng trnh s phi chun cc xc sut pht ny v 1 bng cch chlt cc xc sut ng vi tng thng s cho tng xc sut ca thng s . Trong mt s chng trnh nh MCvic ny c thc hin hon ton t ng, ngi dng ch cn khai bo xc sut thc t ca cc thng s l khng nht thit phi chun xc sut trc khi a vo chng trnh.

V sao kt qu m phng thng hay c chun v dng xc sut (tnh trn 1 ht ngun)?

Thc s th vic chun kt qu m phng v dng xc sut (tnh trn 1 ht ngun) khng phi l vic bt bucnhin n s gip cho ngi dng thun li hn trong vic chuyn i gia kt qu m phng v thc nghim (s trnh by trong nhng phn sau).

Khi m phng ph gamma a nng (c nhiu nh nng lng), lm th no chuyn i gia kqu m phng v thc nghim?

Trong o c thc nghim (TN), i vi mt phn r ht nhn, c th c nhiu hn 1 gamma pht ra, tuy nhin m phng ta khng th m phng c tt cc cc gamma pht ra cng mt lc m phi m phng tng gamma

Khi m phng (MP), kt qu ngi dng nhn c thng l phn b xc sut tnh trn 1 ht pht ra t ngTrong trng hp m phng ph gamma, ht pht ra t ngun thng thng l photon, do nu mun chuygia phn b xc sut m phng v s m tng ng vi thc nghim ta cn phi nhn hoc chia cho tng s phtrong thc t pht ra t ngun ca em. C th hn l:

MP TN: ph m phng tng s photon t ngun

TN MP:ph thc nghim/tng s photon t ngun

Trong :tng s photon t ngun= hot ngun thi gian o tng xc sut pht ca tt c cc tia gamKhi chun v phn b theo xc sut, chng trnh m phng ch chia cho tng s ht ngun m n m phng, kquan tm s lng ht pht ng vi mc nng lng ny hay mc nng lng kia l bao nhiu. Do khi muni qua li gia m phng v thc nghim, ta cn s dng tng xc sut ch khng phi tng xc sut ring l

Trong trng hp m phng ph ring cho tng nh v sau tng hp li thnh 1 ph tng, ta c th s dngsut pht ca tng nh nng lng.

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Trong trng hp c b qua mt s nh nng lng do nm ngoi ngng o ca detector, phitnh xc sut pht nh th no chuyn i qua li gia m phng v thc nghim?

Trong trng hp , ta ch tnh tng xc sut ca nhng nh nng lng m c s dng trong m phng thc trong ngng o ca detector hay khng. Khi , s photon m ta thu c sau khi quy i mi ng vi s phthc t pht ra t ngun.

V d:c 3 nh gamma c xc sut pht l x, y vz (tt c u trong ngng o ca detector), s phn r tsut thi gian o l N. Gi s ta khng m phng nh c xc sut z, khi tng s photon tng ng vi m plc ny s l N (x + y).

Nu ta nhn ph m phng vi tng s photon l N (x + y + z)th s photon pht ra t ngun ng vi mi nthnh ra

N x + Nx

x + y z (thc t s photon phi l N x)

N y + Ny

x + y z(thc t s photon phi l N y)

Tt nhin nu nhn viN (x + y)th ph m phng cng vn cn cha khp vi thc nghim do thiu s ngca nh c xc sut z, nhng phn ng gp ca 2 nh kia l chnh xc.

V sao phi cung cp thng tin nng lng theo knh trong m phng ph gamma?

Sau khi kt thc qu trnh m phng mt ht, chng trnh s tnh ton xem ht mt nng lng l bao ntrong vng nhy. Vic s dng ng chun nng lng theo knh gip quyt nh xem s m s c thmknh no tng ng vi nng lng mt mt .

V sao phi p dng xp x n rng Gaussian trong m phng ph gamma?

Trong qu trnh m phng, khi photon li nng lng Ebn trong vng th tch hot ng ca detector, chtrnh s ghi nhn li chnh xc gi tr Eny. Tuy nhin trong thc t th do cc nh hng ca cc nhiu thnnhiu in t, gi tr nng lng ghi nhn li s thng ging quanh gi tr thc t E, to ra mt nh ph c Gauss. Cc chng trnh nh GEANT4 hay MCNP5 c chc nng ch yu l m phng vn chuyn ht nn ccng lin quan n cc thit b in t s khng c m phng chi tit (v sau ny th cc chng trnh cng c tch hp thm t nhiu cc hiu ng ).

c th thu c dng ph nh trong thc nghim, ngi m phng c s dng mt th thut nh, l thaghi nhn gi tr nng lng Eli, ngi ta s ghi nhn mt gi tr E phn b quanh gi tr Etheo dng phGauss (c xy dng da trn hm F W H M o bng thc nghim). Cui cng ta s thu c ph c bin vi xp x n rng Gauss (Gaussian broadening) cho ph hp vi thc nghim.

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2 MCNP5

Ti c th tham kho ti liu hng dn MCNP5 bng ting Vit u?

Tp ti liu Hng dn s dng MCNP cho h iu hnh Windowsc th c ti v y.

quan trng ca cell l g?

Trong MCNP, mi cell u c gn cho mt i lng c gi l quan trng (importance) ca cell lng ny ch thc s c ngha khi ngi dng s dng cc k thut gim phng sai (variance reduction technKhi , cc cell c quan trngcao hn s nhn c m phng chi tit hn (s lng cc ht c m phnhiu hn) so vi cc cell c quan trngthp.

Theo quy c, khi ht i vo trong cell c quan trngbng 0, qu trnh m phng ht s kt thc. Thng th

cc cell m t vng khng gian bao ngoi khu vc m phng s c gn gi tr ny.

Trng s ht l g?

Tng t nh quan trng ca cell, trng s ht (particle weight) cng l i lng c gn cho mi ht nphc v cho vic thc hin cc k thut gim phng sai. Khi , trng s ca ht s thay i ty thuc vo cthut gim phng sai c p dng, nhm lm cho kt qu khng b chch (bias) ra khi gi tr k vng trongtrnh m phng.

V d:Ta c hai cell, cell 1 v cell 2 c quan trng ln lt l imp = 1 vimp = 3, mt ht c trng s w = 1

cell 1 vo cell 2. Gi s rng gi tr k vng ca nng lng ht li trong cell 2 l 0.5M eV. Khi i t cell 1 san2, do cell 2 c quan trng ln hn nn ht s c chia lm 3 ht con vi trng s ca mi ht con l 1/3.

By gi nu ta xem nh mi ht li nng lng trong cell bng vi k vng l 0.5M eV, th tng nng lng c li trong cell 2 lc ny s l

0.5 + 0.5 + 0.5 = 1.5(MeV)

Tuy nhin do mi ht ch c trng s 1/3 nn kt qu cui cng thc s s l

0.5 13 + 0.5 13 + 0.5 13 = 0.5(M eV)

Gi tr ny tng ng vi kt qu khi khng p dng k thut gim phng sai cho ht i t cell 1 vo cell 2.

Nh vy ta c th thy vic s dng trng s ht gip cho kt qu kho st nng lng li trong cell 2 c thntt hn (kho st 3 ht thay v 1 ht) nhng kt qu vn khng b chch ra khi gi tr k vng.

Lm sao khi phng trnh gii tch ca mt elliptic paraboloid khng ging vi trong MCNP?

Trong MCNP, cng thc tng qut m t cc mt (ellipsoid, hyperboloid, paraboloid,...) l

Ax2 + By2 + Cz2 + Dxy + Eyz + F zx + Gx + Hy + Jz + K = 0

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https://docs.google.com/file/d/0B6xmSiD_R7N-M3RnWkNVZDRVclk/edit?pli=1https://docs.google.com/file/d/0B6xmSiD_R7N-M3RnWkNVZDRVclk/edit?pli=1



Trong cc sch gio khoa, phng trnh elliptic paraboloid c m ta qua cng thc

z

c =

x2

a2 +

y2

b2

Phng trnh trn c th d dng chuyn i sang dng tng ng vi cng thc tng qut trong MCNP

b2c x2 + a2c y2 + 0z2 + 0xy + 0yz + 0zx + 0x + 0y a2b2z + 0 = 0

Cch khai bo ngun hn hp?

khai bo ngun hn hp gm nhiu loi ht khc nhau, ta c th s dng phn b Dncho khai bo PAR, vdng FPAR khai bo cho cc phn b ph thuc vo loi ht pht ra t ngun.

V d: Gi s ta cn khai bo mt ngun im (point source) hn hp gm photon v electron c xc sut pht tng l 0.7 v 0.3. Nng lng pht ca photon l 1M eVv ca electron l 0.5MeV. Cch thc khai bo nh sa

SDEF PAR=D 1 E RG=FPAR=D2 X=1 Y =0.5 Z=3SI1 L P E

SP 1 . 7 . 3D S2 L 1 . . 5

Cch tnh tally tng ca cc cell?

tnh tally tng, ta thm k tTvo cui dng khai bo cc vng ghi nhn tally.

Gi tr bn knh R0trong khai bo tally F5?

Trong tally F5, R0 l bn knh ca mt cu loi tr (exclusion sphere), c tnh theo n v cmnu R0 > 0 v(mean free path) nu R0 < 0.

Nh ta bit, tally F5 s dng phng php c lng s kin tip theo (next-event estimator), theo xcm s kin tip theo xy ra ti v tr ca detector c tnh theo cng thc

W p()e

2R2

viW l trng s ht; p()l gi tr hm mt xc sut ti (cosine ca gc to bi vt ca ht v vectohng ti v tr detector);l tch phn ca qung chy t do (mf p) t v tr va chm n v tr detector; vkhong cch gia v tr va chm vi detector. Vic c lng ny ch c th thc hin c khi v tr va chm kqu gn detector. Do vic khai bo bn knh R0 nhm gip loi tr vic p dng c lng vi cc v tr detector (c th l trong phm vi ca mt cu R0).

Theo khuyn co, gi tr ca bn knhR0 nn nm trong khong t 1/8 n 1/2 mf p, v khng nn bao qu mvt cht (MCNP s khng bo li trong qu trnh chy nhng kt qu c th s khng chnh xc).

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Chng trnh MCNP5 ang chy t nhin b ngng v l do cp in, liu c th chy tip ckhng?

chy tip mt chng trnh MCNP5 b ngng t ngt, ta cn thm chCONTINUEvo trc cc dng khainput file (sau dng u tin) v thm option c vo lnh khi chy li, v d

mc np 5 c n=i n pu t r=runtpe

Chng trnh s tip tc chy vi file runtpec to ra trong qu trnh chy trc .

Ti sao khi dng tally F6 th b li tally volumes or areas were not input or nor calculated mdng tally F4 th khng b?

l do tally F6 l tally dng c lng nng lng trung bnh li trong cell trn mt n v khi l(M eVgm) nn cn phi bit thng tin v th tch cell c th tnh c khi lng. Cn tally F4 ch c ln

di vt trong cell nn khng cn thng tin v th tch.

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3 GEANT4

Ti c th tham kho ti liu hng dn GEANT4 bng ting Vit u?

Tp ti liu Hng dn s dng GEANT4 cho h iu hnh Windowsc th c ti v y.

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https://docs.google.com/file/d/0B6xmSiD_R7N-LUdEUTRyTU5nNGs/edit?pli=1https://docs.google.com/file/d/0B6xmSiD_R7N-LUdEUTRyTU5nNGs/edit?pli=1

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