입자와물질과의상호작용 이효상 인제대학교강의
방사선의성질과물질투과력 방사선 형태 질량 (AMU) 전하 차폐물질 a 입자 4 +2 종이, 피부, 옷 b 입자 1/1836 +-1 플라스틱, 유리, 경금속 g 전자파 0 0 중금속, 콘크리트, 지각
물질에서방사선의주요반응 모든방사선은에너지를지니고있으며방사선이물질을투과하는동안물질의원자에에너지를부여 (energy transfer) 함으로써방사선의에너지는점차감쇄 인체조직에서의에너지부여는인체에유해한영향초래 방사선검출기의검출원리 에너지부여정도와상호작용의종류는차폐설계에서가장중요한인자
주요에너지부여과정 전리 (ionization) 전리란전기적으로입사방사선이중성인원자의궤도전자에전자의결합에너지보다큰에너지를부여함으로써원자로부터전자를제거하는과정 여기 (Excitation) 전자의여기란입사방사선이원자의궤도전자에에너지부여하여안쪽궤도 ( 전자의결합에너지가큰궤도 ) 에있던전자를바깥쪽궤도 ( 전자의결합에너지가작은궤도 ) 로이동시키는과정이며원자는여전히중성인상태로남아있다. 제동복사선 (bremsstrahlung) 고속전자가물질의핵주위를지날때핵의전기장내에서고속전자는회절되면선에너지가감쇄된다. 이때감쇄된전자의에너지에해당하는에너지를지닌광자 ( 제동복사선 ) 가방출된다. 제동복사선은 1895 년에렌트겐 (Roentgen) 이 " 크루크관 (Crooke's tube)" 이라는장치를이용하여발견한최초의전리방사선이다.
하전입자와물질과의상호작용 중하전입자 (a, p, 핵분열생성물 ) 원자의전자들과우선적으로반응 원자의전리및여기에의한충돌과정으로에너지를잃음핵과의반응으로잃는에너지의양은무시할정도 단위충돌에서잃는에너지는작으며물질내에서의진로는거의직선적 반응확률이높으며단위거리당생성되는이온쌍의수가많음 상대적으로짧은거리를이동하는동안에대부분의에너지를잃음 중하전입자는이온화밀도가높은약투과성방사선 Stopping power( 저지능 ) 하전입자가물질내에서단위거리를이동하면서소모하는에너지 de s dx 비정 (range) 하전입자가물질을투과할수있는평균거리 비정 저지능
베타입자 ( 전자 ) 원자의전리및여기에의한충돌과정으로에너지를잃는반면궤도전자와질량이동일하므로알파입자보다단위충돌에서잃는에너지의양이많고진로가크게변하여물질내에서의진로는 'zigzag' 한거동을나타낸다. 중하전입자에비하여베타입자는크기가작고 (-)1 가의전하를띠므로물질내에서반응확률이낮다. 따라서베타입자의비정은동일한에너지를지닌알파입자보다상당히길다. 베타입자가물질의핵주위를지날때강한정전기적힘이작용하여베타입자는회절하면서에너지를잃고제동복사선을방출하게된다. 그러므로베타입자의저지능은충돌 (collision) 에의한저지능과복사 (radiation) 에의한저지능의합으로표현된다. de dx tot de dx col de dx rad (+) 전하를띤베타입자의물질과의상호작용은 (-) 전하를베타입자와유사하지만물질내에서에너지를잃어버리고거의정지상태가되었을때주위의자유전자와결합하여각각의정지질량에해당하는 0.511MeV 의에너지를지닌두개의광자를방출하면서소멸하게된다. 이때방출되는광자를 " 소멸감마선 (annihilation gamma-ray)" 이라부른다.
체렌코프복사 (Cherenkov radiation) 하전입자가광학적으로투명한매질속을통과할때, 입자의속도가그매질속에서의빛의속도보다클경우에빛이발생하는현상 c v n v cos c / n 1 bn 복사선은운동궤적을축으로한원뿔면으로퍼짐 t 가 v 에의존하기때문에발생각도를측정하여하전입자의속력을알수있음 고에너지물리학이나우주선연구에많이이용
광자와물질과의상호작용 광자는전하를띠지않기때문에물질내에서의에너지소모과정은정전기적인힘과는무관하며원자와직접적인물리적접촉을통해에너지를잃는다. 전자또는핵과반응하기전까지물질을자유롭게투과할수있으며물질과반응한확률은매우낮다. 광자가물질의원자또는핵과반응을하면 2 차전리를일으킬수있는하전입자 ( 전자 ) 를생성하므로이런종류의방사선을 " 간접전리방사선 (indirectly ionizing radiation)" 이라부른다. 광전효과 (Photoelectric Effect) 입사광자가흡수체원자의궤도전자에모든에너지를부여하는과정 전자의결합에너지보다큰에너지가전달되므로전자는궤도로부터이탈 이탈된전자를광전자 (photoelectron) 광전자의운동에너지 E e = E r E b 광전효과가일어날확률은광자의에너지가전자의결합에너지에상응할때최대가되므로물질의전자결합에너지만큼낮은에너지영역에서지배적으로일어나며물질의원자번호가높을수록증가
컴프턴산란 (Compton scattering) 입사광자가원자에서느슨하게결합된전자 ( 최외각전자 ) 에부분적으로에너지를전달하는과정 광자는더낮은에너지로산란되고 2차전리를일으킬수있는반도전자또는되튐전자 (recoil electron) 를생성 컴프턴산란은중간에너지영역의광자에서지배적으로일어남 물질의원자번호가높은수록확률은증가 산란된광자의에너지 E g ' E g 1 2 m0c E (1 cos ) 반도전자의운동에너지 E e = E r - E r' g
전자쌍생성 (Pair Production) 입사광자가강한전기장을형성하는핵주위에서소멸되고음전자와양전자를생성하는과정 전자의질량은광자의에너지로부터생성 (E=mc 2 ) 되므로전자쌍생성은입사광자의에너지가전자쌍의정지질량에너지인 1.022MeV 이상인경우에만발생 1.022MeV 이상의에너지를지닌광자가전자쌍생성을일으킨다면여분의에너지는음전자와양전자의운동에너지로전환 전자쌍생성이일어날확률은입사광자의에너지가클수록또는물질의원자번호가높을수록증가 생성된음전자와양전자가각각 2 차전리를일으키다가정지하게되면양전자는주위의음전자와결합하여소멸감마선으로전환
Interactions of photons with water and lead
Gamma interaction
중성자와물질과의상호작용 중성자 : 전기적중성쿨룽힘을통한직접상호작용없음 아주느린중성자라할지라도강한핵력에의한산란이나포획반응을할수있음 낮은에너지중성자핵과비탄성충돌 핵들뜸상태 바닥상태로돌아감 광자를내놓음 표적핵의질량이클경우충돌뒤중성자가대부분의에너지를가지고감 중성자와질량이비슷한수소가많이들어가있는파라핀을이용해속도를줄임 수 MeV 이상의중성자측정방법중성자를다른대전입자로바꾸어검출 n n 6 3 Li H a p 3 3 H, H, n 10 B a 7 n p n p Li
How to detect Quark to Cosmos Cosmos Quark -connection-
Detectors use characteristic effects from interaction of particle with matter to detect, identify and/or measure properties of particle; has tr ansducer to translate direct effect into observable/recordable ( e.g. electrical) signal example: our eye is a photon detector; (photons = light qua nta = packets of light) seeing is performing a photon scattering experiment: light source provides photons photons hit object of our interest -- some absorbed, some scattered, r eflected some of scattered/reflected photons make it into eye; focused onto ret ina; photons detected by sensors in retina (photoreceptors -- rods and con es) transduced into electrical signal (nerve pulse) amplified when needed transmitted to brain for processing and interpretation
Overview of Experiment computer simulation 01011101 Physics detector origin interaction signal processing data handling analysis & control Physics
Particle energy loss in matter X rays PE CS electron(s) scintillator gamma rays PP de/dx loss charged particle de/dx loss thermal neutrons nuclear reaction energetic neutrons de/dx loss proton
Particle Detection Principle
Scintillation detectors and sensors Scintillators In radiation detection in solid state physics: luminescence fast < 1 ms: fluorescence slow > 1 ms: phosphorescence inorganic crystals organic plastics/liquid glass gas Light sensors Photomultiplier Microchannel plate silicon diodes CCDs Avalanch photo diode hybrid photo diode
Inorganic Scintillator examples
Organic scintillator Solvents Solute a) Liquid Scintillator 1,2,4-Trimethylbenzene (Pseudocumene) p-,m-,o-xylene, Toluene, Benzene, MN a) Plastic Scintillator Polystyrene (Polyvinylbenzene, PS) Polyvinyltolunen (PVT) 1 st Solvent PPO, p-terphenyl, PBD, Naphthalene.. 2 nd Solvent POPOP, M 2 -POPOP, bis-msb. Standard Liquid Scintillator : PC + PPO(1.5-4g/l) +POPOP(10-50mg) 65% of anthrecene, safe, Pulse shape discrimination of n/gamma
Scintillator light output readout Basic principles of operation Passage of charged particle generates light in scintillator Charged particle Light guide transmits light to photodetector Photomultiplier tube (PM or PMT) generates electric signal
Light transmission Total reflection sin > n ext /n, sin > 1/n, n ext = air Light guide Optical grease (optical cement) Light reflector Teflon, Al, white paint
Photomultiplier Tube Light detection PMT reflector photo cathode N a 2 dynodes N el 1 3 n electron multiplication scintillator optical coupling ideal case: N el = a N
PMTS PMT types Venetian blind (old) Box-and-grid Focused linear structure Gains - 10 7 Circular grid
Oscilloscope signal 660 kev g Typical signals from CsI(Tl) 10 kev g 660 kev a
CsI crystal R&D Co57 source Am alpha source
Neutron gamma separation with BC501A
Neutron gamma seperation with BC501A Partial Total
Identifying particles
What is happening in detector (Simulation) 1GeV gamma
LSND (neutrino oscillation) Mineral Oil + LSC
ATIC Ballon (HE cosmic ray)
GLAST Sattelate (HE gamma)
PET Scans (Positron Emission Tomography) Scintillating crystal detector and photomultiplier 3-D image Cross Section
Radiation Technology Radioisotope imaging: Planar scintigraphy Phosphonate tagged with 99m Tc, injected into the blood stream, is mainly transported to bones, producing a view of the skeletal system The method is e.g. used to determine whether or not cancer has metastized From: National Geographic 171/1(1987)2-41
Radiation in Aviation Security X-ray Application X-ray tube point source transmission Fan beam 1D position sensitive X-ray detector Conveyor 1D + motion 2D
Radiation in Aviation Security X-ray Application Advanced Multi X-ray Energy Method From: Heimann Systems, Wiesbaden, Germany
Radiation in Border Control Gamma-ray Application Fan beam 1D PSD Gamma-ray source CsI(Tl) crystals integrating Moving truck or container transmission 1D + motion 2D
Radiation in Border Control Gamma-ray Application From: NRC Handelsblad