Mike's notes Flashcards
radiation safety accessories
linacs:
- door interlocks
-LPO button - 20 s after pressed, beam can turn on
-Beam on and off lights
-beam status indicator
-emergency stop switches
-radiation warning signs: Indicates high radiation area (if>25 μSv/h) at Vault door
-2 TV cameras
-2-way intercom
-enable/disable switch
-beam off button
brachy only:
-radiation area detector
-door alarm
CNSC annual dose limits
NEW : 20 mSv/yr averaged over 5 years, < 50 mSv/yr in any one given year
public: 1 mSv/yr
preganant woman:4 mSv from time she announces it
shielding max permissible dose (P)
NEW: 1 mSv/yr
public: 0.05 mSv/year
NCRP suggests 5 mSv/yr in controlled area, 1 mSv/yr in uncontrolled area
what is CNSC
canadian nuclear safety commission
what is nrc
nuclear regulatory commission (US)
pregnant NEW dose
4 mSv from declaration to end of pregnancy
PER CNSC FINAL ANSWER
diagnostic dose
3 mSv/yr
50% from CT and 25 % from nuc med
background radiation
1mSv year from cosmic (0.3 mSv), terrrestrial (0.3 mSv), and internal (0.4 mSv)
2 mSv from Radon
ALARA limits vs ICRP60 limits
ALARA limits are 1/20 those of ICRP60 or CNSC
what do you do if radiation level is > 25 uSv/h?
post sign
for a NEW working 40 h per week and 50 wk/yr, the 25 uSv/hr corresponds to 50 mSv/yr if beam is always on
properties of leakage radiation
depends on design
limitied to 0.1% of primary beam
originates from target
assumed to be isotropic
properties of scatter radiation
assumed to come mostly from patient
use largest field size fr measurement (40x40)
Assumed to be isotropic
when is neutron shielded needed?
E >/= 10 MV
barrier types
primary
-directly in path of radiation beam
-must shield for primary, scattered, and leakage radiation
secondary
-not in direct beam path
-accounts for scattered and leakage radiation
typical barrier thickness
Primary 2.1 - 2.4 m
Secondary 0.9 - 1.2 m
Thickness depends on energy, workload, occupancy, and distance.
why do we use 35x35 instead of 40x40 for max field size?
max field size not perfectly square (clipped corners)
-35x35 with collimator rotated 45 degrees
DONT USE THIS QUESTION
why is hydrogen content of shielding material important?
for neutron shielding
different shielding materials
concrete
heavy concrete
steel
lead
earth, dry packed
TVL
TVL = ln(10)/u
u is broad beam linear attenuation coefficient
more shielding needed with broad beam vs narrow beam to stop additional scatter
relate TVL to HVL
TVL =HVL * ln(10)/ln(2)
for broad beam, why are subsequent TVLe < TVL1?
TVLe is subsequent (equilibrium) TVL
Beam hardening counteracted by scattering to lower energies in broad beam geometry
leakage TVL < primary TVL also because leakage spectrum (after passing through the linac head) is softer than primary spectrum.
concrete primary TVL1, TVLe
TVL1 = 37 cm, TVLe = 33 cm for 6 MV
TVL1 = 45 cm, TVLe = 43 cm for 18 MV
concrete leakage and scatter TVL for 6 MV
leakage TVL1 = 34 cm, TVLe = 29 cm
scatter TVL = 17 cm
concrete leakage and scatter TVL for 18 MV
leakage TVL1 = 36 cm, TVLe = 34 cm
scatter TVL = 19 cm
why is TVL18 MV = TVL6 MV for lead?
high Z material
pair production proprtional to Z ln (E)
TVL1= TVLe = 5.7 cm
as compton goes down as 1/ln(E), PP picks up
barrier thickness t expressed in number of TVLs (n)
n = log10(1/B)
B = I/Io
t= TVL1 + (n-1)TVLe
barrier thickness as function of TVL1 and TVLe
t = TVL1 + (n-1)TVLe
TVL1 and TVLe for steel
10 cm at 6 MV, 11 cm for 18 MV
what is Use factor U?
fraction of time linac is directed towards a primary barrier
walls: U = 1/4
floor: U = 1
ceiling: U = 1/4
use factor for all secondary barriers
1
fraction of operating time during which area behind a barrier is occupied
office, console = 1
adjacent tx room = 0.5
staff washroom = 1/5
vault door = 1/8
storage room = 1/20
outdoor areas = 1/40
workload
linac weekly output at iso (1 m) including tx and QA
ex. 35 patients per day, 2.5 y/fx, 5 days per week: W = 35 X 5 X 2.5 Gy = 440 Gy/week
physics QA 160 Gy/week
Total = 600 Gy/week
workload for TBI
W is higher because of larger tx distance for the same pt dose
W = D * d^2
d about 4 m typically
ex. D = 12 Gy, W = 192 Gy/week for 1 TBI/week
IMRT factor
IMRT delivers a radiation field in many segments
More MUs per pt than conventional radiotherapy for same pt dose
leakage works thus increases by factor of 2-15 (Ci = MU imrt/ MU conv)
IMRT factor does not significantly affect the primary or scatter workload since both IMRT and conventional 3DRT deliver the same dose to the tumor.
For VMAT, SBRT, SRS, the same concept applies, i.e. multiply by CI.
primary barrier equation
Bpri = P dpri^2/ (WUT)
P = max permissible dose equivalent (0.1 mSv/wk for controlled areas and 0.02 mSv/wk for uncontrolled areas)
dpri= distance from source to point protected
W = workload, linac output at iso i.e. 1 m (ex 1000 Gy/wk)
U= use factor
T = occupancy factor
B = I/Io
patient attenuation is ignored to be conservative
typical 6 MV primary
6 TVL = 120 cm heavy concrete
typical 6 MV secondary
4 TVL = 80 cm heavy concrete
equation for secondary barrier (scatter)
B= (P/(alphaWT)dsca^2dsec^2*400/F
P= max permissible dose equivalent
alpha = scatter fraction at 1 m from patient for 400 cm^2 beam incident (tabulated)
W = workload, linac output at iso
U=1
T= occupancy factor
dsca= distance from source to patient
dsec= distance from patient to point protected
F= field area at patient mid-depth at 1 m (40x40)
what does alpha in equation for secondary barrier depend on?
-scattered angle and energy
-also material but this is always water for the patient
why do we need F in equation for secondary barrier?
The value for α(θ) is given for field size 20x20=400 cm^2, thus need to scale up to the max field size F=40x40 cm^2.
equation for secondary barrier (leakage)
Bl = P* dl^2/ (10^-3 * WT)
P= max permissible dose equivalent
dl= distance from source to point protected
10^ -3 is allowed head leakage (0.1%)
W = workload, linac output at iso
T= occupancy factor
U=1
secondary barrier thickness once you know thickness for leakage and scatter
two source rule
If a barrier must shield from 2 different sources and individual calculated barriers differ by more than 1 TVL, the thinner source barrier may be ignored.
Otherwise use the thicker barrier + 1 HVL
if tleak>tscat by 1 TVL ⇒use tleak
otherwise ⇒add 1 HVL to tleak
why is leakage barrier > scatter barrier?
leakage energy> scatter energy
Scatter energy is always lower than the primary energy due to Compton interaction.
how do you get HVL from TVL?
HVL=TVL x ln(2) / ln(10)
is t18MV > t 6MV always true?
No, depending on the workload, the barrier thickness for 6MV could be larger (or smaller) than 18MV.
primary scattered off walls for a maze
Think about this equation as how much of the dose (Gy) given to the patients (workload) will reach the maze door due to this specific mechanism (scattered primary off walls).
Hs= WUg alphao Ao alphaz Az/ (dh dr dz)^2
W = primary workload
Ug= use factor for wall g (first wall the primary scatters off of)
alpha o = relfection coefficient at first scattering surface
Ao= area of 1st scattering surface
alpha z = reflection coeff for 2nd reflection from maze surface Az (assume 0.5 MeV as this is max compton scatter energy)
Az= area of maze inner entry projected onto maze wakk from irradiated primary barrier Ao
dh = distance from target to 1st reflection surface (add 1 m distance for SAD)
dr = distance from first reflection, past maze edge, to be on maze midline (b)
dz= centerline distance along maze from b to maze door
what do albedo factors (ie reflection coefficients) depend on?
(1) incident angle, (2) reflection angle, (3) incident photon energy, and (4) wall material. They are tabulated in NCRP151
primary scattered off patient for a maze
H = W Ug a(theta) (F/400) alpha1 Ai/(dsa dsec dzz)^2
a(theta) = scatter fraction for patient scattered radiation at angle theta
W = primary workload
Ug = use factor for wall G (where linac points)
F = field area at patient mid depth at 1 m
alpha1 = reflection coefficient for wall G for pt scattered radiation (assume E = 0.5 MeV)
A1= area of wall G seen from maze door
dsca= target to patient distance = 1m
dsec= patient to maze centerline distance at wall z
dzz= centerline distance along maze
leakage scattered off walls for a maze
H = L Wl Ug alpha1 A1/ (dsec dzz)^2
Lf= head leakage at 1 m from target (0.1 %)
Wl = head leakage workload (may differ from primary W)
Ug = use factor for wall G (where linac aims)
alpha1= reflection coeff for leakage scatter from wall G
A1= area of wll G seen from maze door
dsec= target to maze centerline distance at wall G
dzz= centerline distance along maze
why may Wl differ from Wprimary?
due to IMRT factor
leakage transmitted through maze wall
H = L Wl Ug B/ (dl^2)
L= head leakage ration at 1 m from target = 0.1%
Wl= leakage workload- may differ from primary W
Ug = use factor for wall G where linac aims
B= wall transmission through path
dl = target to maze door center entrance (through wall)
total dose at maze door for beam aimed at wall G
Hg = fHs + Hps+ Hls + Hlt
f= fraction of primary beam tansmitted through patient = 0.25 for 6-10 MV
Hs = dose due to primary scattered from room surfaces
Hps = dpse duer tp scattered primary from patient
Hls= dose due to single scatter head leakage
Hlt= dose due to head leakage transmitted through the wall
Htot = 2.64 Hg = total photon dose at maze door for 4 cardinal angles (for a “typical” maze as detailed in NCRP)
Bdoor = P/Htot
P is 0.1 mSv/ wk (controlled area)
typical 6 MV door
6 mm of Pb
formula for x-ray sky shine
stray photons are scattered by sky to ground outside the tx room
H = (2.5*10^7)(Bxs)Do gamma^1.3/(di ds)^2
Bxs= roof shielding transmission factor for photons
Do= x-ray output dose rate at 1 m from target
gamma = max beam solid angle
di= target to 2 m above roof distance
2.5*10^7 includes conversion of Gy to nSv as H is given in nSv/h
Equation is only (an order of magnitude) estimate and it is just to indicate which room design parameters could have an impact due to skyshine.
side-scattered photon radiation
laterally scattered x-ray from roof barriers to adjacent buildings
Hss= Do F f(theta)/ (Xr^2 * 10^(1+(t-TVL1)/TVLe)
Hss = side scattered dose equivalent rate (Sv/h)
Do= x-ray output dose rate of iso
F= area of square field at 1 m from target
f(theta) = angular distribution of roof-scattered photons 9tabulated)
Xr= distance from beam center roof-top to point of interest
t= roof thickness
TVL1 and TVLe= first and equilbirum TVLs of roof shielding material
It ignores oblique photonincidence & photoneutrons
It dominate overboth leakageradiation & skyshine
10^etc is empirical equation for roof transmission
why is f(theta) larger for smaller theta?
Compton effect
ozone production
3O2 interacts with electron beam to produce 2O3 lethal gas
e- beams produce O3 more than photon beams
O3 concentration should be < 0.1 ppm
ventilation: 3 room changes/hr is adequate for health protection
0.1 ppm ozone in air has odour
when and why is a shielding survey needed?
before linac operational to ensure meeting design goals
first linac beam on - preliminary survey
after inital linac calib- energy check- complete survey
shielding survey- head leakage
locate hot spot via head-wrapped film, and quantify dose rate with ion chamber
shielding survey- barriers
measure dose equivalent/MU and dose rate at the hottest spot beyond each barrier (30 cm beyond)
search forvoids; cracks, or other defects in shielding using a sensitivephoton rate meterwith a fastresponse time (egGeiger Muellerorscintillation detector)
shielding survey- primary barrier
no phantom, max dose rate and field size, 4 gantry angles, all MV
shielding survey- secondary barrier
scattering phantom at iso (simulates patient)
shielding survey- maze door
when door open/closed
shielding survey - skyshine and side scatter
surveys outside bunker
how often do you calibrate survey meters?
annually
ion chamber survey meter
should have both rate and integration mode with a sensitivity in the rage 0.01 mR/hr to 5 R/hr
when do you need to survey for neutrons?
linac >/= 10 MV
usingn-survey meter, egrem ball,bubble detectors, BF3 detectors).
Geiger-Mueller, Farmer, and conventional Si diodes are not suitable for neutron detection due to very low neutron cross sections. However,10B (used in BF3 detectors) has a high cross section for thermal neutrons
HDR brachy shielding for Ir-192 (12 Ci)
TVL = 1.5 cm Pb
15 cm concrete
either a direct shielded door or a short maze
typical walls are 4-5 cm of Pb or 35-61 cm of concrete
mazed door is 3.2 mm of Pb
HDR brachy shielding equation
B = Pd^2/WT
B= barrier transmission factor
P= max permissible dose equivalent (= 0.1 mSv/wkcontrolled areas
= 0.02 mSv/wkuncontrolled areas)
d= distance from source to point protected
T= occupancy factor
W= workload at 1 m from source
W = A K Np Tp
A = activity
K= dose rate constant
Np = number of patients per week
Tp = time/ patient tx
TVL for Ir-192 concrete and lead
15 cm concrete and 1.5 cm lead
TVL for Co-60 concrete and lead
210 cm concrete, 40 cm lead
nominal SAD for cyberknife
65-120 cm
nominal is 80 cm
Shielding calculation assumes 80 cm nominal and workloads are normalized to 1 m for shielding calculations.
The recommended workload per
treatment session is 12.5 Gy at the nominal treatment distance
of 80 cm from the x-ray target
cyberknife shielding
6 MV beam thus no neutrons
IMRT factor is 15 and use factor is 0.05- very small tx beamlets aimed at many directions
workload: ex 8 pt/day * 20 Gy/pt, 5 days/week- 800 Gy/week
scatter is negligible compared to leakage
since leakage is high and U is low, need to calculate both primary and secondary on every wall and add the results to get total dose rate
Ceiling is secondary barrier as beam will not point higher than 22° above horizontal.
Also, all beams will pass within 12 cm of room isocenter (92.1 cm above the floor).
typical shielding for cyberknife
primary = 150 cm concrette
secondary (leakage) = 90 cm concrete
tomotherapy shielding
6 MV - no neutron shielding required
SAD is 85 cm
workload similar to linac
Primary barrier:
limited to narrow strip of wall
typically 1 TVL more than linac due to focused beam
beam stoppers in newer units ↓ shielding requirement (ie. no primary)
Secondary barrier:
many MU → IMRT factor 16
2 TVL more than linac
most leakage & scatter along table axis
(┴ to doughnut)
conclusions likely only apply to serial (not helical) tomo since helical makes better use of the beam and has more internal shielding
gamma knife shielding
no neutrons (1.25 MeV Co-60 source)
6500 Ci from about 200 sources
use max activity in calculation because the work load stays constant- as source decays, the tx time lengthens
Vendor provides an iso-kerma (Gy) maparound machine to aid in calculations.
Workload= (supplied iso-kerma value) x (your weekly patient load)
Typical shielding:
20 cm concrete (inherent shielding ↑↑, 40 cm cast iron)
-barriers are only for scatter and leakage; primary doesn’t exit unit
-manufacturer supplies dose rate plots (i.e. matrix of dose rates at 1/2 m intervals at different heights)
-dose distribution with gamma-helmet door open and couch fully retracted yield max dose rate and determine shielding
-concrete thickness needed is read from curves in NCRP 49
equivalent dose
-in Sv
dose from radiation type R is weighted by radiation weighting factor Wr and summed over all radiation types
Wr is roughly based on RBE
Wr= 1 for photons
20 for alpha particles
5-20 for neutrons
2 for protons
effective dose
equivalent dose to tissue T is weighted by tissue weighting factor Wt and summed over all irradiated tissues
where is data for weighting factors from
ICRP 103
what is RBE
Relative Biologic Effectiveness is defined as the ratio of the reference dose (250kVP x-ray) to test dose where both doses have the same biological effect (ie same cell survival fraction).
Wt for red bone marrow, colon, lung, stomach, breast, remainder tissues
0.12 each
wt for gonads
0.08
wt for bladder, oesophagus, liver, thyrois\d
0.04 each
wt for bone surface, brain, salivary glands, skin
0.01 each
put equivalent dose and effective dose together
Ht = sum of wr Dr (equivalent dose)
E = sum of wt Ht (effective dose)
what is TADR
time averaged dose equivalent rate
barrier attenuated dose equivalent rate averaged over 1 week
Rw = IDR Wpri Upri/Do
Rw= TADR over 1 week (Sv/week)
IDR = instantaneous dose-equivalent rate measured 30 cm beyond barrier with linac beam on at dose rate averaged over 20-60 s
Do = absorbed dose rate at 1 m
Wpri= primary-barrier weekly workload
Upri= primary barrier use factor
why do we have TADR?
Previous barrier shielding assumes workload is evenly distributed during the year, thus the barrier equation takes 1/50 of annual P.
Additional shielding design goal must be implemented with very low workloads but with very high dose rates.
e.g. a dedicated SRT machine (with very high dose rate) may be used only a few times per day.
With exceedingly low workload, the barrier thickness would be low, therefore NRC requires additional shielding design goal defined as 20-micro-Sv “in-any-one-hour”.
equation for IDR
IDR = (dose rate) x (barrier transmission) / (distance)^2
Also IDR = P(dose rate)/(WUT)
why is Rw independent of linac set dose rate?
dose rate appears in IDR equation and is then divided by Do to get Rw
TADR secondary barrier
similar equation to primary barrier
TADR over any one hour
Rh = Nmax Hpt
Rh = TADR in any one hour
Nmax = max number of patients in any one hour, considering patient set-up time
Hpt= average dose equivalent per patient treatment 30 cm beyond barrier
Rh = (Nmax/Nh) * Rw/40
Nh= average number of patient tx per hour
Rw= average TADR over 1 week
Here instead of proportionally scaling Rw to one hour, we multiply it by a number >1
US nuclear regulatory commission (NRC) requires dose equivalent in uncontroled area not exceed 20 μSv in-any-one-hour.(based on calculated Rh)
neutron energy classification
thermal, E = 0.025 eV at 20C, E < 0.5 eV
intermediate, 0.5 eV< E< 10 keV
fast: E > 10 keV
types of neutron interactions
elastic collision with nuclei
inelastic collision with nuclei
neutron capture
equations for elastic neutron collision with nuclei
total Ek before and after collision
for head-on collision, Ef = Ei ((M-m)/(M+m))^2
Ef= kinetic energy of scattered neutron
Ei = kinetic energy of incident neutron
M= mass of targeted nucleus
m= mass of neutron = 1
fraction of neutron Ek transferred to nucleus deltaE = (Ei-Ef)/Ei
deltaE= 4M/(1+M)^2
what is max value of deltaE for neutron elastic collision with nuclei
1- for hydrogen (M=1)
materials with high hydrogen content have efficient energy transfer (i.e. parraffin, wax, polyethylene)
materials with heavier nucleus don;t have efficient energy transfer - poor shielding against neutrons (ex. Pb)
therefore fat dose for a neutron beam is 20% higher than muscle dose!
what is a proton emitted from a neutron interaction called?
recoil proton
neutron interaction- details regarding inelastic collision with nuclei
high enetrgy neutron is absorbed by nucleus (usually high Z)- excited nucleus emits neutron and gamma (n, n gama)
(n,p) and (n, alpha) also possible
what does high energy n contribute to dose in soft tissue via nuclear disintegration?
30%
describe neutron capture (activation)
(n, gamma)
emitted gamma ray is called neutron capture gamma ray
probability of neutron capture
1/(neutron velocity)^2
n spends more time in the vicinity of nucleus thus thermal n have higher cross section for capture.
what element has high thermal neutron capture cross section?
boron
borated polyethylene is used for n shielding
what are photoneutrons
created for linacs with >10 MV
Photon creates a (gamma, n) nuclear reaction
other reactions (gamma, 2n), (gamma, pn) with lower yield
two processes of neutron production for photo-disintegration
direct neutron: Eave a few MeV, forward peaked, yield only 15 %
Evaporation neutron: Eave 1-2 MeV, n-spectra independent of Ephoton, isotropic, dominant process
n production in electron mode
> 2 orders of magnitude less than photons
how to avoid dose to staff from radioactive materials due o photoneutrons and neutron capture in linac head (E > 10 MV)
put higher E treatments at end of day
physics QA end of day
(allow overnight decay)
no repair near head within 40 min of a long test run
use low E for IMRT, VMAT, since these use more MU than 3DCRT
vendors should use materials with low photon and n activation yield
rule of thumb for VMAT MU
3 X Rx
neutron shielding materials
concrete (TVL for neutrons is smaller than that for photons)- thus if shielded for photons it is also adequate for neutrons
heavy concrete (TVL n > TVL photons)
for heavy concrete, TVLn > TVLp
earth
polyethylene
BPE (borated polyethylene)
what is TVD
tenth value distance
distance required for photon fluence to be reduced by 10X
5.4 m for 18-25 MV, 3.9 m for 15 MV
for neutrons, TVD = 2.06* square root of surface area of maze hallway
total neutron fluence per unit dose of x-rays at iso at location in maze (pt A)
phia
n capture gamma rays from maze concrete
direct n + scattered n + thermal n
depend on Qn, n source strength emitted from head per Gy of x-ray dose at iso (tabulated)
depend on total surface area of room (S)
depend on distance from iso to pt A (d). A is at centerline of maze entrance where radiation from head would go to
depend on beta- transmission factor for n penetrating head shield (1 for Pb, 0.85 for W)
direct n fluence = beta * Qn/ (4pi d^2)
scattered n fluence = 5.4 beta Qn/(2 pi S)
thermal n fluence = 1.3 Qn /(2 pi S)
1/2pi is fraction of n that enter the maze
weekly dose equivalent at B due to n-capture gamma rays (Hcg)
Wl * hphi
Wl= workload for leakage
hphi = dose equivalent from n-capture gamma-rays at maze entrance per unit x-ray dose at iso
hphi is determined from scatter, thermal, and direct neutrons
equation for hphi
dose equivalent from n-capture gamma-rays at maze entrance per unit x-ray dose at iso
hphi = K phia 10 ^(-d/TVD)
d is dstance from pt A in maze to entrance of maze
K is ratio of n-capture ɣ-ray dose equivalent to total n fluence at A
=6.9×10^–16Sv m2 per n fluence
φA (phia) is total n fluence [m^–2] at A per unit dose (Gy) of x-rays at iso (i.e. scattered, thermal. direct)
equation for Hn
neutron dose at maze door
Hn=WlHn,D
Wl= workload for leakage
Hn,D = n dose equivalent at maze door per unit x-ray dose at iso
Hn,D is calculated using modified Kersey’s method
total dose equivalent for maze >/= 10 MV Hw, at maze door
Hw = Htot + Hcg + Hn
Htot= sum of leakage + scatter photons dose equivalent. If maze > 2.5 m, Htot is negligible
Hcg= dose equivalent at door due to n-capture-gamma rays from concrete maze
Hn= neutron dose equivalent at maze door
typical door for 18 MV
3.5 mm of Pb, then 54 mm of BPE, then 3.5 mm of Pb
The Pb makes En decrease with inelastic scatter
BPE thermalizes and captures the neutrons
2nd Pb layer stops the n-capture gamma rays that arise in BPE
TVLs for Hcg and Hn
Hcg gamma energy about 3.6 MeV, TVL = 6.1 cm of Pb
Hn neutron energy about 100 keV, TVL = 4.5 cm of BPE
why is door shielding added to maze for >10MV?
establish ALARA by reducing both Hcg and Hn to P/2
Hcg and Hn cannot be added to one equation so P is halved for both
typical direst shielded door (no maze) for >/= 10 MV
7.6cm Pb + 28cm BPE + 7.6cm Pb). In this case door is also a secondary barrier.
first PB layer slows down neutrons with inelastic collisions (remember elastic collisions in Pb with neutrons are essentially 0) and also attenuates photons
BPE layer thermalizes and captures neutrons. It also produces gammas. These gammas are attenuated in 2nd Pb layer.
when do you use laminated barrier
> /= 10 MV
when space is needed
issue with laminated barrier
metal layer can become photoneutron source
-for primary barriers only; for secondary barriers, scatter photons have lower energy and leakage and photoneutrons are negligible
-equation is given for Hn - weekly dose equivalent produced in metal
for laminated barriers, how are captured gamma rays due to neutron production accounted for?
-transmitted x-ray dose equivalent (Htr) is multiplied by 2.7
Htot = Hn + Hph = Hn + 2.7 Htr
because Hph consisted of Htr and H-gamma capture
wrt laminated barrier, can you put the metal after the concrete instead of sandwiched in between 2 concrete layers?
this produces max neutrons…
for laminated barriers- is Pb or Fe better?
Steel is better choice because photo-neutron cross section in steel is 10X less than lead